fermentation of the natural _d_- forms of the three hexoses, glucose,
mannose, and fructose, but is without effect upon the artificial _l_- forms
of the same sugars. The uniform action of zymase upon these hexoses is
easily explained upon the basis of the same assumption which was used to
account for the formation of identical osazones from these sugars and their
easy transformation into each other; namely, their easy transformation into
an _enolic_ form which is identical for all three.
Further, galactose is fermented by some yeasts (although not by all), but
much less readily than are the other sugars, and the temperature reaction
is quite different with galactose than with the others. Talose and tagatose
are entirely unfermentable. A study of the configuration formulas for these
several sugars shows the explanation for these observed facts. These
formulas are as follows:
CHO CHO CH_{2}OH CHOH
| | | ¦
H-C-OH HO-C-H C=O C-OH
| | | |
HO-C-H HO-C-H HO-C-H HO-C-H
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
Glucose Mannose Fructose Enol
CHO CHO CH_{2}OH
| | |
H-C-OH HO-C-H C=O
| | |
HO-C-H HO-C-H HO-C-H
| | |
HO-C-H HO-C-H HO-C-H
| | |
H-C-OH H-C-OH H-C-OH
| | |
CH_{2}OH CH_{2}OH CH_{2}OH
Galactose Talose Tagatose
It will be noted that in the case of glucose, mannose, and fructose, the
configuration is identical at every point except at the aldehyde end of the
chain, and that here the two groups readily arrange themselves into the
same enolic form for the three sugars. Galactose differs from these three
sugars only in the arrangement of the H and OH groups attached to one of
the other carbon atoms (the third from the alcoholic end); the difficulty
of its fermentation indicates that some molecular rearrangement to bring
this group into its proper configuration must precede the fermentation
process. The fact that it is the third HCOH group which thus undergoes
rearrangement is significant because of the participation of these parts of
molecules in groups of threes in many biological processes, as will be
mentioned elsewhere. Talose is unfermentable, even though the arrangement
of its upper three groups is the same as in the galactose and the lower
three the same as in mannose.
If further proof that fermentability depends upon molecular configuration
were needed, it is furnished by the fact that no pentose is fermentible,
even though the stereo-arrangement of each of the four alcoholic groups in
the molecule is identical with the corresponding groups in a fermentible
hexose.
=Oxidation by Bacteria.=--The bacillus _Bacterium xylinum_ contains an
enzyme, or enzymes, which promote the oxidation of the aldehyde group of an
aldose sugar to COOH, or of one alcoholic CHOH group next the terminal
CH_{2}OH group of a hexatomic alcohol to C=O. But these oxidizing enzymes
affect only those compounds in which the OH groups are on the same side of
the two asymmetric carbon atoms next the end of the molecule where the
oxidation takes place, as indicated in the following groupings.
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH or H-C-OH but not HO-C-H or HO-C-H
| | | |
CHO CH_{2}OH CH_{2}OH CHO
The configuration of the remainder of the molecule is immaterial to action
by these oxidizing bacteria; hence, the enzymes in this case are apparently
concerned only with the configuration arrangement of a portion of the
molecule, instead of with the whole hexose grouping, as in the cases of the
other reactions which have been thus far considered.
It is apparent from these illustrations, and from many more which might be
cited, that there is a very definite relation between the molecular
configuration of a carbohydrate and its biochemical properties, as
represented by the possibilities of the action of enzymes upon it. The
probable nature of this relationship will be better understood after the
general questions involved in the mode of enzyme action have been
considered (see chapter XIV). But for the present, it will be sufficient to
note that it seems to be necessary that the enzyme shall actually fit the
molecular arrangement of the compound at all points, in the same way that a
key fits its appropriate lock; or a still better illustration is that of
the fitting of a glove to the hand. On the basis of the latter
illustration, it is just as impossible for a dextro-enzyme to affect a
levo-sugar, or for [alpha]-glucase to affect a [beta]-glucoside, as it is
to fit a right-hand glove upon a left hand. Further attention will be given
to these matters in later chapters.
POLYSACCHARIDES
The polysaccharides which, like the simpler saccharides, or sugars, which
have thus far been studied, undoubtedly serve as reserve food for plants,
are known under the general name of "starches." They are substances of high
molecular weight, whose constitution is represented by the general formula
(C_{6}H_{10}O_{5})_{n}. It should be noted that an exactly accurate formula
should be (C_{6})_{n}(H_{12}O_{6})_{n-1}; but since the value of _n_ is
very high, the simpler formula is approximately correct. The value of _n_
has not been accurately determined for any of the individual members of the
group, but is probably never less than 30 and may often be 200 or more. The
fact that these compounds are insoluble in most of the solvents which can
be used for molecular weight determinations makes it difficult to determine
their actual molecular constitution.
When completely hydrolyzed, the polysaccharides yield only hexoses. They
are, therefore, technically known as "hexosans." Each individual
polysaccharide which has been studied thus far yields only a single hexose,
although the particular hexose obtained varies in different cases. In fact,
the polysaccharides are often classified according to the hexoses which
they yield on hydrolysis, into the following groups: the dextrosans, which
yield glucose, and include starch, dextrin, glycogen, lichenin, etc.; the
levulosans, which yield fructose, and include inulin, graminin, triticin,
etc.; the mannans; and the galactans. The more common representatives of
each of these groups are discussed below.
(A) THE DEXTROSANS
These are by far the most common type of polysaccharides to be found in
plants.
=Starch.=--It is probable that no other single organic compound is so
widely distributed in plants as is ordinary starch. It is produced in large
quantities in green leaves as the temporary storage form of photosynthetic
products. As a permanent reserve food material, it occurs in seeds, in
fruits, in tubers, in the pith, medullary rays and cortex of the stems of
perennials, etc. It constitutes from 50 to 65 per cent of the dry weight of
seeds of cereals, and as high as 80 per cent of the dry matter of potato
tubers.
Starch occurs in plant tissues in the form of microscopic granules,
composed of concentric layers, there being apparently alternate layers of
two types of carbohydrate material, which have been distinguished from each
other by several different pairs of names used by different authors: thus,
Nägeli uses the terms "granulose" and "amylocellulose"; Meyer, "[alpha] and
[beta] amylose"; Wolff, "amylo-cellulose" and "amylo-pectin"; while Kramer
asserts that the layers are alternate lamella of crystalline and colloidal
starch. Many theories as to the nature of these concentric layers and their
mode of deposition have been advanced, but it would not be profitable to
discuss them in detail here.
For purposes of study, starch may be prepared from the ground meal of
cereals, potatoes, etc., by kneading the meal in a bag or sieve of
fine-meshed muslin or silk, under a slow stream of water. The starch
granules, being microscopic in size, readily pass through the cloth with
the water, and may be caught in any suitable container. The starch is then
allowed to settle to the bottom, the water poured off and the starch
collected and dried.
Starch is insoluble in water; but if boiled in water, the granules burst
and a slimy opalescent mass, known as "starch paste," is obtained. This is
undoubtedly a colloidal suspension of the starch in water. By various
processes, such as boiling with very dilute acids, treatment with acetone,
etc., starch is converted into "soluble starch" which dissolves in water to
a clear solution. Soluble starch is precipitated out of solution by
alcohol, or by lead subacetate solution.
Air-dried starch contains from 15 to 20 per cent of water; but this can be
completely removed, without altering the starch in any way, by heating for
some time at 100° C.
The starch granules from different sources vary considerably in size and
shape, and can generally be identified by observation under the microscope.
The most characteristic reaction of starch is the blue color which it gives
with iodine. The reaction is most marked with starch paste or soluble
starch, but even dry starch granules are colored blue when moistened with a
solution of iodine in water containing potassium iodide, or with tincture
of iodine.
When hydrolyzed, either by boiling with dilute acids or under the influence
of enzymes, starch undergoes a series of decompositions, yielding first
dextrins, then maltose, and finally glucose. These transformations can be
traced by the iodine color reaction, as starch will show its characteristic
blue, dextrins purple or rose-red, and maltose and glucose no color with
iodine.
=Dextrins= may occur in plants as transition products in the transformation
of starch into sugars, or _vice versa_. Most commonly, however, they are
artificial products resulting from the partial hydrolysis of starch in the
laboratory or factory. They are amorphous substances, which are readily
soluble in water, forming sticky solutions which are often used as
adhesives ("library paste" is a common example of a very concentrated
preparation of this kind). They are precipitated from solution by alcohol,
but not by lead subacetate (distinction from starch). They are strongly
dextrorotatory (specific rotatory power +192° to +196°); are not fermented
by yeast alone, but readily undergo hydrolysis to glucose which does
ferment. There are several different modifications, or forms, of dextrins,
depending upon the extent to which the simplification of the starch
molecule by hydrolysis is carried. Three fairly definite forms are
generally recognized, as follows: _amylo-dextrin_, or soluble starch,
slightly soluble in cold water, readily so in hot water, giving a blue
color with iodine; _erythro-dextrin_ easily soluble in water, neutral
taste, red color with iodine; and _achroo-dextrin_, easily soluble in
water, sweetish taste, no color with iodine.
Commercial dextrin, which is much used in the preparation of mucilages and
adhesive pastes, is prepared by heating dry starch to about 250° C. It is
composed chiefly of achroo-dextrin, mixed with varying quantities of
erythro-dextrin and glucose.
=Glycogen=, or "animal starch," is one of the most widely distributed
reserve foods of the animal body; in fact, it is the only known form of
carbohydrate-reserve in animal tissues. But it is present only rarely in
plants. It occurs in certain fungi, particularly in yeasts. In the animal
body, glycogen is found in all growing cells; also in the muscles and
blood; but most largely in the liver, where it is stored in large
quantities. The glycogen found in yeasts is identical with that found in
animal tissues. The quantity of glycogen in a yeast cell increases rapidly
as the yeast grows during the fermentation process.
Glycogen is a white, amorphous compound, readily soluble in hot water,
forming an opalescent solution similar in appearance to the solutions of
soluble starch. It is strongly dextrorotatory (specific rotatory power
+190°), is colored brown by iodine, and is hydrolyzed to dextrin and
maltose, and finally to glucose.
=Lichenin=, =para dextran=, and =para isodextran= are dextrosans which have
been isolated from various lower plants. They all yield glucose when
completely hydrolyzed. They resemble starch in chemical properties, but
differ from it in physical form, etc.
(B) LEVULOSANS
=Inulin= replaces starch as the reserve food carbohydrate in a considerable
number of natural orders of plants, particularly in the Compositae. It is
the carbohydrate of the tubers of the dahlia and artichoke and of the
fleshy roots of chicory. It is often found associated with starch in
monocotyledonous plants, such as many species of _Iris_, _Hyacinthus_, and
_Muscari_. Among the monocotyledons, starch seems to be the characteristic
carbohydrate reserve of aquatic, or moisture-loving, species, while inulin
is more common among those which prefer dry situations.
Inulin may be prepared from the tubers of dahlias or artichokes, by boiling
the crushed tubers with water containing a little chalk (to precipitate
mineral salts, albumins, etc.) filtering and cooling the filtrate
practically to the freezing point, which precipitates the inulin.
Inulin is a white, tasteless, semi-crystalline powder, which is soluble in
hot water, from which it may be precipitated by alcohol or by freezing. It
forms no paste like that of starch or dextrin, and gives no color with
iodine. It is levorotatory, and when hydrolyzed by acids or by the enzyme
_inulinase_ yields fructose; in fact, inulin bears the same relation to
fructose that starch does to glucose.
=Graminin, irisin, phlein, sinistrin, and triticin= are all inulin-like
polysaccharides, which have been found in the plants after which they are
named. Their solutions are, as a rule, sticky or gummy in consistency,
which suggests that these compounds bear the same relation to inulin that
dextrins do to starch.
(C) MANNOSANS, OR MANNANS
=Mannan= bears the same relation to mannose that starch does to glucose and
inulin to fructose. It occurs as a reserve food substance in many plants.
It has been reported as present in moulds, and in ergot; in the roots of
asparagus, chicory, etc.; in the leaves and wood of many trees, such as the
chestnut, apple, mulberry, and many conifers; also as a part of the
so-called "hemi-celluloses" which are present in the seeds of many plants,
notably the palms, the elders, cedar, larch, etc.
It is a white, amorphous powder, which is difficultly soluble in water, is
strongly dextrorotatory (specific rotatory power +285°), and when
hydrolyzed yields mannose.
=Secalin= (or carubin) is a substance which is found in the seeds of
barley, rye, etc., which is similar to mannan, but is optically inactive.
(D) GALACTANS
These bear the same relation to galactose that the preceding dextrosans do
to their constituent hexoses. Four different galactans have been isolated
from plant tissues; they are all white, amorphous solids which dissolve
with difficulty in water, forming gummy solutions.
Both galactans and mannans commonly occur associated with cellulose and
hemi-celluloses in the seeds or other storage organs of plants. They are
practically indigestible by animals, as the proper enzymes to hydrolyze
them are not present in the digestive tract; hence, they are commonly
classed with the indigestible cellulose as the "crude fiber" of plants
which are to be used as food by animals.
