RNA-directed amino acid homochirality.

The phenomenon of L-amino acid homochirality was analyzed on the basis that protein synthesis evolved in an environment in which ribose nucleic acids preceded proteins, so that selection of L-amino acids may have arisen as a consequence of the properties of the RNA molecule. Aminoacylation of RNA is the primary mechanism for selection of amino acids for protein synthesis, and models of this reaction with both D- and L-amino acids have been constructed. It was confirmed, as observed by others, that the aminoacylation of RNA by amino acids in free solution is not predictably stereoselective. However, when the RNA molecule is constrained on a surface (mimicking prebiotic surface monolayers), it becomes automatically selective for the L-enantiomers. Conversely, L-ribose RNA would have been selective for the D-isomers. Only the 2' aminoacylation of surface-bound RNA would have been stereoselective. This finding may explain the origin of the redundant 2' aminoacylation still undergone by a majority of today's amino acids before conversion to the 3' species required for protein synthesis. It is concluded that L-amino acid homochirality was predetermined by the prior evolution of D-ribose RNA and probably was chirally directed by the orientation of early RNA molecules in surface monolayers.