Dilute (0.1 M) solutions of HCN condense to oligomers at pH 9.2. Hydrolysis of these oligomers yields 4,5-dihydroxypyrimidine, orotic acid, 5-hydroxyuracil, adenine, 4-aminoimidazole-5-carboxamide and amino acids. These results, together with the earlier data, demonstrate that the three main classes of nitrogen-containing biomolecules, purines, pyrimidines and amino acids may have originated from HCN on the primitive earth. The observation of orotic acid and 4-aminoimidazole-5-carboxyamide suggests that the contemporary biosynthetic pathways for nucleotides may have evolved from the compounds released on hydrolysis of HCN oligomers.

Summary. A suite of sixteen monocarboxylic acids having carbon numbers 2 to 7, formed by the MillerUrey spark discharge process, were identified and quantified by gas chromatography and mass fragmentography using a deuterium spiking technique. The molar concentration and isomeric distribution of these laboratory synthesized monocarboxylic acids are compared to those previously reported for the Murchison meteorite. They show similar trends, namely, decreasing molar concentration with increasing molecular weight, and, the ratio of normal/branched isomers tend toward smaller with increasing carbon numbers. Urey) process for making carboxylic acids, with the intention of ascertaining how closely this process resembles the conditions that created the carboxylic acids in the meteorites and, if possible, of gaining some insight about the mechanistic reaction pathway that led to their formation. One means of assessing the pathway is through comparison of the isomeric distribution of compounds obtained under controlled simulated conditions with that found in meteorites. In this paper, the identification and quantification results are reported for the suite of monocarboxylic acids formed by a spark discharge (Miller-Urey type) experiment.

Diamincimaleonitrile undergoes a rapid Ni(I1)-catalyzed or a much slower uncatalyzed decomposition to yield 2 equiv of cyanide. This is not an equilibration between diaminomaleonitrile and the dimer and trimer of HCN as shown by the absence of incorporation of H13CN when incubated with diaminomaleonitrile. The formation of urea and oxalic acid is enhanced and the steady-state concentration of diaminomaleonitrile is decreased when the oligomerization of HCN is performed in the presence of oxygen as compared to a pure nitrogen atmosphere. Small but significant yields of oxalic acid and urea were observed when oxygen was eliminated from the reaction solution. An oligomerization pathway is proposed which is consistent with these data. These findings are not consistent with the proposal that HCN condenses to heteropolypeptides via azacyclopropenylidene imine.Hydrogen cyanide oligomers are believed to have had a significant role in the prebiotic synthesis of purines, pyrimidines, and amino HCN condenses in a stegwise fashion to the dimer 1, trimer 2, and tetramer 3. It was pos-tulated that one or more of these simple HCN derivatives condenses further to yield HCN oligomers, a complex mixture of substances with a molecular weight of 500-1000. Purines, pyrimidines, and amino acids are released on hydrolysis of these oligomers. The oligomerization reaction is dependent only on the pH of the reaction mixture and is independent of added nucleophile.4 Urea and oxalic acid are also products of the oligomerization reaction. An investigation of the me chanism of formation of the oligomers was undertaken because these substances may have had a central role in the formation of biomolecules on the primitive earth. Results and DiscussionThe Proposed Equilibrium between Diaminomaleonitrile (3) and Aminomalononitrile (2). We proposed previously that the monomer, dimer ( l ) , trimer (2), and tetramer (3) of HCN readily equilibrate in aqueous ~o l u t i o n .~ The formation of a precipitate of AgCN when Ag+ is added to an aqueous solution of 3 provided support for this h y p o t h~s i s .~ The observation that diaminomaleonitrile releases cyanide 0022-326317811943-3989$01 .OO/O rapidly when treated with Ni2+ in NH40H solution, a method for the determination of cyanide ion, prompted a reinvestigation of the equilibrium proposed between 1,2, and 3. The catalyzed decomposition of diaminomaleonitrile requires the presence of both Ni2+ and NH3 if it is to proceed a t a rapid rate. Approximately 2 equiv of cyanide are released per mole of diaminomaleonitrile. If diaminomaleonitrile is in equilibrium with HCN, 4 equiv of cyanide would be detected as the Ni(CN)42-complex. The same yield of cyanide is obtained when the hydrolysis proceeds 4 X times slower in the absence of Ni2+. The similar yields of HCN in the catalyzed and uncatalyzed reactions suggest that overall decomposition pathways are the same in both reactions.The mechanisms of both the Ni2+-catalyzed and -uncatalyzed decomposition of diaminomaleonitrile are unclear. Hydrolysis of 3 ...

NMR spectral studies on the HCN oligomers suggest the presence of carboxamide and urea groupings. The release of CO2, H2O, HCN, CH3CN, HCONH2 and pyridine on pyrolysis is consistent with the presence of these groupings as well as carboxylic acid groups. No basic primary amine groupings could be detected with fluorescamine. Hydrazinolysis of the HCN oligomers releases 10% of the amino acids normally released by acid hydrolysis. The oligomers give a positive biuret test but this is not due to the presence of peptide bonds. There is no conclusive evidence for the presence of peptide bonds in the HCN oligomers. No diglycine was detected on partial hydrolysis of the HCN oligomers at pH 8.5 suggesting that HCN oligomers were not a source of prebiotic peptides.

The effect of adsorbed ions and pH on the adsorption of several purine and pyrimidine nucleotides on montmorillonite was studied. The cations used to prepare homoionic montmorillonite was Na+, Mn2+, Fe3+, Co2+, Ni+, Cu2+, and Zn2+. The nucleotides studied were 5'-,3'-, and 2'-AMP, and 5'-CMP in the pH range 2 through 12. The results show that preferential adsorption amongst nucleotides and similar molecules is dependent upon pH and the nature of the substituted metal cation in the clay. At neutral pH, it was observed that 5'-AMP was more strongly adsorbed than 2'AMP, 3'-AMP, and 5'-CMP. Cu2+ and Zn2+ clays showed enhanced adsorption of 5'-AMP compared to the other cation clays studied in the pH range 4-8. Below pH 4, the adsorption is attributed to cation and anion exchange adsorption mechanisms: above pH 4, anion exchange may also occur, but the adsorption (when it occurs) likely depends on a complexation mechanism occurring between metal cation in the clay exchange site the biomolecule. It is thus proposed that homoionic clays may have played a significant role in the concentration mechanism of biomonomers in the prebiotic environment, a prerequisite step necessary for the formation of biopolymers in the remaining steps leading to the origin of life.