PHYSIOLOGICAL USE AND BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATES
If the organic compounds produced by plants be classified with reference to
their uses in metabolism into the three groups known, respectively, as
temporary foods, storage products, and permanent structures, it is clear
that the carbohydrates which have been discussed in this chapter may fall
into either one of the first two of these classes. There can be no doubt
that the first products of photosynthesis, whichever ones they may be in
different plants, may be directly used as temporary foods, to furnish the
energy and material for the building up of permanent structures. Also,
there can be no doubt that these same carbohydrates are translocated to
the storage organs and accumulated for later use by the same plant (as, for
example, in the case of the perennials), or by the next generation of the
plant (when the storage is in the endosperm adjoining the embryo of the
seed).
There is no known explanation as to why different species of plants make
use of different carbohydrates for these purposes; or why certain species
elaborate starch out of the same raw materials from which other species
produce sugars, inulin, or glycogen, etc.
In general, starch is the final product of photosynthesis in most green
plants; but there are many exceptions to this. The polysaccharides, which
are generally insoluble, must be broken down into the simpler soluble
sugars before they can be translocated to other organs of the plant for
immediate, or future, use. When they reach the storage organs, they may be
recondensed into insoluble polysaccharides, or stored as soluble sugars.
Examples of the latter type of storage are, sucrose in beet roots, glucose
in onion bulbs, etc. Sometimes, this habit of storage seems to be a species
characteristic; as potatoes store starch, while beets, growing in the same
soil and under exactly the same environment, store sugar. But in other
cases, the nature of the carbohydrate stored undoubtedly is correlated with
the external temperatures at the time of storage. It has been shown that
cold, which tends to physiological dryness, very frequently favors the
storage of sugars instead of starches. Thus, in temperate zones, among
aquatic, or moisture-loving plants, those species which hibernate during
the winter at the bottom of lakes or ponds and are killed by temperatures
below freezing, store starch and no sugar; while in the same ponds, the
species whose storage organs pass the winter above the level of the water
and can withstand temperatures as low as -7° C. contain sugar during the
winter months, even if they contain starch during warmer periods.
Similarly, sugars often appear in the leaves and stems of conifers during
the winter months, only to disappear, or be replaced by starch, when spring
approaches. This same phenomenon is noticeable in arctic plants, which
generally contain but small proportions of starch and relatively large
amounts of sugars.
Similarly, the phenomenon of the turning sweet of potatoes when exposed to
low temperatures has often been noted. The change of the starch in potato
tubers to sugar is most rapid at the temperature of 0° C., and ceases at
7°, or above. Also, if potatoes in which the maximum amount of sugar is
present (not over one-sixth of the total starch can be converted into
sugar) are exposed to a higher temperature the sugar soon disappears.
In general, however, it may be said that each particular species of plant
has its own particular preference for a specific carbohydrate as its
reserve food material, and elaborates the proper enzymes to make it
possible to utilize this particular carbohydrate for its metabolic needs.
Again, the question as to whether the storage of energy-producing materials
for the use of the next generation shall be in the form of carbohydrates or
of fats seems to be definitely connected with the size of the seed, and the
consequent available storage space (see page 138). Animals habitually use
the space-conserving form of fats for their energy-storage, while plants
more commonly use carbohydrates for this purpose, except in the case of
those small seeds in which sufficient energy cannot be stored in
carbohydrate form to develop the young seedling to the point where it can
manufacture its own food. As a general rule, nuts, which contain the embryo
of slow-growing seedlings, and need large proportions of energy reserve,
are characteristically _oily_ instead of _starchy_ in type.
But, aside from temperature reactions and space requirements, there is no
law which has yet been discovered which determines the character of the
energy-storage compound which any given species of plant will elaborate.
The process of photosynthesis would seem to be identical in all cases, at
least up to the point of the production of the first hexose sugar; but the
transformation of glucose into other monosaccharides, disaccharides, and
polysaccharides seems to be a matter which obeys no rule or law.
Finally, there remains to be considered the occurrence and uses of sugars
in the fleshy tissues of fruits. These tissues have, of course, no direct
function in the life history of the plant. They surround the seed, but they
must decay or be destroyed before the seed can come into the proper
environment for germination and growth. In most fruits, starch is the form
in which the carbohydrate material is first deposited in the green tissue,
but as the fruit ripens the starch rapidly changes into sugars, with the
result that the fruit takes on a flavor which makes it much more attractive
as a food for men and animals. This purely biological significance of the
presence of sugars (and of the other substances which give desirable
flavors to fruits, vegetables, etc.), can have no possible relation to the
physiological needs of the individual plant, however.
It is apparent that the production of these immense stores of reserve food
by plants makes them useful as food for animals, and it is, of course, the
storage parts of the plants which are most useful for this purpose. This
biological relationship needs no further emphasis.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 2 ... Die Einfachen
Zuckerarten, Inuline, Cellulosen, ...," 729 pages, Berlin, 1911, and "Band
8--1 Ergänzungsband (same title as Band 2)--" 507 pages; Berlin, 1914.
ARMSTRONG, E. F.--"The Simple Carbohydrates and Glucosides," 233 pages.
_Monographs_ on Biochemistry, London, 1919 (3d ed.).
FISCHER, E.--"Untersuchung ueber Kohlenhydrate und Fermente, 1884-1908,"
912 pages, Berlin, 1909.
MACKENSIE, J. E.--"The Sugars and their Simple Derivatives," 242 pages, 17
figs., London, 1913.
TOLLENS, B.--"Kurzes Handbuch der Kohlenhydrate," 816 pages, 29 figs.,
Leipzig, 1914 (3d ed.).
CHAPTER V
GUMS, PECTINS, AND CELLULOSES
These substances constitute a group of compounds which are very similar to
the polysaccharide carbohydrates in composition and constitution, but which
serve entirely different purposes in the plant. As a class, they are
condensation products of pentoses, known as pentosans and having the
formula (C_{5}H_{8}O_{4})_{n}, or hexosans having the formula
(C_{6}H_{10}O_{5})_{n}, or combined pentosan-hexosans.
In general, these compounds make up the skeleton, or structural framework
material, of the plant, in contrast with the protoplasmic materials or food
substances for which most of the other types of organic compounds
(discussed in other chapters of this book) serve. They are the principal
constituents of "woody fiber," of cell-walls, and of the "middle lamella"
which fills up the spaces between the plant cells. They are, therefore,
found in largest proportions in the stems of woody plants; but they are
also present in every other organ of plants, as the cell-wall or other
structural material.
For purposes of study, these compounds may conveniently be divided into
three groups; namely, the natural gums and pentosans, the pectins and
mucilages, and the celluloses. The segregation into these three groups is
not sharply defined. The distinction between the groups is based upon the
solubility of the compounds in water. The gums and pentosans readily
dissolve in water; the pectins form colloidal solutions which are easily
converted into "jellies"; the mucilages do not dissolve but form slimy
masses; while the celluloses are insoluble in and unaltered by water. Some
authors add a fourth group, known as "humins"; but as these are the
products of decay (usually in the soil) of these structural compounds,
rather than of growth and development, they need not be taken into
consideration in a study of the chemistry of plant growth.
THE NATURAL GUMS AND PENTOSANS
The natural gums, when hydrolyzed, yield large proportions of sugars, but
most of them also contain a complex organic acid nucleus, by means of which
they form salts with calcium, magnesium, etc. Some of them, such as cherry
gum and those which are found in the woody stems of plants (wood gum, and
those found in corn stalks, the straw of cereals, etc.) yield practically
pure pentoses. These are known as pentosans. They bear the same relation to
the pentose sugars as do the dextrosans to glucose, etc. The wound gums,
for example, yield arabinose, and the wood gums yield xylose. But most of
the natural gums yield a mixture of galactose, some pentose, and some
complex organic acid.
The gums are translucent, amorphous substances, whose solutions in water
are levorotatory. They are precipitated out of solution by alcohol and by
lead subacetate solution.
Gums are extremely difficult to hydrolyze, the laboratory process of
hydrolysis usually requiring from eighteen to twenty-four hours of
continuous boiling with acids for its completion. Because of this
difficulty of hydrolysis, gums are practically indigestible by animals and
of little use as food.
The following common examples will serve to illustrate the general nature
of these compounds.
=Gum arabic=, found in the exudate from the stems of various species of
Acacia, is a mixture of the calcium, magnesium, and potassium salts of a
diaraban-tetragalactan-arabic acid. Arabic acid has the formula
C_{23}H_{38}O_{22}, and one molecule of this acid serves as the nucleus for
the union of eight galactose and four arabinose groups, linked together in
some unknown way. The formula for the compound, exclusive of the metallic
elements with which it is loosely united is C_{91}H_{150}O_{78}. This gives
some idea of its complexity.
When boiled with nitric acid, it is oxidized to mucic, saccharic, and
oxalic acids. It gives characteristic reactions with alum, basic lead
acetate, and other common reagents.
Gum arabic comes on the market as a brittle, glassy mass, which is used in
the preparation of mucilages, and as a carrier for essential oils, etc., in
certain toilet preparations.
Recent investigations have shown that the so-called "meta-pectic acid,"
which is often found in sugar beets and interferes with the process of
sugar manufacture, is identical with gum arabic in composition and
properties.
=Gum tragacanth= is the soluble portion of the natural gum which is found
in several species of _Astragalus_. It constitutes only 8 to 10 per cent of
the total gum-like material which is present, the remainder being composed
of insoluble gummy substances of unknown composition. The soluble gum
consists of calcium, potassium, and magnesium salts of an acid which, when
hydrolyzed, yields several molecules of arabinose, six of galactose, and
one of geddic acid (an isomer of arabic acid). It is said to be produced by
the metamorphosis of the medullary rays under unfavorable conditions of
growth. It comes on the market in globular masses of amorphous material,
and is used in the manufacture of cosmetics, etc.
=Wound gum= is frequently found in the tracheæ of plants, and near surface
wounds, which it stanches. It is secreted by the cells surrounding the
injured part. It responds to the reactions of other gums and to some of
those of woody fiber. Its exact composition is not known, but probably lies
between that of the true gums and that of cellulose.
These gums are generally considered to be decomposition products of
celluloses, resulting from the action of some hydrolytic ferment, usually
stimulated by some unfavorable condition of growth, some injury, or some
morbid condition.
The =pentosans=, araban and xylan, occur normally in the stems and outer
seed coats of many common plants. They constitute a considerable proportion
of these tissues, as indicated by the following results of typical
analyses: Wheat bran, 22 to 25 per cent; clover hay, 8 to 10 per cent; oat
straw, 16 to 20 per cent; wheat straw, 26 to 27 per cent; corn bran, 38 to
43 per cent; jute fiber, 13 to 15 per cent; various wood gums, 60 to 92 per
cent.
They are white, fluffy solids, which are difficultly soluble in cold water,
more readily in hot water. They are very difficult to hydrolyze, and
indigestible by animals. When finally hydrolyzed, they yield arabinose and
xylose, respectively. The pith of dry corn stalks is a good illustration of
their general character.
MUCILAGES
These are characterized by forming slimy masses when moistened with water.
They are secreted by hairs on the skin of many plants, so that the external
walls of the leaves, fruit, and seeds are often mucilaginous when damp.
This is particularly true of aquatic plants. The chemical composition of
the mucilages is unknown. When hydrolyzed, they yield arabinose and a
hexose; the latter is sometimes galactose and sometimes mannose.
When present on the surface of plant tissues, the mucilages probably serve
to prevent the too rapid diffusion of materials through the skin, in the
case of the aquatic plants, and too rapid transpiration, in the case of
young vegetative tissues or in other plants when growing under extremely
dry conditions. When found in tubers, or other storage organs, it has been
supposed that they may serve as reserve food materials, but it seems that
such difficultly hydrolyzable compounds as these can hardly function as
normal reserve foods.
PECTINS
Many fruits, such as currants, gooseberries, apples, pears, etc., and many
fleshy roots of vegetables, such as carrots, parsnips, etc., contain
substances known as _pectins_. These are readily soluble in water, and when
dissolved in concentrated solutions in hot water, they set into "jellies"
when the solution is cooled. These jellies carry with them the soluble
sugars and flavors which are present in the fruits, and constitute a
familiar article of diet.
There are undoubtedly several different modifications of the pectins, to
which the names "meta-pectin," "para-pectin," "pectic acid," "meta-pectic
acid," and "para-pectic acid," have been applied. These all seem to be
products of hydrolysis of a mother substance known as "pectose," which
constitutes the middle lamella of unripe fruit, etc. As the fruit ripens,
the pectose is hydrolyzed into the various semi-acid, or acid, bodies
mentioned above. The intermediate products of the hydrolysis are the
pectins, which swell up in water and readily form jellies; while the final
meta-pectic acid is easily soluble in water and resembles the true gums in
its properties. When the middle lamella reaches the pectic acid stage, the
fruit becomes soft and "mushy" in texture.
