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Isotopic evidence for extraterrestrial non- racemic amino acids in the Murchison meteorite

Abstract

Many amino acids contain an asymmetric centre, occurring as laevorotatory, L, or dextrorotatory, D, compounds. It is generally assumed that abiotic synthesis of amino acids on the early Earth resulted in racemic mixtures (L- and D-enantiomers in equal abundance). But the origin of life required, owing to conformational constraints, the almost exclusive selection of either L- or D-enantiomers1,2, and the question of why living systems on the Earth consist of L-enantiomers rather than D-enantiomers is unresolved3. A substantial fraction of the organic compounds on the early Earth may have been derived from comet and meteorite impacts4,5,6. It has been reported previously that amino acids in the Murchison meteorite exhibit an excess of L-enantiomers7, raising the possibility that a similar excess was present in the initial inventory of organic compounds on the Earth. The stable carbon isotope compositions of individual amino acids in Murchison support an extraterrestrial origin8—rather than a terrestrial overprint of biological amino acids—although reservations have persisted (see, for example, ref. 9). Here we show that individual amino-acid enantiomers from Murchison are enriched in 15N relative to their terrestrial counterparts, so confirming an extraterrestrial source for an L-enantiomer excess in the Solar System that may predate the origin of life on the Earth.

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Figure 1: Gas chromatogram showing the resolution of D-alanine and L-alanine in the Murchison meteorite.

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Acknowledgements

We thank S. Fulkerson for his support of this work, and V. Andrusevich and T.Brockwell for their assistance with the analyses. This work was supported by the US NSF. Instrument time and travel funds were provided by Micromass Inc.

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Engel, M., Macko, S. Isotopic evidence for extraterrestrial non- racemic amino acids in the Murchison meteorite. Nature 389, 265–268 (1997). https://doi.org/10.1038/38460

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