Amino acid enantiomer excesses in meteorites: Origin and significance

Abstract

The presence of small l-excesses in both of the two enantiomeric pairs of 2-amino-2,3-dimethylpentanoic acid, in 2-amino-2-methylbutanoic acid (Isovaline), and in 2-amino-2-methylpentanoic acid from the Murchison meteorite has also been observed in these amino acids as extracted from the Murray meteorite. As in Murchison, the α-hydrogen analogues of the latter amino acids, 2-amino butanoic acid and 2-amino pentanoic acid, were found to be racemates. In addition, l-excesses were observed in 2-amino-2,3-dimethylbutanoic acid and 2-amino-2-methylhexanoic acid from both the Murchison and Murray meteorites. The l-excesses observed in the amino acids from Murray were smaller than those found in their Murchison analogues. The substantial excess of l-alanine reported by others was not observed in fractionated (reversephase chromatography) extracts of either Murchison or Murray. Several amino acids were identified in the Murchison extract that can interfere with determination of the alanine enantiomer ratio on Chirasil-L-Val unless removed by a prior fractionation step. The role of ultraviolet circularly polarized light in generating meteorite enantiomeric excesses and the possible significance of such excesses to the origin of terrestrial homochirality are discussed.