| Abstract | 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 (i.e., whole-molecule) 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 position-specific C isotopic distributions within glycine, L-alanine, L -serine, L -leucine, and L -phenylalanine sorbed to an ice surface from 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, in sorbed amino acids with non-polar side chains, we observed a depletion of 13C in the carboxyl carbon, consistent with hydrogen bonding between the carboxyl group and hydroxyl groups on the ice surface. In contrast, the 13C depletion was observed at another site within serine, which has a polar side chain. Hydrogen bonding at the carboxyl carbon lessens its electron density and promotes peptide bond formation via nucleophilic attack by another amino acid, which could explain the dominance of amino acids with non-polar side chains in modern proteins. |
