Prebiotic Access to Enantioenriched Amino Acids via Peptide-Mediated Transamination Reactions

The first critical observation against classification of this reaction as prebiotically plausible is that it utilizes stereospecific dipeptides which are having higher chiral bias to bring about kinetic resolution of racemic alanine. Although, dipeptides are prebiotically plausible, as peptide formation is constrained by thermodynamics and even prebiotic formation of peptide is not expected to be stereo selective as racemization cannot be excluded. Hence, it is likely that formation of stereospecific LL-dipeptides could have occurred in early earth later than chirally pure amino acids. As chirally pure amino acids are required to assemble stereospecific LL-dipeptides, then, this clearly represents a case of synthetic access. Further, The kinetic resolution results depicted in table 2 are not demonstrated for other amino acids except for rac-alanine, with the reported substrate scope too narrow. It is true that the transamination reaction reported in table 1 is catalytic as employed peptide concentration is 100 mM and substrate concentration is 200 mM, however reaction reported in table 2 reveals that the concentration of substrate rac-alanine employed is 100 mM and employed dipeptide also has concentration 100 mM. Then, the reported kinetic resolution of rac-alanine , is it really catalytic or just stoichiometric? Further more, the kinetic resolution reaction sacrifices (via mediation not by catalysis) two stereo center containing dipeptide to result in kinetic resolution of alanine with yield of 28 % conversion with highest enantiomeric excess 15 % of L-Alanine. That means 72% rac-alanine has undergone reaction with pyridoxal resulting in pyruvic acid and pyridox amine, which represents major reaction, suggesting that, the kinetic resolution of rac-alanine is a minor side reaction. The results reported in table 2 does not represent statistically significant result as reflected by , the six entries reports % Ala ee less than 1% out of 14 entries.