Unusual nonterrestrial L-proteinogenic amino acid excesses in the Tagish Lake meteorite

The distribution and isotopic and enantiomeric compositions of amino acids found in three distinct fragments of the Tagish Lake C2-type carbonaceous chondrite were investigated via liquid chromatography with fluorescence detection and time-of-flight mass spectrometry and gas chromatography isotope ratio mass spectrometry. Large l-enantiomeric excesses (lee similar to 4359%) of the a-hydrogen aspartic and glutamic amino acids were measured in Tagish Lake, whereas alanine, another a-hydrogen protein amino acid, was found to be nearly racemic (d l) using both techniques. Carbon isotope measurements of d- and l-aspartic acid and d- and l-alanine in Tagish Lake fall well outside of the terrestrial range and indicate that the measured aspartic acid enantioenrichment is indigenous to the meteorite. Alternate explanations for the l-excesses of aspartic acid such as interference from other compounds present in the sample, analytical biases, or terrestrial amino acid contamination were investigated and rejected. These results can be explained by differences in the solidsolution phase behavior of aspartic acid, which can form conglomerate enantiopure solids during crystallization, and alanine, which can only form racemic crystals. Amplification of a small initial l-enantiomer excess during aqueous alteration on the meteorite parent body could have led to the large l-enrichments observed for aspartic acid and other conglomerate amino acids in Tagish Lake. The detection of nonterrestrial l-proteinogenic amino acid excesses in the Tagish Lake meteorite provides support for the hypothesis that significant enantiomeric enrichments for some amino acids could form by abiotic processes prior to the emergence of life.