The pectins more nearly approach to the composition, properties, and
functions of the celluloses than do any of the other groups of organic
compounds. They have been extensively studied in connection with the
parasitism of certain fungous diseases which cause the soft rots of fruits
and vegetables. These parasites usually penetrate the tissues of the host
plant by dissolving out the middle lamella material, which may sometimes
serve as food material for the fungus; but more often the parasite secures
its food supply from the protoplasm of the cell contents. In such cases,
the parasite secretes both a pectose-dissolving enzyme, known as "pectase"
and a "cellulase" which attacks the cell-wall material in order to provide
for the entry of the fungus into the cells. Other enzymes, known as
"pectinases," which coagulate the soluble pectins or pectic acids into
insoluble jellies in the tissues of the plants seem to aid the plant in
resisting the penetration by the parasite.
CELLULOSES
Used in its general sense, this term includes all those substances which
are elaborated by protoplasm to constitute the cell-wall material.
Cellulose proper is a definite chemical compound, whose properties are well
established. In plants, however, this true cellulose is nearly always
contaminated by various encrusting materials; and in the process of
wood-formation, the cell-wall material continually thickens by the
conversion of the cellulose into ligno-cellulose and the protoplasm of the
cell as continuously diminishes in volume. Thus the protoplasm of the cell
produces a number of different kinds of material which are deposited in the
walls of the cell. All of these, taken together, constitute the general
group known as the celluloses.
These may be divided into three classes: namely, (1) the hemi-celluloses,
(2) the normal celluloses, and (3) the compound celluloses.
The =hemi-celluloses= (pseudo-, or reserve celluloses) include a series of
complex polysaccharides which occur in the cell-walls of the seeds of
various plants. They are found in the shells of nuts, rinds of cocoanuts,
shells of stony fruits, etc., and in the seedcoats of beans, peas and other
legumes. They are much more easily hydrolyzed than the other members of
this group, and when hydrolyzed yield various sugars, chiefly galactose,
mannose, and the pentoses. They bear the same relation to these sugars that
starch does to glucose, and are generally supposed to serve as reserve food
material, although it is difficult to conceive how the shells, etc., in
which they appear can be utilized by a growing seedling. They differ in
structure from the fibrous celluloses and are probably not cell-wall
building material. They appear to be a form of reserve carbohydrates, which
differ from the glucose-polysaccharides in being condensed in, or as a part
of, the external structural material rather than in the internal storage
organs. They are soluble in water and exhibit the properties of gums, and
are often classified with the gums and described under the names
"galactans," "mannosans," "pentosans," etc.
The =normal celluloses=, of which the fibers obtained from cotton, flax,
hemp, etc., are typical examples, are widely distributed in plants and form
the commercial sources for all textile fibers of vegetable origin. Ordinary
cotton fiber contains 91 per cent of cellulose, about 7.5 per cent of
water, 0.4 per cent of wax and fat, 0.55 per cent of pectose derivatives,
and 0.25 per cent of mineral matter; or a total of only 1.2 per cent of
non-cellulose solids. Filter paper is practically pure cellulose.
Pure cellulose is a white, hygroscopic substance, which is insoluble in
water and in most other solvents. If heated with water under pressure to
about 260° C., it dissolves completely without decomposition. If boiled
with a strong solution of zinc chloride, or treated in the cold with zinc
chloride and concentrated hydrochloric acid, or with an ammoniacal solution
of copper hydroxide (Schweitzer's reagent), it dissolves to a clear
solution from which it may be reprecipitated without chemical change by
neutralizing or diluting the solution.
Cellulose has the formula (C_{6}H_{12}O_{5})_{n}. When hydrolyzed under the
influence of the enzyme _cytase_, it breaks down, first into cellobiose, an
isomer of maltose, and then into glucose. It is, therefore, chemically
like, but not identical with, starch; and structurally it is arranged in
fibrous form instead of in granules. Under the action of fermentative
enzymes, as when vegetable matter decays under stagnant water, in swamps,
etc., cellulose breaks down into carbon dioxide and marsh gas, according to
the equation
(C_{6}H_{12}O_{5})_{n} + {n}_H_{2}O = 3_{n}CO_{2}+3_{n}CH_{4}.
Cellulose is acted upon by caustic alkalies in a variety of ways. When
fused with a mixture of dry sodium and potassium hydroxides, it is
decomposed into oxalic and acetic acids. When heated with a 10 to 15 per
cent solution of caustic soda, cellulose fibers thicken and become
translucent, thus resembling silk fibers. This process, known as
"Mercerizing," is largely used for the production of commercial fabrics.
Acids also act on cellulose in a variety of ways. When heated with nitric
acid (sp. gr. 1.25), it is converted into _oxycellulose_; while dilute
sulfuric acid, under similar conditions, yields _hydro-cellulose_, a
substance having the formula C_{12}H_{22}O_{11}, which retains the fibrous
structure of the original cellulose but which, when dry, may be rubbed up
into a fine powder. Concentrated nitric acid, or better, a mixture of
concentrated nitric and sulfuric acids, acts upon cellulose, converting it
into various nitro-derivatives, several of which have great industrial
value. The number of NO_{3} groups which unite with the cellulose molecule
under these conditions depends upon the temperature, pressure, etc.,
employed during the nitration process; di-, tri-, tetra-, penta-, and
hexanitrates are all known. _Pyroxylin_, or _collodion_, is a mixture
of the tetra- and penta-nitrates, which is soluble in alcohol and is
used in surgery, in photography, and in the manufacture of celluloid,
which is a mixture of collodion and camphor. The hexanitrate,
C_{12}H_{14}(NO_{3})_{6}O_{4}, is the violent explosive known as
_gun-cotton_.
Gentler oxidizing agents, such as "bleaching powder," etc., have no effect
upon cellulose, and hence are extensively used in the treatment of cotton
and other vegetable fibers, in preparation for their use in the manufacture
of textiles, paper, etc.
Cellulose is indigestible in the alimentary tract of animals, but the
putrefactive bacteria which are generally present there ferment it, with
the production of acids of the "fatty acid" series, carbon dioxide,
methane, and hydrogen. Excessive fermentations of this kind are responsible
for the distressing phenomenon known as "bloat."
The =compound celluloses= comprise the larger proportion of the material of
the woody stems of plants. They consist of a base of true cellulose, which
is either encrusted with or chemically combined with some non-cellulose
constituent. Depending upon the nature of the non-cellulose component, the
compound celluloses are divided into three main groups, known respectively
as (1) ligno-celluloses, (2) pecto-celluloses, and (3) adipo-, or
cuto-celluloses. As the names indicate, the non-cellulose component in the
first group is lignin; in the second, pectic substances; and in the third,
fats or waxes.
=Ligno-celluloses.=--In the young plant cell, the cell-walls consist of
practically pure cellulose; but as the plant grows older, this becomes
permeated with lignin, or woody fiber, until in the stem of a tree, for
example, the proportion of cellulose in the tissue is only 50 to 60 per
cent. In the preparation of wood pulp for the manufacture of paper, the
lignin materials are dissolved off by means of various chemical reagents,
leaving the cellulose fibers in nearly pure form for use as paper. The
lignin material generally consists of two types of substances, one of which
contains a closed-ring nucleus of unknown composition and the other is
probably a pentosan. These materials are so extremely difficult to
hydrolyze that their composition has not yet been definitely determined.
=Pecto-celluloses= are found in various species of flowering plants; those
which are present in the stems and roots being true pecto-celluloses, while
those which are found in fruits and seeds contain mucilages rather than
pectose derivatives, and are generally designated as "muco-celluloses." The
exceedingly inert character of these compounds makes their study difficult
and their functions uncertain.
The term =cuto-celluloses= is applied to the group of substances, including
suberin and cutin, which constitute waterproof cell-walls. These were
formerly supposed to consist of true cellulose impregnated with fatty or
wax-like materials. Recent investigations seem to indicate, however, that
there is really no cellulose nucleus in such walls as these, but that they
are compound glyceryl esters resembling the true fats (see chapter X) in
composition. If this view should finally be established as a fact, this
sub-group of supposed compound celluloses should be dropped from
consideration as such.
PHYSIOLOGICAL USE OF CELLULOSES
There seems to be no question that the sole use of celluloses is to serve
as structure-building materials. They are undoubtedly elaborated from the
carbohydrates as the cell grows. In only rare cases, however, is there any
evidence that they can be reconverted into carbohydrates to serve as food
material. Certain bacteria can make use of cellulose as food, and secrete
an enzyme, cytase, which aids in the hydrolysis of cellulose to sugars for
this purpose. But this enzyme seems rarely, if at all, to be present in the
tissues of higher plants. It has been reported that some cellulose is
hydrolyzed during the malting of barley, indicating that this might have
some food use for the growing seedling; but this observation has not been
confirmed and later investigations seem to throw doubt upon its accuracy.
Bacteria of decay also act upon cellulose materials, converting them
chiefly into gaseous products; but this seems to be a provision of nature
for the destruction of the cell-wall material of dead plants, rather than
an arrangement for the constructive use of it as food for the bacterium.
When fibrous plant residues decay in the soil, the cellulose compounds are
first converted into a series of complex organic acids, known as "humins,"
which undoubtedly have a significant effect upon the chemical and physical
properties of the soil, but these have little interest or significance in a
study of the chemistry of plant growth.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 2, Gummisubstanzen,
Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin, 1911; and "Band
8--1 Ergänzungsband (same title as Band 2)--," 507 pages, Berlin, 1914.
SCHWALBE, C. G.--"Die Chemie der Cellulose," 665 pages, Berlin, 1911.
CHAPTER VI
GLUCOSIDES
Strictly speaking, the term _glucoside_ should be applied only to such
compounds as contain glucose as the characteristic basic group. But in
common usage, it refers to any compound which, when hydrolyzed, yields a
sugar as one of the products of the hydrolysis. In all the natural
glucosides which occur in plant tissues, the other organic constituent,
which is represented by the R in the formula for glucosides
(R·C_{6}H_{11}O_{5}, or R·(CHOH)_{5}CHO) is some aromatic group, or
closed-ring benzene derivative.[3] The different organic constituents of
glucosides are of a great variety of types, such as phenols, alcohols,
aldehydes, acids, oxyflavone derivatives, mustard oils, etc. It is
noteworthy, however, that no nitrogenous groups of the protein type have
been found combined with sugars in glucosides.
Some glucosides contain more than one saccharide group, possibly as di- or
trisaccharides. Under proper conditions of hydrolysis, one or more of the
saccharide groups can be removed from such compounds, resulting in
glucosides of simpler structure.
Most of the common glucosides are derived from _d_-glucose. Some are known,
however, which are derivatives of galactose or rhamnose; while in some
cases the exact nature of the sugar which is present has not yet been
determined.
FOOTNOTES:
[3]
HC
// \
HC CH
The structural formula for benzene, C_{6}H_{6}, | ¦ is one which it is
HC CH
\\ /
CH
difficult and inconvenient to reproduce in type. On that account, it is
/\
customary to indicate this formula by a plane hexagon, thus | |.
\/
It is understood, in all such cases, that the figure represents six carbon
atoms arranged in a closed ring, with alternate double and single bonds,
and with a hydrogen atom attached to each carbon. The printing of some
other group as OH, CH_{3}, adjacent to an angle of the hexagon means that
this group replaces the H atom in the compound which is being illustrated.
HYDROLYSIS OF THE NATURAL GLUCOSIDES
All natural glucosides are hydrolyzed into a sugar and another organic
residue by boiling with mineral acids; although they vary widely in the
ease with which this hydrolysis is brought about.
In most cases, the glucoside is easily hydrolyzed by an enzyme which occurs
in the same plant tissue, but in different cells than those which contain
the glucoside. Injury to the tissues, germination processes, and perhaps
other physiological activities of the cells, result in bringing the enzyme
in contact with the glucoside and the hydrolysis of the latter takes place.
A large number of such enzymes have been found in plants, many of which
hydrolyze only a single glucoside. However, two enzymes, namely, the
emulsin of almond kernels, and _myrosin_ of black mustard seeds, each
hydrolyze a considerable number of glucosides. In general, _emulsin_ will
aid in the hydrolysis of any glucoside which is a derivative of
[beta]-glucose, and myrosin will help to split up any sulfur-containing
glucoside. Glucosides which are derivatives of rhamnose require a special
enzyme, known as _rhamnase_, for their hydrolysis.
