Comparison of isoleucine epimerization and leaching potential in the molluskan genera Astarte, Macoma, and Mya

Abstract

Amino acid racemization geochronology is widely applied to provide relative age control for carbonate fossils, however, there are significant chemical differences between genera which affect diagenetic reactions in proteins such as hydrolysis and racemization. In order to evaluate intergeneric influences on isoleucine epimerization, laboratory heating experiments were undertaken to compare the rate of epimerization of L-isoleucine to D-alloisoleucine and the potential for leaching of amino acids in three molluskan genera: Astarte, Macoma, and Mya. Shell fragments from these genera were heated for up to 816 h at 100°C to simulate aging. After 96 h of heating, the three genera experienced similar degrees of epimerization in the total acid hydrolysate, but with additional heating, Astarte and Macoma epimerized more extensively than Mya. In the free amino acid fraction, the relative rates of epimerization were reversed: Mya shows the fastest apparent rate and Astarte the slowest.All three genera lost amino acids from both the free and bound fractions by leaching, but more significantly, alloisoleucine leached proportionately much faster than isoleucine. If leaching of amino acids under natural conditions continues after significant alloisoleucine is produced, then the preferential loss of alloisoleucine over isoleucine will result in lowering of the alloisoleucine/isoleucine ratio and, therefore, lead to significant underestimation of ages. An important implication for all studies utilizing amino acid racemization geochronology is that even coeval shells of the same genus may give different alle/Ile values if different individual shells experienced different burial conditions and degrees of leaching.