The second is the polymerization of aminoacetonitrile on a solid
surface, probably absorbed on clay, followed by the hydrolysis of the
polymer to polyglycine and ammonia.
x H₂N—CH₂—CN ————> (—NH—CH₂—C—)ₓ
||
||
NH
|
| + x H₂O
|
V
(—NH—CH₂—CO—)ₓ + x NH₃
The third step is the introduction of side chains into polyglycine by
the reaction with aldehydes or with unsaturated hydrocarbons. Akabori
has demonstrated experimentally the formation of cystinyl and cysteinyl
residue in his above-postulated mechanism.
Fox’s theory of thermal copolymerization ([ref.23]) suggests that
proteins or like molecular units could have been formed in the Earth’s
crust, under geothermal conditions. The accumulated amino acids were
heat polymerized and transported into the primary oceans for further
modifications. Fox has obtained polymers consisting of all 18 amino
acids usually present in proteins. The polymerization is generally done
at 160° C to 200° C, although in the presence of polyphosphoric acid it
can be accomplished at temperatures below 100° C. Molecular weights
increased from 3600 in a proteinoid made at 160° C to 8600 in one made
at 190° C.
Fox showed that when hot saturated solutions of thermal copolymers
containing the 18 common amino acids were allowed to cool, large numbers
of uniform, relatively firm, and elastic spherules separate. These range
from 0.2µ to 60µ in diameter and are quite uniform within each
preparation. Various chemical observations suggest the presence of
peptide bonds in the structural organization of these proteinoids.
Continuing observations of these microspheres have established further
characteristics that point to the possibility of their interpretation as
a kind of primitive protein macromolecule with self-organizing
properties, such that a primitive form of cell, with boundary and other
properties, might form.
In laboratory experiments the behavior of gram-negative and
gram-positive microspheres in dilute alkali parallels that of
gram-negative and gram-positive bacteria ([ref.23]). Furthermore,
time-lapse studies indicate that the proteinoid microspheres undergo a
septate kind of fission, mimicking cell division as shown in figure 1.
Cytochemical studies show that the microsphere’s boundary is
membranelike in having a primitive selectivity. Electron micrographs of
sections of stained microspheres also indicate the presence of a
boundary.
Oparin ([ref.15]) states that the type of organization peculiar to life
could only result from the evolution of a multimolecular organic system
separated from its environment by a distinct boundary but constantly
interacting with this environment. In his concept of coacervates as
precell models, Oparin ([ref.24]) indicates that present-day protoplasm
possesses a number of features similar to coacervate structure. These
coacervates could represent the starting point for evolution leading to
the origin of life. Moreover, in the course of their evolution the
initial systems may gradually become more complex. Oparin also showed
([ref.15]) that mixing solutions of different proteins and other
substances of high molecular weight produced these coacervate droplets.
These droplets are characterized by the formation of a surface layer
with altered structure and mechanical properties, thus providing a
somewhat selective barrier in which to house a molecular system capable
of replication. However, these coacervates are unstable structurally.
[Illustration: Figure 1.—_Protenoid microspheres undergoing septate
fission. Small microspheres and filamentous associations thereof are
also shown ([ref.25])._]
The NASA program has further provided considerable impetus for
continuing research with respect to the chemical evolution of life,
since its life-detection experiments may encounter prebiological
molecules in their search for extraterrestrial life on other planetary
surfaces.
In the area of exobiological research, the significant accomplishments
to date have been—
(1) The reconstruction of some of the pathways which may have led to
the origin of life, by means of laboratory simulation of processes
yielding prebiological organic molecules
(2) The developments in experimental and theoretical biology;
specifically, the role of nucleic acid-protein interactions in
storage and transmission of information both within living cells
and from generation to generation of cells
(3) The suspected role of DNA in information storage and the
development of new concepts of the coding mechanism in DNA that
may lead to a universal biological theory embracing evolutionary,
as well as homeostatic, adaptation to environment and learned
behavioral systems
With the essential biochemical constituents of life and the mechanism of
replication beginning to be understood, the challenge for the synthesis
of living matter by abiogenic experimental techniques has become to many
scientists the ultimate goal of the scientific era.
NASA has established an exobiology laboratory at Ames Research Center in
addition to the sizable support of research at various academic centers
of excellence for the continuation of abiogenic synthesis.
Although research on organochemical evolution is in its infancy, the
data from relatively few experiments have already created an immense
enthusiasm for knowledge of the biochemical pathways of evolution. This
kind of research will ultimately elucidate the terrestrial evolution of
life and, perhaps, the nature of life on other planetary bodies and the
distribution of life in our galaxy.
This program, with its vast demands on the scientific community at
large, is coordinated with related endeavors of a number of Federal
agencies. It is allied with certain biochemical studies at the National
Institutes of Health for the eventual elucidation of the dynamic
pathways in cosmochemical synthesis of life’s essential biochemical
constituents.