Cryo-bioorganic chemistry: freezing effect on stereoselection of DL-alanine-N-carboxyanhydride oligomerization in dioxane solution

The possibility of stereoselection through the DL-alanine-N-carboxyanhydride (NCA-DL-Ala) oligomerization, and the effect of freezing on it have been investigated. To this end, the chirality of peptides obtained by oligomerization for 1 and 3 days, respectively, in liquid (+22°C) and frozen (-18°C) dioxane solutions, was analyzed. These water-soluble samples were fractionated by gel filtration, aliquots of the fractions were completely hydrolyzed and then derivatized with Marfey reagent (1-fluoro-2,4-dinitrophenyl-5-L-alanine amide). These derivatives were traced and evaluated by RP-HPLC analysis. The relatively best effects appeared in a given fraction, where after 1 day of oligomerization the L-alanine enantiomeric excess (ee) was 3.8% in liquid and 8.6% in frozen conditions. After 3 days, however, the ee contents decreased to 2.0% and 4.1%. The mass spectrometric data of the peptides pointed to the formation of open chain and cyclic peptide mixtures, where the residue numbers of 8-11 and 4-5 dominated. The formation of some chiral peptides from a racemic amino acid suggests the possibility of preferential incorporation of the L-enantiomer into the growing chain, beside the achiral statistical succession of residues. Here we provide the first example of the role of freezing in the increased formation of chirality from racemic amino-acid through oligomerization, together with some speculations about the implications of our model in the events of prebiotic chemistry.