The possible role of hydrogen cyanide in chemical evolution

Abstract

The effect of kaolinite and montmorillonite clays on the oligomerization of hydrogen cyanide was studied. It was found that these clays inhibited the oligomerization by catalyzing the decomposition of diaminomaleonitrile to hydrogen cyanide. It was concluded that these clays could not have served as prebiotic catalysts for the formation of biomonomers from hydrogen cyanide.

The possible role of hydrogen cyanide in chemical evolution was investigated. Aqueous solutions of hydrogen cyanide (0.1 M) were prepared and adjusted to pH 9.2 with ammonium hydroxide. These solutions were stored in the dark for at least one year, during which time the solutions turned darker and a black precipitate formed. A fraction containing acidic components was used in further studies of "HCN oligomers."

The oligomeric material was hydrolyzed using 6 N hydrochloric acid at 110°C for 24 hours. Amino acid analyses were performed on the oligomeric material and on the hydrolysate. The major amino acids identified in this manner were aspartic acid, meso- and dl-diaminosuccinic acids, glycine, alanine, β-alanine, and α-aminoisobutyric acid. In addition, a large number of amino acids were obtained in smaller amounts, some of whose elution times corresponded with those of known amino acids.

The oligomeric material and the hydrolysate were silylated and gas chromatographic analysis of the trimethylsilyl derivatives revealed the presence of glycine, oxalic acid, and 4,5-dihydroxypyrimidine. The presence of glycine and oxalic acid was confirmed by combined gas chromatographymass spectrometry.

Hydrolysis of the oligomeric material was also carried out in pH 8.5 sodium hydroxide solution. Silylation and gas chromatographic analysis indicated a composition similar to that obtained from acid hydrolysis; however, 4,5-dihydroxypyrimidine was not detected and a larger yield of glycine was obtained.

These results suggest that hydrogen cyanide could readily have served as a source of biomonomers in primitive bodies of water. In addition, the formation of pyrimidines and amino acids from hydrogen cyanide, in combination with previous work, in which adenine was formed from hydrogen cyanide, represents the first evidence that all three major classes of nitrogen-containing biomonomers - amino acids, purines, and pyrimidines - can be obtained from hydrogen cyanide under prebiotic conditions.

A number of structural studies, both spectral and chemical, were performed on the oligomeric material, and indicated that the "HCN oligomer" was, in fact, a complex mixture of relatively low molecular weight oligomers, rather than a polymer, as suggested by other workers. No evidence could be found for the presence of peptide bonds, also suggested by other workers. The major functional groups detected in the oligomeric material were amide and/or urea groups.

In order to realize a more realistic model for chemical evolution on the primitive earth, the "HCN oligomerization" reaction was carried out in the absence of oxygen. It was found that similar yields of amino acids were obtained as in the presence of oxygen. Decreased yields of oxidized products, such as urea and oxalic acid, were obtained in the absence of oxygen. The possible role of diiminosuccininitrile as an intermediate in the oxidation reactions was also suggested in these studies. These results indicate that hydrogen cyanide could readily have condensed to biomonomers in the reducing atmosphere of the primitive earth.

It is suggested that diaminomaleonitrile is a key intermediate in the oligomerization reactions of hydrogen cyanide and that this 'compound could have served a a precursor to biomonomers on the primitive earth. Several experiments were performed to study the decomposition of diaminomaleonitrile in aqueous solution. The effect of nickel(II) chloride as a catalyst in these reactions was also investigated.