N-CARBAMOYL-ALPHA-AMINO ACIDS RATHER THAN FREE ALPHA-AMINO-ACIDS FORMATION IN THE PRIMITIVE HYDROSPHERE - A NOVEL PROPOSAL FOR THE EMERGENCE OF PREBIOTIC PEPTIDES

Our previous kinetic and thermodynamic studies upon the reactional sys tem HCHO/HCN/NH3 in aqueous solutions are completed. In the assumed pr ebiotic conditions of the primitive earth ([HCHO] and [HCN] near 1 g L -1, T = 25 degrees C, pH = 8, [NH3] very low), this system leads to 99 .9% of alpha-hydroxyacetonitrile and 0.1% of alpha-aminoacetonitrile ( precursor of the alpha-amino acid). The classical base-catalyzed hydra tion of nitriles, slow and not selective, can not modify significantly this proportion. On the contrary, we found two specific and efficient reactions of alpha-aminonitriles which shift the initial equilibrium in favor of the alpha-aminonitriie pathway. The first reaction catalyz ed by formaldehyde generates alpha-aminoamides, precursors of alpha-am inoacids. The second reaction catalyzed by carbon dioxide affords hyda ntoins, precursors of N-carbamoyl-alpha-aminoacids. In the primitive h ydrosphere, where the concentration in carbon dioxide was estimated to be higher than that of formaldehyde, the formation of hydantoins was consequently more efficient. The rates of hydrolysis of the alpha-amin oacetamide and of the hydantoin at pH 8 being very similar, the synthe sis of the N-carbamoyl-alpha-amino acid seems then to be the fatal iss ue of the HCHO/HCN/NH3 system that nature used to perform its evolutio n. These N-protected alpha-amino acids offer new perspectives in prebi otic chemistry, in particular for the emergence of peptides on the pre biotic earth.