Site-specific D-H exchange of amino acids under asteroidal hydrothermal conditions

Abstract

Amino acids detected in carbonaceous chondrites are commonly enriched in heavy isotopes of hydrogen compared to terrestrial counterparts. This is interpreted as the consequence of synthesis processes happening in cold extraterrestrial environments. However, the magnitude of this enrichment is variable among classes of chondrites and among individual amino acid in a given chondrite. In this study, we investigated the evolution of the D/H isotope ratio of amino acids experimentally exposed to pure D₂O at 150 °C. We observed that not all the hydrogen-specific sites are prone to deuterium-hydrogen exchange under hydrothermal conditions. Ab-initio modeling pinpoints the higher acidity of the carbon in α position (C_α) leading to a site-specific preferential D-H exchange, affecting the hydrogen atoms bonded to C_α (α-H). This explains the low exchange rate of 2-aminoisobutyric acid and isovaline, these branched amino acids lacking α-H, and the rather high exchange rate of glycine, α-alanine and β-alanine, their α-H exchanging faster. By extrapolating these results, it can be assumed that chondritic amino acids lacking α-H and containing only primary hydrogen (i.e., –CH₃ group) have better retained their pre-accretional D/H values despite hydrothermal alteration on the parent body.