Intrinsic asymmetries of amino acid enantiomers and their peptides: a possible role in the origin of biochirality.

L-amino acids and D-carbohydrates were incorporated into the first forms of life over 3.5 billion years ago, presumably from racemic mixtures of organic solutes produced by abiotic synthetic pathways. The process by which this choice occurred has not been established, but a consensus view is that it was a chance event, such that life could equally well have used D-amino acids and L sugars. In this review we will explore a second, less plausible alternative that minute differences in the physical properties of certain enantiomers made it more likely that L-amino acids and D-carbohydrates would be incorporated into early life. By all classical criteria, chiral isomers are perfect mirror image structures and, therefore, are expected to be identical in their macroscopic properties. However, scattered reports in the literature suggest that there may be slight differences in the physical properties of L- and D-amino acids and their polymers, which could lead to a preferred incorporation of L-amino acids into primitive forms of life. Here we present a literature survey of this issue and discuss its possible role in the origin of biochirality. (c) 2007 Wiley-Liss, Inc.