Kinetics of Amino Acid racemization (epimerisation) in the dentine of fossil and modern bear teeth

Abstract

The present study examines the question of whether heating experiments on modern bear teeth dentine model the pattern of D/L racemization in fossil teeth. Using samples of modern bear teeth dentine heated at 65°C, 85°C (up to 53 days), and 105°C (up to 71 days), and three independently dated fossil bear teeth, we have compared the modes of racemization induced by temperature in the modern samples and by time on the fossil samples. We have studied seven amino acids (aspartic and glutamic acids, alanine, valine, leucine, isoleucine, and phenylalanine) that follow a reversible first-order kinetic model of racemization (epimerization) either at low or high temperature. We have estimated the Arrhenius parameters, the activation energy E3 and the frequency factor A, first based on the heating experiments, and later including the fossil data. Valine shows no appreciable differences in Ea and A in both estimations, and could then be used with confidence in dating studies. In a lesser extension this also applies to alanine, phenylalanine, leucine, and glutamic acid. Aspartic acid shows a great difference between the temperature-induced and the time-induced racemization kinetic models, and it should be used with special care in dating studies, since diagenetic racemization in aspartic acid is extremely sensitive to the thermal history of the site.