The following reactions for the hydrolysis of arbutin and of amygdalin are
typical of this action, and will serve to illustrate the general structure
of these compounds:
------O-------
| |
CH_{2}OH·CHOH·CH·CHOH·CHOH·CH·O·C_{6}H_{4}OH + H_{2}O
Arbutin
= C_{6}H_{12}O_{6} + HOC_{6}H_{4}OH
Glucose Hydroquinone
C_{6}H_{5}
|
(_a_) C_{6}H_{11}O_{5}·O·C_{6}H_{10}O_{4}·O·CH + H_{2}O
Amygdalin |
CN
C_{6}H_{5}
|
= C_{6}H_{11}O_{5}·O·CH + C_{6}H_{12}O_{6}
Mandelo-nitrile | Glucose
glucoside CN
C_{6}H_{5}
|
(_b_) C_{6}H_{11}O_{5}·O·CH+H_{2}O = C_{6}H_{5}·CHOH·CN+C_{6}H_{12}O_{6}
Mandelo-nitrile | Mandelo-nitrile Glucose
glucoside CN
(_c_) C_{6}H_{5}·CHOH·CN + H_{2}O = C_{6}H_{5}·CHO + HCN
Mandelo-nitrile Benzaldehyde Hydrocyanic
acid
GENERAL PROPERTIES OF GLUCOSIDES
As a rule, glucosides are easily soluble in water. They are generally
extracted from plant tissues by digestion with water or alcohol. In most
cases, the enzyme which is present in other cells of the same tissue must
be killed by heating the material, in a moist condition, to the temperature
of boiling water, before the extraction is begun, as otherwise the
glucoside will be hydrolyzed as rapidly as it is extracted from its parent
cell. Maceration or otherwise bruising the tissue, after the enzyme has
been destroyed, facilitates the extraction. The glucosides, after
extraction and purification by recrystallization, are generally colorless,
crystalline solids, having a bitter taste and levorotatory optical
activity. This latter property is remarkable, as most of them are compounds
of the strongly dextrorotatory _d_-glucose.
Many of the natural glucosides have marked therapeutic properties and are
largely used as medicines; others are the mother-substances for brilliant
dyes; for example, indican, from which indigo is obtained, and the alizarin
glucosides.
Several hundred different glucosides have been isolated from plant tissues,
and their properties described, and this number is being added to
constantly, as the methods of isolation and study are improved. They may be
classified into groups, according to the nature of the organic compound
other than sugars which they yield when hydrolyzed. The following
descriptions of the occurrence, constitution, products of hydrolysis, and
special properties of typical members of each of the several different
classes of glucosides will serve to illustrate their general relationship
to plant growth.
THE PHENOL GLUCOSIDES
=Arbutin=, C_{12}H_{16}O_{7}, is obtained from the leaves of the bear berry
(_Arctostaphylos uva-ursi_), a small evergreen shrub. When hydrolyzed by
mineral acids or emulsin, it yields glucose and hydroquinone.
C_{12}H_{16}O_{7}+H_{2}O = C_{6}H_{12}O_{6}+C_{6}H_{4}(OH)_{2}.
Hydroquinone has strongly antiseptic properties. Arbutin is both an
antiseptic and a diuretic, and is used in medicine.
=Phloridzin=, C_{21}H_{24}O_{10}, is found in the bark of apple, pear,
cherry, plum, and similar trees. Mineral acids (but not emulsin) hydrolyze
it to glucose and _phloretin_ (C_{15}H_{14}O_{5}), according to the
equation
CH_{3}
C_{21}H_{24}O_{10} + H_{2}O = C_{6}H_{12}O_{6} + |
(OH)_{3}C_{6}H_{2}·CO·CH·C_{6}H_{4}OH.
It is used in medicine as a remedy for malaria, having marked anti-periodic
properties.
=Glycyphyllin=, C_{21}H_{24}O_{9}, found in leaves of Smilax, yields
rhamnose and phloretin, when hydrolyzed.
=Iridin=, C_{24}H_{26}O_{13} (glucose and irigenin), found in rootstocks of
Iris, is used in medicine as a cathartic and diuretic.
=Baptisin=, C_{26}H_{32}O_{14}·9H_{2}O (two rhamnose and baptigenin), found
in roots of wild indigo (_Baptisia_), has strong purgative properties.
=Hesperidin=, C_{50}H_{60}O_{27} (one rhamnose+two glucose+hesperitin), is
found in the pulp of lemons and oranges.
The characteristic phenol group which is present in these glucosides has
the following structural formula, in each case, the X indicating the H atom
which is replaced by the sugar molecule to form the glucoside:
Phloretin
/\ /\
/ \----C----CH------/ \
| | ¦ | | |OX
HO| |OH O CH_{3} | |OH
\ / \ /
\/ \/
Irigenin
O
/ \
-CH_{2}-C-O--/\
/\ \ / \O·CH_{3}
/ \ O-| |
| | | |OX
CH_{3}O-| | \ /
\ / \/
O
|
CH_{3}
Hesperitin
/\ /\
/ \ / \
HO| |-CH=CH-C-| |OX
CH_{3}O| | ¦ | |
\ / O \ /
\/ \/
THE ALCOHOL GLUCOSIDES
=Salicin=, C_{13}H_{18}O_{7} (glucose+saligenin, or _o_-oxy benzyl alcohol)
is found in the bark, leaves, and flowers of most species of willow, the
proportion present depending upon the season of the year, and the sex of
the tree. It is used as a remedy against fevers and rheumatism, causing
less digestive disturbances than the salicylic acid which is the oxidation
product of saligenin and which is sometimes used as a remedy for
rheumatism.
=Coniferin=, C_{16}H_{22}O_{8} (glucose and coniferyl alcohol), is found in
the bark of fir trees. The coniferyl alcohol obtained from coniferin by
hydrolysis can be easily oxidized to _vanillin_, and is, therefore, the
source for the artificial flavoring extract used as a substitute for the
true extract of the vanilla bean.
=Populin=, C_{20}H_{22}O_{8} (glucose+saligenin+benzoic acid), found in the
bark of poplar trees, is used in medicine as an antipyretic. It can be
hydrolyzed, by a special enzyme, into salicin and benzoic acid.
The structure of the two typical closed-ring alcohols which are present in
these glucosides is indicated by the following formulas;
Coniferyl alcohol
Saligenin
CH=CH·CH_{2}OH
/\ /\
/ \ / \
| |CH_{2}OH | |
| |OX | |OCH_{3}
\ / \ /
\/ \/
OX
THE ALDEHYDE GLUCOSIDES
=Salinigrin=, C_{13}H_{16}O_{7} (glucose and _m_-oxy benzaldehyde), is
found in the bark of one species of willow (_Salix discolor_). Its isomer,
known as _helicin_ (glucose and _o_-oxy benzaldehyde, or salicylic
aldehyde), does not occur naturally in any plant, but is easily produced
artificially by the gentle oxidation of salicin. Their relationships are
shown on the following formulas;
Salicin Helicin Salinigrin
/\ /\ /\
/ \ / \ / \
| |CH_{2}OH | |CHO | |CHO
| |OX | |OX | |
\ / \ / \ /
\/ \/ \/
OX
=Amygdalin=, also contains a benzaldehyde group, but there is linked with
it a hydrocyanic acid group; hence, this glucoside is usually classed with
the cyanophoric glucosides (see page 86).
THE ACID GLUCOSIDES
The most common example of this group is =gaultherin=, C_{14}H_{18}O_{8},
which is found in the bark of the black birch and is a combination of
glucose with methyl salicylate. Both the glucoside itself and the methyl
salicylate ("oil of wintergreen") which is derived from it are used as
remedies for rheumatism.
=Jalapin=, C_{44}H_{56}O_{16} (glucose and jalapinic acid), and
=convolvulin=, C_{54}H_{96}O_{27} (glucose+rhodeose+convolvulinic acid),
are glucosides of very complex organic acids, found in jalap resin, which
are used in medicine as cathartics or purgatives.
THE OXY-CUMARIN GLUCOSIDES
Cumarin itself is widely distributed in plants. No glucoside containing
cumarin as such has yet been isolated; but several glucosides of its
oxy-derivatives are known. The following are common ones:
=Skimmin=, C_{15}H_{16}O_{8} (glucose and skimmetin), is found in _Skimmia
japonica_; =æsculin=, C_{15}H_{16}O_{9} (glucose and æsculetin), is found
in the bark of the horse-chestnut, _Æsculus hippocastanum_, and its isomer,
=daphnin= (glucose and daphnetin), in several species of _Daphne_; and
fraxin, C_{16}H_{18}O_{10} (glucose and fraxetin), is found in the bark of
several species of ash.
The structural arrangement of the oxy-cumarin groups which are found in
these glucosides is shown in the following formulas. It is not known to
which OH group the sugar is attached, in each case.
Skimmetin Æsculetin
CH=CH·CO CH=CH·CO
/\ | /\ |
/ \_O_| / \_O_|
| | | |
| | HO| |
\ / \ /
\/ \/
OH OH
Daphnetin Fraxetin
CH=CH·CO CH=CH·CO
/\ | /\ |
/ \_O_| / \_O_|
| | HO| |
| |OH HO| |
\ / \ /
\/ \/
OH OCH_{3}
=Scopolin=, C_{22}H_{28}O_{14}, found in _Scopolia japonica_, contains two
glucose molecules united to a monomethyl ether of æsculin; while
=limettin=, found in certain citrus trees, is the dimethyl ether of
æsculin.
THE PIGMENT GLUCOSIDES
Many, if not all, of the red, yellow, violet, and blue pigments of plants
either exist as, or are derived from, glucosides. These are of three types:
the madder, or alizarin, reds are derivatives of various
oxy-anthraquinones; most of the soluble yellow pigments are glucosides
derived from flavones or xanthones; and the soluble red, blue, and violet
pigments of the cell-sap of plants are mostly anthocyan derivatives. The
four basic groups, or nuclei, which are present in these different types of
compounds are complex groups consisting essentially of two benzene rings
linked together through a third ring in which there are either two oxygen
atoms in the ring, or one oxygen in the ring and a second attached to the
opposite carbon in the (C=O) arrangement, as shown by the following
diagrammatic formulas:
Xanthone Anthraquinone
O O
¦ 1 1 ¦ 1'
/\ C /\ /\ C /\
/ \ / \ / \ / \ / \ / \
| | | |2 2| | | |2'
| | | |3 3| | | |3'
\ / \ / \ / \ / \ / \ /
\/ O \/ \/ C \/
4 4 ¦ 4'
O
Flavone Anthocyan
1 1
/\ O 5'____4' /\ O 5'____4'
/ \ / \____/ \ / \ / \____/ \
2| | | \____/3' 2| | | \____/3'
3| | |5 1' 2' 3| | |6 1' 2'
\ / \ / \ / \ /
\/ C \/ 5
4 ¦ 4
O
The red dyes which were formerly obtained from madder, the powdered roots
of _Rubia tinctoria_, but are now almost wholly artificially synthetized,
consist of at least four different glucosides, the organic group of which,
in each case, is an hydroxy-derivative of anthraquinone. The most important
of these is _ruberythric acid_, composed of two molecules of glucose linked
with one of alizarin (1,2, dioxyanthraquinone). _Xanthopurpurin_ contains
1,3, dioxyanthraquinone, which is isomeric with alizarin; and _rubiadin_ is
a monomethyl (the CH_{3} being in the 4 position), derivative of this
compound. _Purpurin_ is a glucoside of 1,2,4, trioxyanthraquinone.
The soluble yellow pigments are generally glucosides of hydroxy-derivatives
of xanthone or flavone, known as oxyxanthones or oxyflavones. The sugars
which are united to these nuclei vary greatly, so that there are a great
variety of yellow, white, or colorless flavone or xanthone pigment
compounds. These compounds are almost universally present in plants. For
example, one typical set of examinations of the wood, bark, leaves, and
flowers of over 240 different species of tropical plants showed that
flavone derivatives were present in every sample which was tested, the
pigments being usually located in the powdery coating of the epidermis of
the tissues.
The following typical examples will serve to illustrate the composition and
properties of the glucosides of this type.
=Quercitrin=, C_{21}H_{20}O_{11}, is found in oak bark, in the leaves of
horse-chestnut, and in many other plants, often associated with other
pigments. It is a brilliant yellow crystalline powder. Industrially, it
ranks next to indigo and alizarin in importance as a natural dye stuff. It
is a glucoside of rhamnose with 1,3,3',4', tetraoxyflavonol (i.e., the
flavone nucleus with five OH groups replacing the hydrogens in the 1, 3, 5,
3', and 4' positions). =Quercetin=, C_{15}H_{10}O_{7}, which is the
tetraoxyflavonol itself, without any sugar in combination with it, is found
in the leaves of several species of tropical plants and in the bark of
others. =Isoquercitrin=, C_{21}H_{20}O_{12}, is derived from the same
flavone, but contains glucose instead of rhamnose, as the sugar constituent
of the glucoside.
=Apiin=, C_{26}H_{20}O_{9}, the yellow glucoside found in the leaves of
parsley, celery, etc., contains apiose (a pentose sugar of very unusual
structure, represented by the formula,
CH_{2}OH
\
COH·CHOH·CHO), and apigenin, which is a 1,3,4',trioxyflavone.
/
CH_{2}OH
=Xanthorhamnin=, C_{34}H_{42}O_{20}, is a very complex glucoside containing
two rhamnose and one galactose groups, united with rhamnetin, which is
quercitin with the H of the OH in either the 1, or 3, position replaced by
a methyl group. There are several similar pigments which differ from
xanthorhamnin only in the number or position of the methoxy groups (i.e.,
the OH groups with a CH_{3} replacing the H), or in the nature of the sugar
which is present in the compound. Rhamnetin itself is found in the fruits
of certain species of _Rhamnus_, and is used in dyeing cotton.
