COMPARATIVE RACEMIZATION KINETICS OF AMINO ACIDS IN SIMPLE PEPTIDES AND MOLLUSK FRAGMENTS

Rate constants for amino acid racemization in geological samples represent the bulk kinetics of a complex mixture of peptides and free amino acids. This mixture changes in molecular weight distribution during diagenesis (natural or simulated), and racemization likely occurs primarily when amino acids reside at terminal peptide positions (e.g. Mitterer, 1993). Activation energies determined from elevated temperature apparent rate constants from heated carbonate samples usually fall in the range of 27 to 30 kcal/mole, regardless of amino acid. Smaller apparent activation energies (<20 kcal/mole) may be observed when data from a limited temperature range (<100) are used, but the larger values are frequently reported, at least partly because of the wide temperature range (~1000C) represented by the comparison of common laboratory kinetic experiments (conducted at ~110-1450) with ambient temperature (~0-200) age-calibrated results.

Selected peptides, primarily angiotensins, with four to ten amino acid residues, heated under conditions identical to those for mollusk fragments, provide new insights into relative rates of racemization of different amino acids, apparent activation energies, and the linkages between racemization and the actual position of each amino acid within the peptide. Extensive racemization is usually observed for those amino acids initially in terminal positions, and also for those that become terminal during peptide cleavage, as confirmed by ESI tandem mass spectrometry that identifies selected peptide fragments generated during experimental diagenesis. For most amino acids, the D/L values from peptides are lower than those observed in mollusks heated under identical conditions, implying an abundance of terminal positions within the natural specimens, or longer residence times at terminal positions. Aspartic acid, whether or not initially terminal, is always the most extensively racemized amino acid in all heated peptides. With suitable constraints, D/L values serve as an indirect measure of peptide preservation.