Parity-violating energy differences and the origin of biomolecular homochirality

Abstract

The violation of parity by the weak interactions ensures that enantiomeric chiral molecules have inequivalent energies. These parity-violating energy differences have been calculated, using ab initio methods, for the series of α -amino acids glycine, alanine, valine, serine and aspartic acid, and for a set of polypeptide/protein fragments in both the α -helix and β -sheet secondary structures. In each case the energy differences are found to favour the existence of the natural left-handed l -enantiomers over their unnatural mirror-image right-handed d -enantiomers. The variation of the parity-violating energy difference along a prebiotic reaction path leading to the formation of a possible precursor of alanine has also been determined. Under equilibrium conditions the energy difference is found to preferentially select the stereochemical reaction channel corresponding to the eventual formation of l -alanine rather than that for its unnatural d -enantiomer. The significance of these results in accounting for the prebiotic origins of the terrestrial biomolecular homochirality are discussed.