The structural arrangement of the characteristic groups of these flavone
pigments will be dealt with more in detail in the chapter dealing with
Pigments (Chapter VIII).
The best-known yellow pigment which is a _xanthone_ derivative is
=euxanthic acid=, known as "Indian yellow," which is a "paired" compound of
glucuronic acid (see page 42) and euxanthone. The latter is a 2, 3',
dioxyxanthone. The pigment is found in the urine of cattle which have been
fed on mango leaves.
The soluble red, blue, and violet pigments are glucosides of various
hydroxy-derivatives of the anthocyan nucleus. Their constitution and
properties will be discussed in detail in the chapter dealing with the
Pigments. These compounds are isomeric with similar flavone and xanthone
derivatives, and the transition from one color to the other in plants takes
place very easily under the action of oxidizing or reducing enzymes. This
accounts for the change of reds and blues to yellows and browns, and vice
versa, under changing temperature conditions.
The following red or blue plant pigments, which are anthocyan
glucosides, have been isolated and studied (for the structural
arrangement of the characteristic groups, see pages 116): from
cornflower and roses, _cyanin_, C_{28}H_{31}O_{16}Cl (2 molecules
glucose + cyanidin); from cranberries, _idain_, C_{21}H_{21}O_{10}Cl
(galactose + cyanidin); from geranium, _pelargonin_, C_{27}H_{30}O_{15}Cl
(2 molecules glucose + pelargonidin); from pæony, _pæonin_,
C_{28}H_{33}O_{16}Cl (2 molecules glucose + pæonidin, a monomethyl
cyanidin); from blue grapes, _[oe]nin_, C_{23}H_{25}O_{12}Cl (glucose +
[oe]nidin); from whortle berry, _myrtillin_, C_{22}H_{23}O_{12}Cl
(glucose + myrtillidin); from larkspur, _delphinin_, C_{41}H_{39}O_{21}Cl
(2 molecules glucose + 2 molecules _p_-oxybenzoic acid + delphinidin);
and from mallow, _malvin_, C_{29}H_{35}O_{17}Cl (2 molecules glucose +
malvidin).
The blue dye, indigo, is derived from a glucoside of an entirely different
type, known as _indican_. Indican is readily extracted from the leaves of
various species of indigo plants. When hydrolyzed, it yields glucose and
_indoxyl_ (colorless). Indoxyl is easily oxidized to _indigotin_ (the deep
blue dye known as "indigo"). The equations illustrating these changes are
as follows:
(_a_) C_{14}H_{17}O_{6}N + H_{2}O = C_{6}H_{12}O_{6} + C_{8}H_{7}ON
Indican Glucose Indoxyl
(_b_) 2C_{8}H_{7}ON + O_{2} = C_{16}H_{10}O_{2}N + 2H_{2}O
Indoxyl Indigotin
The structural relationships of indoxyl and indigotin may be illustrated by
the following formulas:
O O
/\ /\ ¦ ¦ / \
/ \___COH / \___C C____/ \
| | ¦ | | | | | |
| | C-H | | C=C | |
\ /\ / \ /\ / \ / \ /
\/ N \/ N N \/
| | |
H H H
Indoxyl Indigotin
Natural indigo dye is prepared by fermentation of indigo leaves, the decay
of the cell-walls liberating the enzymes in the tissues, which bring about
the chemical changes illustrated in the above equations.
THE CYANOPHORE GLUCOSIDES
Several glucosides which yield hydrocyanic acid as one of the products of
their hydrolysis are of common occurrence in plants. These are generally
spoken of as the "cyanogenetic" glucosides; but as they do not actually
produce cyanogen compounds, but only liberate them when hydrolyzed, the
recently suggested term "cyanophore" undoubtedly more correctly indicates
their properties.
The best known and most widely distributed of these is =amygdalin=.
Amygdalin was first discovered in 1830, and was one of the first substances
to be recognized as a glucoside. It is found in large quantities in bitter
almonds and in the kernels of apricots, peaches, and plums; also in the
seeds of apples, etc., in fact in practically all the seeds of plants of
the Rose family. It is the mother substance for "oil of bitter almonds,"
which is widely used as a flavoring extract.
Amygdalin has been the object of very extensive studies, and even yet the
exact nature of the linkage between its constituent groups is not certainly
known. When completely hydrolyzed, it yields two molecules of glucose and
one each of benzaldehyde and hydrocyanic acid. Recent studies indicate that
the two sugar molecules are separately united to the other constituents,
rather than united with each other in the disaccharide relationship. In
other words, amygdalin is a true _glucoside_ rather than a _maltoside_.
This is indicated by the fact that when submitted to the action of all
known hydrolyzing agents which affect it, it has never been found to yield
maltose as one of the products of hydrolysis. Furthermore, the rate of
hydrolysis of amygdalin is not affected by the presence of maltose; and the
segregation of the two glucose molecules is accomplished by enzymes other
than maltase, which is the only enzyme which is known to break up a maltose
molecule. Since the exact nature of the linkage is not known, it is
customary and convenient to indicate the unit groups as linked together in
the following order:
C_{6}H_{11}O_{5}-O-C_{6}H_{10}O_{4}-O-C_{6}H_{5}·CH-C[trb]N
(1) (2) (3)(4)
A study of the hydrolysis reactions of amygdalin shows that there are three
different linkages in the molecule which may be broken by the simple
interpolation of a single molecule of water and a fourth which may be split
by a different type of hydrolysis, namely, the C[trb]N linkage. These are
indicated by the numbers below the corresponding portion of the formula
above. Most hydrolyzing agents break the molecule first at (1), yielding
one molecule of glucose and one of mandelo nitrile glucoside (see page 77).
The next step usually breaks the latter at the point indicated by (2),
yielding glucose and benzaldehyde cyanhydrin, or mandelo nitrile. The
latter in turn breaks down at (3) into benzaldehyde and HCN. But when
amygdalin is boiled with concentrated hydrochloric acid, the first change
is the splitting off at (4) of the nitrogen in the form of ammonia and the
consequent conversion of the CN group into a COOH group, producing
amygdalinic acid. On further hydrolysis, this breaks up in the same order
as before. Similarly, it is possible to convert mandelo nitrile into
mandelic acid by splitting off the nitrogen to form a COOH group, instead
of splitting off the HCN group leaving benzaldehyde.
The mandelo nitrile glucoside contains an asymmetric carbon atom which is
wholly outside its glucose group, thus C_{6}H_{10}O_{5}-O-C_{6}H_{5}·CH·CN.
Hence, it may exist in dextro, levo, and racemic forms. In the amygdalin
molecule, it exists in the dextro form, which has been named "prunasin."
The levo form, known as "sambunigrin," has been obtained by hydrolysis
of a compound isomeric with amygdalin, whose composition has not been
definitely worked out; while the racemic form, known as "prulaurasin,"
has been prepared from isoamygdalin, by the action of alkalies. Hence,
all the possible compounds indicated by the presence of the asymmetric
carbon have been found and identified.
The crude enzyme preparation which is obtained from almond seeds, known as
"emulsin," contains two enzymes, _amygdalase_, which breaks the amygdalin
molecule at linkage (1), and _prunase_, which breaks it at (2). The action
of amygdalase must always precede that of prunase. In other words, it is
never possible to break off a disaccharide sugar from the molecule, either
by the action of prunase alone, or by means of any other hydrolytic agent.
=Dhurrin=, C_{14}H_{17}O_{7}N, is another glucoside of fairly general
occurrence in plants, which yields HCN as one of the products of its
hydrolysis. It is found in the leaves and stems of several species of
millets and sorghums. Frequent cases of poisoning of cattle from eating of
these plants as forage have been reported. On hydrolysis, dhurrin first
yields glucose and paraoxy-mandelo nitrile; the latter then breaks down
into paraoxy-benzaldehyde and HCN.
=Vicianin=, C_{19}H_{25}O_{10}N, is a cyanophoric glucoside, found in the
seeds of wild vetch, etc. On hydrolysis, it yields glucose, arabinose, and
_d_-mandelo nitrile. It is, therefore, similar to amygdalin, except that
one glucose molecule is replaced by arabinose.
THE MUSTARD OIL GLUCOSIDES
The seeds of several species of plants of the Cruciferæ or mustard family
contain glucosides in which the other characteristic group is a
sulfur-containing compound. These glucosides yield "mustard oils" when they
are hydrolyzed by the enzyme _myrosin_, which accompanies them in the
plant. The following glucosides, found in the seeds of white and black
mustard, are the best-known representatives of this class.
=Sinigrin=, C_{10}H_{16}O_{9}NS_{2}K, found in black mustard seeds, when
hydrolyzed yields glucose, acid potassium sulfate, and allyl
isosulfocyanide (mustard oil), as indicated by the equation.
C_{10}H_{16}O_{9}NS_{2}K+H_{2}O = C_{6}H_{12}O_{6} +
C_{3}H_{5}N[trb]C=S+KHSO_{4}.
The acid potassium sulfate group separates first and most readily, leaving
a compound known as _merosinigrin_, for which the following formula has
been suggested:
-----O------
| |
CH_{2}OH·CHOH·CH·CHOH·CH·CH
| |
O S
| /
|/
C=N[trb]C_{3}H_{5}
This compound usually breaks down into glucose and mustard oil; but by
special treatment it is possible to obtain from it thioglucose,
C_{6}H_{11}O_{5}·SH. This indicates that in the original glucoside the
glucose is linked with the mustard oil through the sulfur atom.
=Sinalbin=, C_{30}H_{42}O_{15}N_{2}S_{2}, from white mustard seeds, when
hydrolyzed by myrosin, yields glucose, sinalbin mustard oil (a
paraoxybenzyl derivative of allyl isosulfocyanide) and sinapin acid
sulfate; according to the equation
C_{30}H_{42}O_{15}N_{2}S_{2}+H_{2}O = C_{6}H_{12}O_{6}+C_{7}H_{7}O·NCS
Sinalbin Glucose Sinalbin mustard oil
+ C_{16}H_{24}O_{5}N·HSO_{4}.
Sinapin acid sulfate
The sinalbin mustard oil may be represented by the formula
____
/ \
HO-CH CH-CH_{2}NCS. Hydrolysis of the sinapin acid sulfate converts
\____/
it into sinapinic acid, C_{6}H_{2}OH·(OCH_{3})_{2}·CH=CH·COOH, choline,
N(CH_{3})_{4}C_{2}H_{4}OH (see page 152), and H_{2}SO_{4}. It is,
therefore, a very complex glucoside.
TEE DIGITALIS GLUCOSIDES
The five, or more, glucosides which are present in the leaves and seeds of
the foxglove (_Digitalis purpurea_) have been extensively studied, as they
are the active principles in the various digitalis extracts which are used
in medicine as a heart stimulant.
=Digitoxin=, C_{34}H_{54}O_{11}, which is the most active of these
glucosides in its physiological effects, when hydrolyzed, yields
digitoxigenin, C_{22}H_{32}O_{4}, and a sugar having the formula
C_{6}H_{12}O_{4}, which is known as "digitoxose" and is supposed to be a
dimethyl tetrose.
=Digitalin=, C_{35}H_{56}O_{14}, is also strongly active. When hydrolyzed,
it yields digitaligenin, C_{22}H_{10}O_{3}, glucose, and digitoxose.
=Digitonin=, C_{54}H_{92}O_{28}, constitutes about one-half of the total
glucosides in the extract which is obtained from most species of the
digitalis plants. It is much less active than the others. It is a saponin
(see page 90) in type. On hydrolysis, it yields 2 molecules of glucose, 2
of galactose, and one of digitogenin.
=Gitonin=, C_{49}H_{80}O_{23}, containing 3 molecules of galactose, one of
a pentose sugar, and one of gitogenin; and =gitalin=, C_{28}H_{48}O_{10},
containing digitoxose and gitaligenin, have also been isolated from
digitalis extracts.
The structural arrangement of the characteristic groups in these glucosides
has not yet been definitely worked out.
=Cymarin=, the active principle of Indian hemp (_Apocynum cannabinum_), is
similar in type to the digitalis glucosides. When hydrolyzed, it yields a
sugar known as "cymarose," C_{7}H_{14}O_{7}, which seems to be a monomethyl
derivative of digitoxose, and cymarigenin, C_{23}H_{30}O_{5}, a compound
which is either identical or isomeric with the organic residue obtained
from other members of this group.
THE SAPONINS
The saponins constitute a group of glucosides which are widely distributed
in plants, whose properties have been known since early Grecian times. They
have been found in over four hundred different species of plants, belonging
to more than forty different orders.
The most characteristic property of saponins is that they form colloidal
solutions in water which produce a soapy foam when agitated, and are
peculiarly toxic, especially to frogs and fishes. In dry form, they have a
very bitter, acrid taste, and their dust is very irritating to the mucous
membranes of the eye, nose, and throat.
On hydrolysis, the saponins yield a variety of sugars,--glucose, galactose,
arabinose, and sometimes fructose, and even other pentoses--and a group of
physiologically active substances, known as "sapogenins."
The more toxic forms of these glucosides are known as "sapotoxins."
