Evolution of the genetic code: chemical studies on the genesis of coded α-amino acids

Abstract

The choice of the twenty coded L-α-amino acids from innumerable such possibilities has been correlated to selection based on structural versatility and the ability to accommodate new entrants as a function of time. This, in turn, favours an evolutionary profile for the genetic code, an idea that finds much support on the basis of a close examination of the pattern of degeneracy that is present. The focus of the experiments reported in the present paper is to delineate pathways that would lead to a centrally controlled synthesis of α-amino acids and to the selective replacement of one amino acid with another in intact peptides. These endeavours would also be appropriate in the context of the current demonstration that the replacement of a single amino acid residue with another could bring about profound biological changes. N-trityl serine methyl ester-O-mesylate and N-benzoyl serine methyl ester-O-mesylate have been demonstrated as alanyl transfer synthons. The latter synthon is quite effective and it has been already transformed to N-benzoyl phenylalanine methyl ester, N-benzoyl leucine methyl ester and N-benzoyl tryptophan methyl ester via reaction with appropriate organo cuprate reagents. In principle, as many as 14 of the twenty coded amino acids could be derived in a similar manner from this synthon. N-benzoyl phenyl alanine methyl ester (BzN-Phe-OMe), BzN-Phe-Phe-OMe and BzN-Phe-Phe-Phe-OMe have been transformed to, respectively, Bz-Asp-OMe, Bz-Asp-Asp-OMe and BzN-Asp-Asp-Asp-OMe in good yields via controlled oxidation with RuO₄ in aqueous acetonitrile. Benzoyl aspartic acid cyclohexyl amide-β-methyl ester has been transformed to NBz-Leu-OMe via Grignard addition and selective dehydroxylation. Thus, the methodology for the Phe → Leu change, of importance with respect to evolution of code, has been delineated. Whilst the structural relationship between phenyl alanine and leucine is obvious, that in the case of the pair histidine-glutamine is not. Glutamine precursors have been obtained in a single stepvia oxidation of Bz-His-OMe with RuO₄. The mechanism of formation of the products has been delineated by model studies. A formamidomethyl transfer synthon, of potential use in the Gin → His change, has been prepared, and a cycloaddition strategy for the transformation is described.