Chiral amplification of oligopeptides in the polymerization of α-amino acid N-carboxyanhydrides in water

This article is concerned with the chiral amplification of oligopeptides formed in the polymerization of chiral, nonracemic mixtures of the N-carboxyanhydride (NCA) of Leu and Glu in aqueous solution. Labeling (deuteration) of one enantiomer and reversed-phase and normal-phase high-performance liquid chromatography mass spectrometry (RP- and NP-HPLC/MS, respectively) were used to determine the product distribution, both with respect to oligopeptide chain length and stereoisomer distribution. Starting the polymerization with an enantiomeric excess (ee) of 20% of the L-enantiomer (L-amino, acid/D-amino acid 6:4) leads to an ee of 73% at the level of the homochiral enantiomeric (Leu)5, and of 71% at the level of the homochiral enantiomeric (Glu)(7). For the Len system and in the presence of a solid support (quartz), the ee reached values of up to 100%. We argue that such amplification processes could be relevant for the chemical evolution towards single-handedness.