The chemical composition of the saponins varies so widely that it is
scarcely possible to cite typical individuals. Sarsaparilla, the dried root
of smilax plants, contains a mixture of non-poisonous saponins, from which
at least four individual glucosides have been isolated and studied. Corn
cockle contains a highly poisonous sapotoxin which, on hydrolysis, yields
four molecules of a sugar and one of sapogenin, C_{10}H_{16}O_{2}. Other
sapotoxins are obtained from the roots of soapwort and from several species
of _Gypsophila_. Digitonin and digito-saponin are glucosides of this type
which are found in the extracts from various species of _Digitalis_.
THE PHYSIOLOGICAL USES OF GLUCOSIDES
It is scarcely conceivable that substances which vary so widely in
composition as do the different types of glucosides can possibly all have
similar physiological uses in plants. The cyanophoric glucosides, the
pigment glucosides, the mustard oil glucosides, and the saponins, for
example, can hardly be assumed to have the same definite relationships to
the metabolism and growth of the plant. To be sure, they are alike in that
they all contain one or more sugar molecules, and it is probable that the
carbohydrates which are held in this form may serve as reserve food
material, especially when the glucoside is stored in the seeds; but it is
obvious that the simpler and more normal form of such stored food is that
of the polysaccharides which contain no other groups than those of the
carbohydrates. It seems much more probable that the physiological uses of
glucosides depend upon their ability to form temporarily inactive "pairs"
with a great variety of different types of organic compounds which are
elaborated by plants for a variety of purposes.
It has been noted that in most, if not all, instances, the glucosides are
accompanied in the same plant tissue (although in separate cells) by the
appropriate enzyme to bring about their hydrolysis and so set free both the
sugar and the other characteristic component whenever the conditions are
such as to permit the enzyme to come in contact with the glucoside. This
occurs whenever the tissue is injured by wound or disease, and also during
the germination process.
Injury to the plant tissue seems to be a necessary preliminary to the
functioning of the active components of the glucoside, except in the case
of the seeds. This leads naturally to the supposition that at least some of
these glucosides are protective or curative agents in the plant tissues.
This conception is further supported by the facts that many of the
non-sugar components of glucosides are bactericidal in character and that
the glucosides commonly occur in parts of the plant organism which are
otherwise best suited to serve as media for the growth of bacteria. Thus,
it is known that in the almond, as soon as the tissue is punctured,
amygdalin is hydrolyzed and all bacterial action is inhibited. Similarly,
the almost universal presence of glucosides containing bactericidal
constituents in the bark of trees insures natural antiseptic conditions
for all wounds of the outer surfaces of the stem of the plant. In fact, it
is easily conceivable that at least one of the reasons for the failure of
the processes of decay of plant tissues to set in until after the death of
the cells, is that during living, respiratory activity these antiseptic
glucosides are so generally present in the tissues.
Further, it has been fairly well established that the "chromogens," or
mother-substances of the pigments, which, under the influence of oxidase
enzymes, serve to regulate the respiratory activities of the plant are
essentially glucosidic in character. This, and other, functions of the
pigments, most of which are glucosides, will be discussed at some length in
the chapter dealing with the Pigments (Chapter VIII).
Many gaseous anæsthetics are known to have a marked effect in stimulating
plant growth. In a number of cases, it has been shown that the contact of
plant tissues with these anæsthetics brings about an interaction of the
enzyme and glucoside which are present in the tissue, with the consequent
hydrolysis of the latter, setting free its characteristic components. This
observation has led to the supposition that many of the organic
constituents of glucosides are definite plant stimulants, to which the name
"hormones" has been applied. There is considerable experimental evidence to
support this conception that glucosides may be the source of stimulating
hormone substances, which will be discussed more in detail in the chapter
dealing with these plant stimulants (Chapter XVII).
Glucosides may also serve as the mechanism for putting out of action of
harmful products which may appear in the tissues as the result of abnormal
conditions. These harmful substances may be rendered soluble by combination
with sugars and so transposed by osmosis to some other part of the plant.
The abnormally large percentages of glucosides which are present in certain
species of plants during unfavorable climatic conditions lends some support
to this view.
Finally, it may be assumed that easily oxidizable substances, such as
aldehydes and acids, are possibly protected against too rapid, or
premature, oxidation by being transformed into glucosides.
In general, it may be said that the glucosides seem to serve as the
regulatory, protective, and sanatory agencies of the plant mechanism.
BIOLOGICAL SIGNIFICANCE OF GLUCOSIDES
The bitter taste of glucosides and their almost universal presence in the
bark of plants undoubtedly helps to prevent the destructive gnawing of the
bark by animals.
Glucosides having either a strong bitter taste, or pronouncedly poisonous
properties, likewise undoubtedly serve to protect such important organs of
plants as the seeds and fruits from being prematurely eaten by birds and
animals. The common disappearance of these bitter substances as the seed or
fruit ripens adds to the attractiveness of the material for food for
animals at the proper stage of ripeness to provide for wider distribution
of the seeds for further propagation. Further, the very general occurrence
of these protective glucosides in many of the vegetative parts of plants
during the early stages of growth, followed by their disappearance after
the seeds of the plant have been formed, certainly serves to protect these
plants from consumption as forage by animals before they have been able to
develop their reproductive bodies. The lack of palatability, and even the
production of digestive disorders resulting from the eating of unripe fruit
may be due, in part at least, to the presence of protective glucosides in
unripe fruits and vegetables.
On the other hand, the almost universal presence of the brilliant pigment
glucosides in the external parts of flowers undoubtedly serves to attract
the insects which are biologically adapted to provide for the
transportation of pollen from one blossom to another and so to insure the
cross-fertilization which is so important in maintaining the vigor of many
species of plants.
It is apparent that this important group of compounds, with its exceedingly
varied and complex constituent groups, may play a variety of significant
rôles in plant growth.
REFERENCES.
ARMSTRONG, E. F.--"The Simple Carbohydrates and Glucosides," 239 pages,
_Monographs_ on Biochemistry, London, 1919 (3d ed.).
VAN RIJN, J. J. L.--"Die Glykoside," 511 pages, Berlin, 1900.
CHAPTER VII
TANNINS
Using the term in its general application to a group of substances having
similar chemical and physical properties, rather than in its limited
application to a single definite chemical compound known commercially as
"tannin," the _tannins_ are a special group of plant substances, mostly
glucosides, which have the following characteristic properties. First, they
are non-crystalline[4] substances, which form colloidal solutions with
water, which have an acid reaction and a sharp astringent taste. Second,
they form insoluble compounds with gelatine-containing tissues, as shown by
the conversion of hide into leather. Third, they form soluble, dark-blue or
greenish-black compounds with ferric salts, the common inks. Fourth, they
are precipitated from their solutions by many metallic salts, such as lead
acetate, stannous chloride, potassium bichromate, etc. Fifth, they
precipitate out of solution albumins, alkaloids, and basic organic coloring
matters. Finally, most tannins, in alkaline solutions, absorb oxygen from
the air and become dark brown or black in color.
FOOTNOTES:
[4] The needle-like forms, in which commercial "tannin" comes on the
market, are not true crystals, but are broken fragments of the threads into
which the colloidal tannin is "spun-out" from the syrupy extracts of
nutgalls, etc.
OCCURRENCE
Tannins occur widely distributed in plants. Practically every group of
plants, from the fungi up to the flowering plants, contains many species of
plants which show tannin in some of their tissues. Among the higher plants,
tannins occur in a great variety of organs. Thus, they are found in the
roots of several species of tropical plants; in the sterns, both bark and
wood, of oaks, pines, hemlock, etc.; in the leaves of sumac, rhododendron,
etc.; in many fruits, especially in the green, or immature, stages; and in
the seeds of several species, either before or after germination. Tannins
are also found in certain special structures, such as gland cells, cells of
the pulvini, laticiferous tissues, etc. Further, they are especially
abundant in the pathological growths known as galls, which often contain
from 40 to 75 per cent of tannin and constitute the most important
commercial source for these materials.
The principal commercial sources of tannin, which is used in the
manufacture of inks, in the tanning of leather, in certain dyeing
operations, etc., are oak-galls, the bark and wood of oak, hemlock, acacia,
and eucalyptus, the bark of the mangrove, the roots of canaigre, and the
leaves of several species of sumac.
CHEMICAL CONSTITUTION
Tannins are either free phenol-acids or, more commonly, glucosides of these
acids. Common "tannin," when hydrolyzed, yields from 7 to 8 per cent of
glucose, which indicates that it is a penta-acid ester of glucose, i.e.,
each glucose molecule has five acid groups attached to it. The formula for
such a tannin is, therefore, as follows,
------O-------
| |
CH_{2}OR·CHOR·CH·CHOR·CHOR·CHOR
in which the R represents a complex phenol-acid like tannic acid, or
digallic acid. These acids are derivatives of the common phenols, whose
constitution will be brought to mind by the following series of formulas:
Ordinary phenol Pyrocatechol Resorcinol Hydroquinone
/\ /\ /\ /\
/ \ / \ / \ / \
| |OH | |OH | |OH | |OH
| | | |OH | | HO| |
\ / \ / \ / \ /
\/ \/ \/ \/
OH
Pyrogallol Oxyhydroquinone Phloroglucinol
OH OH OH
/\ /\ /\
/ \ / \ / \
| |OH | |OH | |
| |OH | | HO| |OH
\ / \ / \ /
\/ \/ \/
OH
These phenols themselves do not occur as constituents of tannins, although
they are often found in other glucosides, gums, etc. The following
mono-carboxyl acid derivatives of these phenols are, however, found both
free and in glucoside formation as constituents of many of the common
tannins.
_Pyrocatechuic acid_, derived from pyrocatechol, represented by the
formula,
OH
/\
/ \
| |OH
| |
\ /
\/
COOH
_Gallic acid_, derived from pyrogallol, and represented by the formula,
OH
/\
/ \
HO| |
HO| |COOH
\ /
\/
COOH
In most of the common tannins, however, the characteristic acids are
oxy-derivatives of the so-called "tannon" group, represented by the
formula, C_{6}H_{5}·CO·O·C_{6}H_{5}. For example, _digallic acid_, which is
a constituent of many common tannins, is a tetra-oxy, mono-carboxyl
derivative of this group, having the structural formula,
/\ /\
/ \ / \
HO| |--CO·O--| |COOH
HO| | HO| |
\ / \ /
\/ \/
OH OH
_Ellagic acid_, which is an hydrolysis product of many of the pyrogallol
tannins (see below) and which produces the characteristic "bloom" on
leather tanned by this type of tannins, has the following formula,
-----CO·O-----
/\ /\
/ \__________/ \
HO| | | |OH
HO| |___CO·O___| |OH
\ / \ /
\/ \/
CLASSES OF TANNINS
The tannins are divided into two general classes, known respectively as the
_pyrogallol tannins_ and the _catechol tannins_. These differ in their
characteristic reactions as follows:
Pyrogallol variety Catechol variety
Ferric salts Dark blue Greenish black
Bromine water No precipitate Yellow or brown precipitate
Leather Produce a "bloom" No "bloom"
Conc. sulfuric acid Yellow or brown Red or pink
Lime water Gray or blue ppte. Pink to brown ppte.
Pyrogallol tannins contain approximately 52 per cent of carbon; while the
catechol tannins usually contain 59 per cent to 60 per cent, the difference
being due to the absence of glucose from the molecule in the latter types.
The two types are distributed in plants as follows: pyrogallol tannins in
oak-galls, oak wood, sumac, chestnut, divi-divi, and algaro billa; catechol
tannins in the barks of pines, hemlocks, oaks, acacias, mimosas, cassia,
and mangrove, in quebracho wood, canaigre roots, cutch and gambier. The
so-called "pseudo-tannins" (i.e., compounds which do not tan leather but
possess other properties like tannins) are found in hops, tea, wine,
fruits, etc.
SOME COMMON TANNINS
Ordinary commercial "_tannin_," or "_tannic acid_," is a compound of one
molecule of glucose with five of digallic acid. It is found in many plants,
and is prepared commercially from the Turkish oak-galls and the Chinese
sumac-galls. It exhibits all the characteristic properties which have been
listed above for tannins in general and responds to all the characteristic
reactions of a pyrogallol tannin. It is extensively used for the
manufacture of blue-black ink, and in many technical processes.
=Catechu tannin= and =catechin= are compounds of the catechol tannin type.
The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc.
It is not a true tannin, since it does not convert hide into leather; but
when heated to 120° or above, it is easily dehydrated, forming catechu
tannin which is identical with that which is obtained directly from gambier
and Bombay cutch (products made by evaporating water extracts from the bark
of various tropical trees). This latter is a true tannin, which is much
used in dyeing and other technical processes.
"=Quercitannic acid=," obtained from oak bark, etc., is likewise a catechol
tannin. It yields no glucose on hydrolysis.
A great many other tannins are known, and their possibilities for technical
use in tanning, dyeing, etc., have generally been investigated; but so
little has been learned about their composition and relation to the plant's
own needs, that it seems unnecessary to discuss them in detail here.
