Impact of prebiotic synthesis and diagenesis on the distribution, stereochemistry, and stable isotope composition of amino acids in carbonaceous meteorites

Abstract

Simulation experiments for prebiotic synthesis result in racemic mixtures (D/L = 1.0) for protein and non-protein amino acids, irrespective of the composition of the precursor gas mixtures or the energy sources employed. It is therefore not surprising that it was commonly assumed that if amino acids in carbonaceous meteorites were formed by similar reactions they would also be racemic. Engel and Nagy¹ and more recently Glavin et al.2 have shown that protein amino acids in carbonaceous meteorites often exhibit a moderate to strong L-amino acid excess (D/L ˂ 1.0). Engel³ hypothesized that this extraterrestrial L-excess might be the precursor material from which life as we know it originated. Stable isotope analyses^{e.g 4} confirmed that the L-amino acid excess in meteorites was indigenous rather than the result of contamination subsequent to impact on Earth. A key question that arises is that since there are no known mechanisms for the direct prebiotic synthesis of amino acids with an L-enantiomer excess, how did these compounds come to exist in carbonaceous meteorites? It has been proposed⁵ ^{and references therein} that a series of diagenetic reactions subsequent to synthesis are responsible for the L-enantiomer excess. In this paper, this hypothesis is further explored with respect to explaining the range of D/L values for amino acids in the various types of carbonaceous meteorites as well as in different stones of the same meteorite. Possible effects of diagenesis on the stable isotope compositions of these compounds are also addressed.