M. Járó - L. Költő szerk.: Archaeometrical research in Hungary (Budapest, 1988)

Dating - CSAPÓ János, KÖLTŐ László , PAP Ildikó: Archaeological age determination based on the racemization and epimerization of amino acids

the sample. For the half-time we get the following formula, wich is derived from the formula describing the racemization speed: ln2 r ki + k

2 or In 2 T

'~ (l*K')k, The D/L-amino acid ratio for the half-time can be given by the formula: The condition ^ = k

2 is not true for the amino acid most frequently used for geochronology, for isoleucine. The L- isoleucine has an asymmetric centre at both the cv and the ß carbons. The racemization, or as already stated for the diastereomers the epimerization, only reacts on the a carbon; there was no racemization observed on the ß carbon neither during the model experiments nor on the fossils. In the epimerizational equilibrium, the reaction constant for the L-isoleucine formation (k

l ) is greater than for the reformation (k

2

), and therefore the equilibrium constant (K) is greater than 1. Different authors give a value of 1—1.4 for K, but in order to avoid the errors caused, it is advisable to determine for each series of experiments the values for Kjj

g

. Although several experts have proven the assumption of k

l - k

2 for amino acids with a single asymmetry centre, Petit (1974) proposes that the speed of formation and reformation might be quite different for amino acids of a protein chain. According to this theory in a given protein environment the mutual effects between the D- and L-enantiomers may be quite different, and this can influence the speeds of formation and reformation. Neuberger (1948) described the following mechanism for the racemization of amino acids catalysed by bases. As the first step the proton in position a is bound by a base and a planar structured anion is formed from the tetrahedral configuration. The anion is later stabilized by the taking up of a proton. According to Neuberger any substitution in the carboxyl group increases the racemization because this enables the freeing of the proton in position a . A similar effect can also be reached when an electronegative substituent is tied to a carbon atom in position ß. Manning (1970) proved the dislocation and recombination of the a proton as the first step of racemization by the measurement of the built-in cv positioned tritium. By subsequent experiments these suppositions were proved to be correct, and Smith et al. (1976) firmly stated that the ratio of relative racemization in a protein could only be assessed by taking into account the simultaneous effect of several factors, such as sterics, neighbour, effect of thinner. Another assumption of Neuberger, according to which the racemization of amino acids in peptide bonds is always much quicker than in free amino acids, was also proven later. This applies to reactions catalysed by both acids and bases. It follows that amino acids in dipeptides racernize quicker than free amino acids, and the increasing racemization speed is further increased by increasing the length of the peptide chain. Therefore one must definitely be able to recognize the racemizational processes as free and bound amino acids. A totally contradictory observation is that in fossils free amino acids better racernize than amino acids in proteins (Dungworth et a. 1973; Bada 1975). This was explained by