PHYSIOLOGICAL USES OF TANNINS
Tannins are probably not direct products of photosynthesis. They are,
however, elaborated in the green leaves of plants and translocated from
there to the stems, roots, etc. Their close association with the
photosynthetic carbohydrates has led many investigators to seek to
establish for them some significant function as food materials, or as
plastic substances in cell metabolism. Many conflicting views have been
advanced, but a careful review of these leads inevitably to the conclusion
that tannins probably do not serve in any significant way as food material.
The glucose which is generally present in the tannin molecule may, of
course, serve as reserve food material, but it seems probable that it
functions as a constituent of the tannins only to assist in making them
more soluble and hence more easily translocated through the plant tissues.
Some fungi, and perhaps other plants as well, can actually utilize tannins
as food material under suitable conditions and in the absence of a proper
supply of carbohydrates. But this does not prove that tannins can normally
replace carbohydrates as food material for these species of plants.
There seems to be ample evidence that tannins are elaborated where intense
metabolism is in progress, such as occurs in green leaves during the early
growing season; in the rapid tissue formation which takes place after the
stings of certain insects, producing galls, etc.; during germination, and
as a result of any other unusual stimulation of metabolism. It may be,
therefore, that tannins serve as safety accumulations of excessive
condensations of formaldehyde, or other photosynthetic products, under such
conditions. It seems certain that in all such cases tannins are the result
of, and not (as some investigators have supposed) the causative agents
for, the abnormally rapid metabolism.
It seems to be fairly well demonstrated that tannins are intermediate
products for the formation of cork tissue. This may account for their
common occurrence in the wood and bark of trees. Indeed, it has been shown
that gallic and tannic acids are present in considerable proportions in
those parts of the plant where cork is being formed. Further, that they
bear direct relation to cork-formation has been demonstrated in two
different ways. First, cork-like substances have been artificially produced
by passing a stream of carbon dioxide through mixtures of formaldehyde with
various tannic acids. Second, by various treatments of cork, decomposition
compounds showing tannin-like properties may be obtained.
Some investigators have held that not only cork tissue but also other
lignose, or cell-wall material, may be developed from tannins. Certain
observations with _Spirogyra_ seem to indicate that tannin may play an
important part in the formation of new cell walls during conjugation, as
cells which are ready to conjugate are rich in tannin, which gradually
diminishes in quantity until it is practically absent at the time of
spore-formation. There seems to be no evidence that tannins perform any
such function as this in higher plants, however.
Again, tannins may play a very important part in pigment-formation. They
are very similar in structure to the anthocyanin pigments, both being made
up of practically identical decomposition units, the phenolic bodies. The
disappearance of tannins during the process of ripening of fruits may be
connected, in part at least, with the development of the brilliant red,
blue, and yellow pigments which give such rich colors to the thoroughly
ripe fruits.
Finally, certain of the tannins undoubtedly serve as protective agents to
prevent the growth of parasitic fungi in fruits, etc. Recent investigations
show that at least some of the varieties of fruits which are resistant to
the attacks of certain parasitic diseases utilize tannins for this purpose.
This protective effect may be accomplished in two different ways. Either
the tannin actually serves as an antiseptic to prevent the growth of the
parasitic fungus within the tissues of the host plant, or it assists in the
development of a corky layer which "walls-off" the infected area and so
prevents further spread of the disease through the tissue. Examples of
both types of protective action have recently been reported.
It is obvious that the different forms of tannins may play different rôles
in plant life, and the same tannin substance may possibly serve different
purposes under different conditions.
BIOLOGICAL SIGNIFICANCE OF TANNINS IN FRUITS
The presence of tannins in fruits and the changes which they undergo during
the ripening process cannot fail to attract attention to their biological
significance in serving to protect the fruit from premature consumption as
food by animals.
Tannins are of frequent occurrence in green fruits, imparting to them their
characteristic astringent taste. They nearly always disappear as the fruit
ripens. The fact that during the ripening process both sugars and fruit
esters, as well as attractive surface pigments, are developed has led
certain investigators to the conclusion that tannins serve as
mother-substances for these materials in the green fruits and are converted
into these attractive agencies during ripening. There is nothing in the
chemical composition of tannins which indicates, however, that they are
precursors of sugars or fruit esters, although (as has been pointed out)
they may give rise to anthocyan pigments.
Further, recent researches concerning the tannin of persimmons (the
best-known and most striking example of the phenomena under discussion)
clearly show that the tannin is not actually used up during the ripening
process; that instead it remains in the ripe fruit in practically
undiminished quantity; but that when the fruit is ripe, the tannin is
enclosed in certain special large cells or sacs, which are surrounded by an
insoluble membrane, so that when the fruit is eaten by animals the
astringent tannin, enveloped in these insoluble sacs, passes by the organs
of taste of the animal without causing any disagreeable effects. This
walling-off of the astringent tannins can be stimulated in partially ripe
fruits by treating them with several different chemical agents, the
simplest method being that of placing the unripe fruit in an atmosphere of
carbon dioxide gas for a short period. The artificial "processing" of
persimmons to render them edible for a longer period before they become
naturally fully ripe and subject to decay is now a commercial enterprise.
This process is of interest because of its possible connection with the
conversion of tannins into cork, under the influence of carbon dioxide gas,
as mentioned in a preceding paragraph.
From these facts, it is apparent that in persimmons, and probably in other
tannin-containing fruits, the process of natural selection has developed a
mechanism for the secretion of tannin in green fruits, followed by a
process for walling it off in harmless condition when the fruit is ripe,
which serves most admirably to protect the fruit from consumption by
animals before the enclosed seeds have fully developed their reproductive
powers.
REFERENCES.
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe,
Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze,
Kautschuk," 822 pages, Berlin, 1912.
ALLEN'S Commercial Organic Analysis, Vol. 5, "Tannins, Dyes and Coloring
Matters, Inks," 704 pages, 6 figs., Philadelphia, 1911 (4th ed.).
COOK, M. T. and TAUBENHAUS, J. J.--"The Toxicity of Tannin," Delaware
College Agricultural Experiment Station _Bulletin_ No. 91, 77 pages, 43
figs., Newark, Del., 1911.
DEKKER, J.--"Die Gerbstoffe," 636 pages, 3 figs., Berlin, 1913.
GORE, H. C.--"Experiments on the Processing of Persimmons to Render them
Nonastringent," U. S. Department of Agriculture, Bureau of Chemistry
_Bulletin_ No. 141, 31 pages, 3 plates, 1911; and No. 155, 20 pages, 1912.
LLOYD, F. E.--"The Tannin-Colloid Complexes in the Fruit of the Persimmon,
_Diospyros_," in _Biochemical Bulletin_, Vol. 1, No. 1, pages 7 to 41, 34
figs., New York, 1911.
CHAPTER VIII
PIGMENTS
Practically all plant structures contain pigments. These may be considered
as of two types: (_a_) the vegetative pigments, which have a definite
energy-absorbing rôle in the metabolic processes of the tissues which
contain them, and (_b_) the ornamental pigments. It is probable that the
same chemical compound may serve in either one of these capacities under
different conditions, but, in general, it is possible to assign either a
definite vegetative, or physiological, use, or else a simple ornamental, or
biological, significance to each of the common pigments. The first type is
found widely distributed through the protoplasm, or cell-sap, of the plant
structures; while the ornamental pigments are located chiefly in the
epidermal cells, especially of flowers.
With respect to their colors, the plant pigments may be grouped as follows:
Green--the chlorophylls.
Yellow--the carotinoids, flavones, and xanthones.
Red--phycoerythrin, lycopersicin, anthocyanin.
Blue--anthocyan derivatives.
Brown--phycophæin, fucoxanthin.
Of these, the chlorophylls, the carotinoids, phycoerythrin (in red
sea-weeds) and phycophæin (in brown sea-weeds) are generally vegetative
pigments; while the others form the basis for most of the ornamental
pigments, although they may have a definite energy-absorbing effect, in
some cases.
THE CHLOROPHYLLS
The importance of the green coloring matter in plants has been understood
for more than a century, its connection with photosynthesis having been
known as far back as 1819. But definite knowledge as to its chemical
constitution is of very recent origin. As recently as 1908, it was asserted
that chlorophyll is a lecithin-like body, yielding choline and
glycero-phosphoric acid on hydrolysis. It is now known, however, that
chlorophyll contains neither choline nor phosphorus, the earlier
observations being due to mixtures of various other materials with the true
chlorophyll in the extracts which were examined. Beginning with 1912,
Willstätter and his collaborators, in a series of classic papers which were
finally collected in book form, clearly demonstrated the chemical
constitution of the green pigments of plants, which had been previously
designated under the single name "chlorophyll." In 1912, Willstätter and
Isler first showed that the green coloring matter which is extracted from
plants by alcohol, ether, etc., is made up of two definite chemical
compounds, to which they assigned the names "chlorophyll _a_" and
"chlorophyll _b_," associated with two yellow pigments, carotin and
xanthophyll, and, in some cases, with the reddish-brown fucoxanthin. The
percentages of total pigment materials, and the relative proportions of the
five different pigments, in several types of plants, are as follows:
-------------------------------------------------------------------------
| Land | Brown | Green
| Plants, | Seaweeds, | Algæ,
| Per Cent. | Per Cent. | Per Cent.
-------------------------------------------------------------------------
Total pigment in the dry matter | 0.99 | 0.29 | 0.21
Proportion of: | | |
Chlorophyll _a_ | 63 | 55 | 44
Chlorophyll _b_ | 22 | 4 | 31
Carotin | 6 | 11 | 7
Xanthophyll | 9 | 10 | 18
Fucoxanthin | | 20 |
The two chlorophylls have the following formulas: chlorophyll _a_,
C_{55}H_{72}O_{5}N_{4}Mg, and chlorophyll _b_, C_{55}H_{70}O_{6}N_{4}Mg.
Hence, they differ only in having two hydrogen atoms in the one replaced by
one oxygen atom in the other. Both are amorphous powders, from which
crystalline chlorophyll (see below) can be obtained by hydrolysis.
Chlorophyll _a_ is blue-black, is easily soluble in most organic solvents,
and when saponified by alcoholic potash gives a transient pure yellow
color. Chlorophyll _b_ is dark green, is somewhat less soluble than the
other form, and when saponified by potash gives a transient brilliant red.
=Amorphous and Crystalline Chlorophyll.=--When the chlorophyll of plants is
extracted by alcohol and the alcoholic extract evaporated nearly to
dryness, beautiful dark green crystals are obtained. Willstätter has shown,
however, that in these crystallized forms the ethyl group (from the ethyl
alcohol used) has replaced the phytyl group (see below) which is present in
the pigments as they exist in the plant tissues; and that, when extracted
by other solvents than alcohol, the pigments may be obtained in the
amorphous forms in which they exist in the plant.
This change from amorphous to crystalline compounds may be understood from
the preliminary statement that the chlorophylls are esters of tri-basic
acids, in which one acid hydrogen is replaced by the methyl (CH_{3}) group
and a second by the phytyl (C_{20}H_{39}, from phytol, or phytyl alcohol,
C_{20}H_{39}OH) group. When treated with ethyl alcohol (C_{2}H_{5}OH) for
the purpose of extracting the pigments, the ethyl (C_{2}H_{5}) group
replaces the phytyl group, thus yielding a methyl-ethyl ester, and these
esters are the crystalline forms of the chlorophylls. This replacement is
made possible through the action on the original pigment in the tissues of
an enzyme, _chlorophyllase_, which is also present in the tissues, which
splits off the phytyl group, forming phytyl alcohol, and leaving a free
COOH group in the pigment, with which the alcohol used in the extraction
forms the ethyl ester (see Chapter IX for a discussion of the formation and
hydrolysis of esters).
While the chlorophylls are tri-basic acids, only two of the acid COOH
groups actually function in ester-formation. The third acid group seems not
to exist as a free acid group; but in chlorophyll _a_, it is in what is
known as the "lactam" arrangement, represented by the --CONH-- group, and
in chlorophyll _b_, it is probably in the "lactone" arrangement,
represented by the --COO-- group; the two bonds in each case being attached
to different structural units in the molecule (see page 106).
The change from amorphous to crystalline forms may be represented by the
following formulas, in which the R represents the whole of the complex
group to which the acid ester groups are united:
COO·CH_{3} COO·CH_{3} COOH
/ / /
R R R
\ \ \
COO·C_{20}H_{39} COO·C_{2}H_{5} COOH
Amorphous chlorophyll Crystalline chlorophyll Chlorophyllin
or or
methyl-phytyl methyl-ethyl
chlorophyllide chlorophyllide
"Chlorophyllin," the compound in which the ester groups have been converted
into free acid groups, as indicated above, may be obtained from either
amorphous or crystalline chlorophyll by treatment with caustic potash
dissolved in methyl alcohol.
=Phytol.=--This alcohol, which furnishes the characteristic ester group in
the chlorophyll of plants, is a compound of very unusual composition, which
has never been found in any other form or in any other type of compound
which is present in either plant or animal tissues. Careful studies of its
addition and oxidation products prove that it has the following structural
arrangement:
H H H H H H H H H H
| | | | | | | | | |
H-C-----C-----C-----C-----C-----C-----C-----C-----C=====C-----C--CH_{2}OH
| | | | | | | | | | |
CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}
As this formula indicates, the compound contains one unsaturated,
double-bond linkage, one primary alcohol group, and eleven methyl groups.
