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.
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.