Position-specific carbon isotopes of Murchison amino acids elucidate extraterrestrial abiotic organic synthesis networks

Abstract

The Murchison meteorite is a well-studied carbonaceous chondrite with relatively high concentrations of amino acids thought to be endogenous to the meteorite, in part because they are characterized by carbon isotope (δ¹³C) values higher than those typical of terrestrial amino acids. Past studies have proposed that extraterrestrial amino acids in the Murchison meteorite could have formed by Strecker synthesis (for α-amino acids), Michael addition (for β-amino acids), or reductive amination, but a lack of constraints have prevented confident discrimination among these possibilities, or assignment of specific formation pathways to each of several specific amino acids. Position-specific carbon isotope analysis differentiates amongst these mechanisms by relating molecular sites to isotopically distinct carbon sources and by constraining isotope effects associated with elementary chemical reactions. Prior measurements of the position-specific carbon isotopic composition of α-alanine from the Murchison CM chondrite demonstrated that alanine's high δ¹³C_(VPDB) value is attributable to the amine carbon (δ¹³C_(VPDB) = +142 ± 20‰), consistent with Strecker synthesis drawing on ¹³C-rich carbonyl groups in precursors (L. Chimiak et al., Geochim. Cosmochim. Acta 292, 188–202, 2021). Here, we measured the δ¹³C composition of fragment ions generated by electron impact ionization of derivatized ⍺-alanine, β-alanine, and aspartic acid from Murchison via gas chromatography-Fourier transform mass spectrometry. α-Alanine's amine carbon yielded δ¹³C_(VPDB) = +109 ± 21‰, which is consistent with the previously measured value and with formation from ¹³C-rich precursors. β-Alanine's amine carbon presents a lower δ¹³C_(VPDB) = +33 ± 24‰, which supports formation from ¹³C-rich precursors but potentially via a Michael addition mechanism rather than Strecker synthesis. Aspartic acid's amine carbon has δ¹³C_(VPDB)= –14±5‰, suggesting synthesis from precursors distinct from those that generated the alanine isomers. These measurements indicate that Murchison amino acids are a mixture of compounds made from different synthesis mechanisms, though some subsets likely drew on the same substrates; this conclusion highlights the complexity of extraterrestrial organic synthesis networks and the potential of emerging methods of isotope ratio analysis to elucidate the details of those networks.

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