Oligopeptides and copeptides of homochiral sequence, via β-sheets, from mixtures of racemic α-amino acids, in a one-pot reaction in water;: Relevance to biochirogenesis

As part of our studies on the biochirogenesis of peptides of homochiral sequence during early evolution, the formation of oligopeptides composed of 14-24 residues of the same handedness in the polymerization of DL-leucine (Leu), DL-phenylalanine (Phe), and DL-valine (Val) in aqueous solutions, by activation with N,N'-carbonyldiimidazole and then initiation with a primary amine, in a one-pot reaction, was demonstrated by MALDI-TOF MS using deuterium enantio-labeled alpha-amino acids. The formation of long isotactic peptides is rationalized by the following steps occurring in tandem: (i) creation of a library of short diasteroisomeric oligopeptides containing isotactic peptides in excess in comparison to a binomial kinetics, as a result of an asymmetric induction exerted by the N-terminal residue of a given handedness; (ii) precipitation of the less soluble racemic isotactic penta- and hexapeptides in the form of beta-sheets that are delineated by homochiral rims; (iii) regio-enantiospecific chain elongation occurring heterogeneously at the beta-sheets/solution interface. Polymerization of L-Leu with L-isoleucine (Ile) or L-Phe with L-N-1-Mehistidine yielded mixtures of copeptides containing both residues. In contrast, in the polymerization of the corresponding mixtures of L- + D-alpha-amino acids, the long oligopeptides were composed mainly from oligo-L-Leu and oligo-D-Ile in the first system and oligo-D-Phe in the second. Furthermore, in the polymerization of mixtures of hydrophobic racemic alpha-amino acids DL-Leu, DL-Val, and DL-Phe and with added racemic DL-alanine and DL-tyrosine, copeptides of homochiral sequences are most dominantly represented. Possible routes for a spontaneous mirror-symmetry breaking process of the racemic mixtures of homochiral peptides are presented.