KINETICS OF RACEMIZATION IN PROTEINS FROM PLEISTOCENE BONES OF SOUTHERN FRANCE

The racemization ratios of aspartic and glutamic acids in fossil bone proteins were studied. The bones originated from eight prehistoric sit es of southern France from which twelve stratigraphic levels were sele cted. Their ages are included between 20,000 to 1,000,000 years. First ly, the racemization ratios were determined in total extract (whole of amino acids present in bone obtained by 6 M HCI hydrolysis of the bon e powder). The results of such an approach are scattered and they are of no use to geochronology. Next, fossil proteins were selected accord ing to their molecular weights by extraction with 0.5 M EDTA and 10,00 0 Dalton dialysis or according to their solubilities by dissolution in 1 M HCI, 8 ml.g(-1) and centrifugation. The racemization ratios deter mined on both dialysed extract and HCI insoluble part exhibit evolutio ns in two parts. Each of the two parts corresponds to a ''pseudo first -order kinetics, but in the first period (until ca. 50,000 years) race mization ratios increase rapidly, while in the second one ten. 100,000 - 1 My) the apparent rate constants are lower. Such a behaviour was p reviously described in various series of carbonated fossils (shells, c orals...). We tested the fit df a sum of two exponential functions to the four series of values (Asp and Glu, EDTA + dialysis or HCI insolub le). The fits are good for aspartic acid, while the results for glutam ic acid are more scattered. The bi-exponential kinetics could be cause d by steric and electrostatic interactions between amino acids inside proteins. The corroboration of this result on a large number of sites should allow the absolute dating of fossil bones within a wide time in terval.