Studies in prebiotic synthesis. II. Synthesis of purine precursors and amino acids from aqueous hydrogen cyanide.
There is much evidence suggesting that hydrogen cyanide was important in prebiotic synthesis. Here we discuss the relevant aqueous solution chemistry, since we believe it to be pertinent to prebiotic synthesis. In mildly alkaline solutions (pH 7 to 10) two modes of destruction compete. Hydrolysis to formamide and formic acid is first-order in HCN, while polymerization involving the formation of C C bonds is quadratic in HCN. Within the pH range 8 to 9 and the temperature range 0 to 60°C, the two reactions become equally important for concentrations between 0·01 M and 0·1 M ; hydrolysis dominates in more dilute and polymerization in more concentrated solutions. Concentration of HCN in the liquid phase by freezing out ice permits efficient polymerization from initially very dilute solutions. Intermediates in the polymerization proceBS include a very reactive trimer, aminomalononitrile, and a relatively stable tetramer, diaminomaleonitrile. The latter is converted to 4-aminoimidazole-5-carbonitrile and related compounds either by photochemical rearrangement or by reaction with formamidine. The imidazole derivatives are then readily converted to purines including adenine, guanine and hypoxanthine. We have also made some observations on the formation of ex-amino acids from HCN oligomers and polymers. Various theories of prebiotic purine formation are considered in the light of the quantitative kinetic data. We conclude that the arguments for a photochemical step are persuasive and that polymerization subsequent to concentration in a eutectic phase is an attractive posiibility.