Freezing Effect on Chirality Generation of DL-Alanine-N-Carboxy-Anhydride Oligomerization in Aqueous Solution

This article is concerned with a study of the role of ice in the synthesis of oligopeptides containing L- or Denantiomeric excess (ee) from racemic alanine. With this aim, the oligomerization of DL-alanine-N-carboxyanhydride was investigated by keeping this activated derivative in liquid (+22°C) or frozen (-20°C) aqueous solutions for 30 days. The aqueous solution of the peptide mixtures were gel-filtered and the aliquots of the fractions were completely hydrolyzed to alanine monomers. These monomers were then derivatized with 1-fluoro-2,4-dinitrophenyl-5-L-alanine amide (Marfey's reagent) and analyzed by RP-HPLC to reveal the occasional enantiomeric excess of L- or D-Ala. The mass spectrometry of the gel-filtered fractions pointed to open-chain peptide mixtures together with a slight amount of cyclic ones, where the residue numbers ranged between 5-8. Our studies indicated that an enantiomeric excess of L- or D-Ala appeared in some oligopeptide fractions. Their excesses were significantly larger in the frozen than liquid solution. Speculations are made as concerns the implications of our findings in the events of prebiotic chemistry.