The mechanism and protobiochemical relevance of dicyanamide-mediated peptide synthesis

Abstract

The mechanism by which dicyanamide promotes the synthesis of diglycine from glycine in aqueous solution has been investigated in detail using 14C- a nd 3H-labeled compounds. It was found that protonation of both the dicyanamide anion and the glycine carboxylate group are required for dipeptide synthesis. Peptide bond formation is apparently initiated by reaction of the glycine carboxyl group with dicyanamide to form an unstable intermediate which undergoes nucleophilic attack by the amino group of a second glycine molecule, resulting in the production of diglycine and cyanurea. N-cyano- N′-glycylurea occurs in the reaction mixture as a prominent side product, suggesting that dicyanamide induces peptide bond formation via an intermediate similar to that postulated in the case of the dialkylcarbodiimides. The limiting factor in dicyanamide-mediated diglycine synthesis is the aminecatalyzed dimerization of dicyanamide to N, N, N′-tricyanoguanidine, a compound inactive in peptide bond formation. The primary intermediate involved in this reaction is the amine adduct of glycine and dicyanamide, i.e., N-cyano- N′-carboxymethylguanidine. Enhanced yields of diglycine, as well as appreciable yields of triglycine and tetraglycine, were obtained by slowly adding dicyanamide to reaction mixtures initially containing glycine and HCl. The possible role of dicyanamide-mediated polypeptide synthesis in prebiological chemical evolution is discussed.