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.

Low-frequency cooperative dynamics in L-, D-, and DL-alanine crystals: a 13C and 15N cross-polarization magic-angle-spinning NMR study.

Knowledge of the dynamical changes in molecular configurations in various amino acid structures over a wide range of time scales is important since such changes may influence the structural transformations and the diverse biological functionalities of proteins. Using the temperature dependence of the rotating-frame NMR spin-lattice relaxation times T(1rho) of protons as a probe, we have investigated the low-frequency (approximately 60-100 kHz) dynamics in the crystal structures of L-, D-, and DL- alanine (C(12)H(28)O(8)N(4)) polymorphs. The proton relaxation times T(1rho) were obtained from (13)C <-- (1)H and (15)N <-- (1)H cross-polarization magic-angle-spinning NMR experiments over a temperature range of 192-342 K. The data reveal that the time scales of these low-frequency dynamical processes are distinctly different from the localized, high-frequency rotational motion of methyl and amine groups. The strongly asymmetric T(1rho) versus temperature curves and the subtle dynamical differences between the DL-alanine and the L- and d-enantiomorphs indicate that these low-frequency processes are cooperative in nature and are sensitive to molecular packing.