Calvin ([ref.16]) irradiated water and carbon dioxide in a cyclotron, obtaining formaldehyde and formic acid. Miller ([ref.17]) found that when methane, ammonia, water, and hydrogen were subjected to a high-frequency electrical discharge, several amino acids were produced along with a variety of other organic compounds.
Corroborating experiments established that the synthesis of amino acids occurred readily. The apparent mechanism for the production of amino acids is as follows: aldehydes and hydrogen cyanide are synthesized in the gas phase by the electrical discharge. These substances react together and also together with ammonia in the water phase of the system to give hydroxy and amino nitriles, which are then hydrolyzed to hydroxy and amino acids. Among the major constituents were aspartic acid, glutamic acid, glycine, α-alanine, and β-alanine.
The "Miller-Urey" reaction mixture has been extended and several modifications introduced. Oró ([ref.18]) introduced hydrogen cyanide into the system as the primary gas component. Adenine was obtained when Oró heated a concentrated solution of hydrogen cyanide in aqueous ammonia for several days at temperatures up to 100° C. Adenine is an essential component of nucleic acids and of several important coenzymes. Guanine and urea were the two other products identified in the hydrogen cyanide reaction. Oró further obtained guanine and uracil as products of nonenzymatic reactions by using certain purine intermediates as starting materials.
Ponnamperuma ([ref.19]) also obtained adenine upon irradiation of methane, ammonia, hydrogen, and water, using a high-energy electron beam as the source of energy of irradiation. These results indicate that adenine is very readily synthesized under abiotic conditions. Adenine, among the biologically important purines and pyrimidines, has the greatest resonance energy, thus making its synthesis more likely and imparting greater radiation stability to the molecule.
The formation of adenine and guanine, the purines in RNA and DNA, by a relatively simple abiological process lends further support to the hypothesis that essential biochemical constituents of life may have originated on Earth by a gradual chemical evolution and selection. In this respect, the examination of planetary surfaces—specifically Mars—presents practical implications for current research on the problem of chemical evolution.
When Ponnamperuma et al. ([ref.14]) exposed adenine and ribose to ultraviolet light in the presence of phosphate, adenosine was produced. When the adenine and ribose were similarly exposed in the presence of the ethyl ester of polyphosphoric acid, adenosine diphosphate (ADP) and adenosine triphosphate (ATP) were produced. The abiological formation of ATP was a major stride along the path of chemical evolution, since ATP is the principal free energy source of living organisms.
Oparin ([ref.15]) postulated that α-amino acids could have been formed nonbiologically from hydrocarbons, ammonia, and hydrogen cyanide at a time when the Earth's atmosphere contained these substances in high concentrations. Oparin's hypothesis has received strong experimental support, as evidenced by the work of Miller ([ref.12]). Bernal ([ref.20]) has emphasized the role played by ultraviolet light in the formation of organic compounds at a certain stage of the Earth's evolution.
Generally it has been believed that the first proteins or foreprotein were nonbiologically formed by the polycondensation of preformed free amino acids ([ref.21]). Akabori ([ref.22]) proposed a hypothesis for the origin of the foreprotein and speculated that it must have been produced through reactions consisting of the following three steps.
The first step is the formation of aminoacetonitrile from formaldehyde, ammonia, and hydrogen cyanide.
CH2O + NH3 + HCN ————> H2N—CH2—CN + H2O