Plural origins of molecular homochirality in our biota Part II. The relative stabilities of homochiral and mixed oligoribotides and peptides.

By computer simulations--molecular mechanics and molecular dynamics with the amber force field (Weiner et al, (1986), J. Comp. Chem. 7, 230-252)--we have determined the stabilities of oligoribotide strands built with D- and L-riboses, and of peptide chains with D- and L-amino acid residues. In particular, complementary double-chains of oligoribotides were studied, since they are an important feature of the growing mechanism of modern nucleic acids. Peptide chains on the other hand, grow without need of a template. We found that mixed oligoribotides are less stable than homochiral ones, and that this chiral effect is less noticeable in peptide chains. The results support the interpretation that L-riboses act as terminators to the template-assisted growth of oligo-r-GD (enantiometric cross-inhibition; Joyce et al., (1987), Proc. Natl. Acad. Sci. USA 84, 4398-4402). Based on this effect, a chemical pathway is proposed which could, under assumed prebiotic conditions, bypass the hindrance of homochiral growth.