ASYMMETRIC AMPLIFICATION OF PROTEIN AMINO ACIDS IN CARBONACEOUS METEORITES

Structural proteins of living organisms consist of L-amino acids. However, laboratory simulation experiments result in racemic mixtures. Owing to molecular-level constraints of biosynthesis, the expansion of life as we know it was preceded by or coincided with the magnification of the L-isomers from these initial mixtures of amino acid racemates. Engel and Nagy (Nature, 296:837, 1982) reported that seven protein amino acids in the Murchison meteorite exhibited a significant L-enantiomer excess. Other common protein amino acids were not detected and those that were detected had stable isotope signatures indicative of an extraterrestrial origin (e.g. Engel and Macko, Nature, 389:265, 1997). These observations confirmed a possible prebiotic origin for the asymmetry during the early stages of the formation of the solar system. Several mechanisms have been proposed for the heightened levels of L-amino acids in the Murchison meteorite, including preferential destruction of D-amino acids by circularly polarized light originating in neutron stars or adsorption of L-amino acids by mineral surfaces. However, neither of these mechanisms is likely to account for the magnitude of the excesses observed in Murchison. Alternative pathways have been recently suggested that result in similar magnitudes for excesses observed in protein amino acids of Murchison (Klussmann et al. Nature, 441:621, 2006; Noorduin et al. JACS, 130:1158, 2008). The enhanced amplification could result during equilibrium of solid-liquid phases of amino acids in solution. Preliminary results indicate that the relative extent of L-enantiomer excess for amino acids in Murchison is consistent with what is predicted for solution enantiomeric excess at the eutectic point in water for the amino acids for which this data is available. A model is presented for the initial synthesis and subsequent aqueous alteration on the parent body of Murchison that resulted in the amino acid stereochemistry that is presently observed.