As has been said, this alcohol occurs nowhere else in nature, and its
presence and function in the chlorophyll molecule are, as yet, wholly
unexplainable. Phytol itself is a colorless, oily liquid, with a high
boiling point (145° in vacuo, 204° at 10 mm. pressure).
THE CONSTITUTION OF THE CHLOROPHYLLS
As has been mentioned, chlorophyll _a_ differs from chlorophyll _b_ by
having one more oxygen and two less hydrogen atoms in the molecule, and in
having one of its nitrogen atoms in the "lactam" arrangement. These
differences in structure are represented by the following formulas which
are commonly used to represent the two compounds, but which do not show the
arrangements of the major groups of the complex molecules:
COO·C_{20}H_{39} COO·C_{20}H_{39}
/ /
MgC_{31}H_{29}N_{3}-COO·CH_{3} MgC_{32}H_{28}O_{2}N_{4}-COO·CH_{3}
|\ Chlorophyll _b_
NH-CO
Chlorophyll _a_
The chlorophylls are unstable compounds, readily acted upon by acids or
alkalies, and by the enzyme chlorophyllase, which splits off the phytyl
alcohol group. The progressive action of acids and of alkalies in breaking
down the molecule, and the products of its oxidation and reduction, have
served to establish the chemical composition of the compound in each case.
Because of the importance of these pigments in the whole metabolic
processes of the plant, it seems to be desirable to consider the nature of
these reactions in some detail, as follows:
=Decomposition of the Chlorophylls by Alkalies.=--The first action of
dilute alkalies on the chlorophylls is to split off, by hydrolysis, the
alcoholic groups of the esters, producing the crystalline tri-basic acids,
or _chlorophyllins a_ and _b_. Each of these chlorophyllins exists in two
forms, the normal and the iso, in which the attachment of the COOH groups
to the other groups in the molecule is in different positions. Hence,
chlorophyll _a_ yields chlorophyllin _a_ and isochlorophyllin _a_, and
chlorophyll _b_ yields chlorophyllin _b_ and isochlorophyllin _b_, all four
of which are tri-basic acids.
These compounds, when heated with alkalies, split off carbon dioxide in
successive stages, losing one COOH group at each step, thus yielding a
series of simpler compounds of the following types: First, di-basic acids;
second, monobasic acids; and finally, _ætiophyllin_, a compound in which no
COOH group is present. In all of these compounds, derived from chlorophylls
by the action of alkalies, the Mg remains in the molecule, and all the
Mg-containing derivatives from the chlorophylls are known as "phyllins." At
the stage at which only one COOH group remains in the molecule, only one
group arrangement is possible, and the derivatives from chlorophyllin _a_
and isochlorophyllin _b_, and those from chlorophyllin _b_ and
isochlorophyllin _a_, are identical. At the final stage, the derivatives
from all four forms are identical. This may be graphically illustrated by
the following diagram indicating the progressive decomposition of the two
chlorophylls under the action of alkalies:
[Illustration: Decomposition of Chlorophyll a and Chlorophyll b]
=Decomposition of Chlorophylls by Acids.=--The first action of dilute acids
upon chlorophylls is to remove the magnesium, without otherwise changing
the molecule. Two hydrogens go in in the place of the magnesium. Dilute
acids act in precisely the same way upon each of the "phyllins" shown in
the above scheme. In this way, a whole series of compounds, corresponding
to each of the chlorophylls and their alkali-decomposition products, but
with the magnesium lacking in each case, has been prepared. Thus,
COO·C_{20}H_{39}
/
Chlorophyll _a_, MgC_{31}H_{29}N_{3}-COO·CH_{3},
\\
(NHCO)
COO·C_{20}H_{39}
/
becomes Phæophytin _a_, C_{31}H_{31}N_{3}-COO·CH_{3},
\\
(NHCO)
COO·C_{20}H_{39}
/
Chlorophyll _b_, MgC_{31}H_{29}O_{2}N_{4}
\
COO·CH_{3}
COO·C_{20}H_{39}
/
becomes Phæophytin _b_, C_{32}H_{30}O_{2}N_{4}
\
COO·CH_{3}
Similarly,
Isochlorophyllin _a_, becomes Phytochlorin _e_,
Chlorophyllin _a_, becomes Phytochlorin _f_, and _g_,
COOH
/
C_{32}H_{32}ON_{4}
\
COOH
Isochlorophyllin _b_, becomes Phytorhodin _g_
Chlorophyllin _b_, becomes Phytorhodin _i_ and _k_,
COOH
/
C_{32}H_{30}O_{2}N_{4}
\
COOH
And bodies known as "porphyrins" are similarly derived from all the other
known phyllins.
For example: cyanophyllin, MgC_{31}H_{32}N_{4}(COOH)_{2}, becomes
cyanoporphyrin, C_{31}H_{34}N_{4}(COOH)_{2}; ætiophyllin,
MgC_{31}H_{34}N_{4}, becomes ætioporphyrin, C_{31}H_{36}N_{4}, etc.
Phytochlorin _e_ and phytorhodin _g_ are the chief products of the
decomposition by acids of the chlorophylls. Indeed, it was the production
of these compounds which led to the discovery of the existence of the two
chlorophylls. When treated with alkalies, they lose their carboxyl groups
and become ætioporphyrin.
=Decomposition of the Chlorophylls by Oxidation and Reduction.=--When acted
upon by oxidizing agents, such as chromic acid, the porphyrins yield two
chief oxidation products, which are pyrrole derivatives having the
following formulas,
CH_{3}-C-CO CH_{3}-C-CO
¦ \ ¦ \
¦ NH ¦ NH
¦ / ¦ /
CH_{3}-CH_{2}-C-CO HOOC-CH_{2}-CH_{2}-C-CO
Methylethylmalein imide Hæmatinic acid imide
By reduction, there have been obtained from the chlorophylls and the
various porphyrins, three isomeric pyrrole derivatives having the following
formulas,
CH_{3} H CH_{3}
| | |
C_{2}H_{5}-C=C C_{2}H_{5}-C=C C_{2}H_{5}-C=C
| \ | \ | \
| NH | NH | NH
| / | / | /
CH_{3}-C=C CH_{3}-C=C CH_{3}-C=C
| | |
CH_{3} CH_{3} H
Phyllopyrrole Hæmopyrrole Isohæmopyrrole
As a result of the study of these decomposition units, Willstätter has
suggested the following formulas for the structural arrangement of
ætiophyllin and ætioporphyrin, the compounds which result from the removal
of all of the acid groups and finally of the magnesium from the
chlorophylls,
H HC===CH
| | |
CH_{3}-C--C C---C
¦ \\ // ¦
¦ N N ¦
¦ / · · \ ¦
C_{2}H_{5}-C--C · · C---CH
\ · · //
C------------C
/ · · \
C_{2}H_{5}-C==C · · C==C-C_{2}H_{5}
| \ ·· / |
| N-----Mg-----N |
| / \ |
CH_{3}-C==C C==C-C_{2}H_{5}
| |
CH_{3} CH_{3}
Ætiophyllin
H HC===CH
| | |
CH_{3}-C--C C---C
¦ \\ // ¦
¦ N N ¦
¦ / \ ¦
C_{2}H_{5}-C--C C---CH
\ //
C------------C
/ \
C_{2}H_{5}-C==C C==C-C_{2}H_{5}
| \ / |
| N N |
| / \ |
CH_{3}-C==C C==C-CH_{3}
| |
CH_{3} CH_{3}
Ætioporphyrin
The COOH groups which are attached to these compounds to form the various
phyllins and porphyrins, as well as the original chlorophylls, are supposed
to be attached to the C_{2}H_{5} groups in the above formulas, the
different modifications, or compounds, depending upon the position in which
one or more of these attachments are made.
SIMILARITY OF CHLOROPHYLL AND HÆMOGLOBIN
It seems to be desirable, at this point, to call attention to the
remarkable similarity in the chemical composition of chlorophyll, the most
important pigment of plants, and hæmoglobin, the all-important
respiration-regulating pigment in the blood of animals. Hæmoglobin is a
complex compound, consisting of about 96 per cent of albumin (a protein,
see Chapter XIII) united with about 4 per cent of _hæmatin_, a brilliant
red pigment which has the formula FeClC_{32}H_{32}O_{4}N_{4}. When treated
with acids, the iron (and its accompanying Cl) is removed, and
hæmatoporphyrin, C_{32}H_{36}O_{4}N_{4}, is obtained. When either hæmatin,
or hæmatoporphyrin is oxidized, hæmatinic acid imide identical with that
obtained from ætioporphyrin is obtained. Also, when hæmatoporphyrin is
reduced, hæmopyrrole identical with that from ætioporphyrin is obtained.
Thus, it would appear that the unit structural groups in hæmatin and in
chlorophyll are identical; although chlorophyll may exhibit more variations
in isomeric arrangement of these structural units than have been found in
hæmatin. Hence, it is apparent that the only essential difference in
composition between chlorophyll and hæmatin is that in the former the
structural units are linked together by iron, while in the latter, the same
units are united through magnesium as the linking element. Further, it is
known that while iron is not a constituent element in the chlorophyll
molecule, it is, in some unknown way, absolutely essential to the
production of chlorophyll in plants; plants furnished with an iron-free
nutrient solution rapidly become etiolated and photosynthesis stops.
The following skeleton formulas have been suggested to indicate the way in
which these elements are linked between the structural units in their
respective compounds.
-C C- -C C-
\ / \ /
N N N N
/ \ / \ / \ / \
-C \ / C- -C \ / C-
M g F e
-C / \ C- -C / | \ C-
\ / \ / \ / | \ /
N N N C l N
/ \ / \
-C C- -C C-
Chlorophyll Hæmatin
It is understood, of course, that the mineral element does not furnish the
definite means of holding the structural units together as otherwise it
would not be possible to remove the iron, or magnesium, without breaking
down the molecule, as is done in the case of the porphyrins. The actual
binding linkage is undoubtedly between carbon atoms, as indicated in
Willstätter's formulas for ætiophyllin and ætioporphyrin (see page 109).
The attachment of the magnesium to each one of the four nitrogen atoms in
the skeleton formula assumes the existence of subsidiary valences of 2-4
for magnesium (and of 3-5 for iron), or of possible _oscillating_ valences
similar to those supposed to be exhibited by carbon in its closed-ring
arrangements.
PROPERTIES OF THE CHLOROPHYLLS
The phytyl esters, or natural chlorophylls, are amorphous solids; while the
methylethyl esters (chlorophyllins) and the free acids (phyllins) are
crystalline compounds. All of these compounds are easily soluble in ether
and alcohol, but insoluble in water. The chlorophylls and chlorophyllins
are practically insoluble in petroleum ether and chloroform; but the
monobasic acids (pyrrophyllin and phyllophyllin) and the neutral
ætiophyllin dissolve easily in chloroform.
Solutions of the chlorophylls are fluorescent, being green by transmitted,
and red by reflected light.
Chlorophyll _a_ is a blue-black solid, which gives dark green solutions in
all of its solvents. Chlorophyll _b_ is a dark-green solid, which yields
brilliant green solutions. Solutions in ether of glaucophyllin and of
cyanophyllin are blue; of rhodophyllin, deep violet; of rubiphyllin, light
violet; of erythrophyllin, red; and of pyrrophyllin and phyllophyllin,
bluish-red. Solutions of the porphyrins are all red, the di-basic ones
being usually a bluish-red, and the simpler ones a brilliant red to deep
brownish-red in color.
The several chlorophyll derivatives are further distinguished by
characteristic differences in their absorption spectra. These differences
have been pictured by Willstätter in his book dealing with the results of
his investigations concerning the chlorophylls, and reproduced in one or
two other texts which treat in detail with the physical-chemical properties
of these pigments, but need not be presented in such detail here.
THE CAROTINOIDS
The characteristic brilliant green of healthy plant tissues is due to the
fact that there are always associated with the dark bluish-green
chlorophylls two (or more) yellow pigments. These are known as the
"carotinoids." This group includes the two brilliant yellow pigments,
carotin and xanthophyll, and the reddish brown fucoxanthin and the
brilliant red lycopersicin, which are similar in their chemical
composition. The first two are found universally distributed in plants,
associated with the chlorophylls, and may be regarded as vegetative
pigments, although the characteristic ornamental yellow and orange colors
of many flowers and fruits, as well as that of the roots of carrots, etc.,
due to these pigments.
=Carotin.=--This pigment occurs in various forms in plants, both amorphous
and crystalline. It crystallizes out of solution in flat plates, which are
orange-red by transmitted light, and greenish-blue by reflected light, and
have a melting point of 168°. Carotin is insoluble in water, only very
slightly soluble in acetone or cold alcohol, readily soluble in petroleum
ether, ether, chloroform, and carbon disulfide. Its solutions are strongly
fluorescent.