Extraterrestrial amino acids and L-enantiomeric excesses in the CM2 carbonaceous chondrites Aguas Zarcas and Murchison

Abstract

The abundances, distributions, enantiomeric ratios, and carbon isotopic compositions of amino acids in two fragments of the Aguas Zarcas CM2 type carbonaceous chondrite fall and a fragment of the CM2 Murchison meteorite were determined via liquid chromatography time-of-flight mass spectrometry and gas chromatography isotope ratio mass spectrometry. A suite of two- to six-carbon aliphatic primary amino acids was identified in the Aguas Zarcas and Murchison meteorites with abundances ranging from similar to 0.1 to 158 nmol/g. The high relative abundances of alpha-amino acids found in these meteorites are consistent with a Strecker-cyanohydrin synthesis on these meteorite parent bodies. Amino acid enantiomeric and carbon isotopic measurements in both fragments of the Aguas Zarcas meteorites indicate that both samples experienced some terrestrial protein amino acid contamination after their fall to Earth. In contrast, similar measurements of alanine in Murchison revealed that this common protein amino acid was both racemic (D approximate to L) and heavily enriched in C-13, indicating no measurable terrestrial alanine contamination of this meteorite. Carbon isotope measurements of two rare non-proteinogenic amino acids in the Aguas Zarcas and Murchison meteorites, alpha-aminoisobutyric acid and D- and L-isovaline, also fall well outside the typical terrestrial range, confirming they are extraterrestrial in origin. The detections of non-terrestrial L-isovaline excesses of similar to 10-15% in both the Aguas Zarcas and Murchison meteorites, and non-terrestrial L-glutamic acid excesses in Murchison of similar to 16-40% are consistent with preferential enrichment of circularly polarized light generated L-amino acid excesses of conglomerate enantiopure crystals during parent body aqueous alteration and provide evidence of an early solar system formation bias toward L-amino acids prior to the origin of life.