Position-specific isotope fractionation in amino acids sorbed to ice: Implications for the preservation of isotopologue biosignatures

Sorption to mineral surfaces is a key process that protects amino acids from oxidation and aids their polymerization into complex biomolecules. Sorption of neutral amino acids is driven by a combination of intermolecular forces, commonly through hydrogen bonds with surface functional groups. Substitutions of heavy isotopes are known to influence the strength of intermolecular interactions, but global C isotope fractionation associated with sorption is small (<1‰). However, larger fractionation of C isotopes (>2‰) was observed for specific positions within a molecule during chromatographic separation, indicating that fractionation during sorption is likely more significant at positions that interact with a surface. We used quantitative isotopic ¹³C NMR to measure the relative difference in position-specific C isotopic distributions within glycine, L-alanine, L-serine, L-leucine, and L-phenylalanine sorbed to an ice surface versus in an aqueous solution. Isotopic differences up to 8.5‰ at functional sites were observed between sorbed and free amino acids, suggesting that sorption can alter primary isotopic patterns associated with their synthesis. Further, postion-specific isotope patterns appear to reflect the orientation of amino acids on the ice surface. As a result, position-specific isotope analysis may provide further understanding of mineral-mediated polymerization reactions.