Achaeometrical Research in Hungary II., 1988

PROSPECTING and DATING - János CSAPÓ - Zsuzsanna CSAPÓ-KISS - János CSAPÓ JR.: How the amino acids and amino acid racemization can be used and with what limits for age determination of fossil materials in archaeometry

János CSAPÓ* - Zsuzsanna CSAPÓ-KISS* - János CSAPÓ JR. ** HOW THE AMINO ACIDS AND AMINO ACID RACEMIZATION CAN BE USED AND WITH WHAT LIMITS FOR AGE DETERMINATION OF FOSSIL MATERIALS IN ARCHAEOMETRY Abstract: Racemization of free amino acids is considerably lower than that of amino acids bound in peptide. Under the same experimental conditions, the rate of racemization of free amino acids is only 20-80% ofthat of peptide bound amino acids. When using traditional protein hydrolysis, racemization was 1.2-1.6 times as high as that obtained at high temperatures (160-180 °C), under conditions ensuring total hydrolysis of the protein. This lower degree of racemization may be explained by the fact that, at high temperatures, the protein hydrolyses more rapidly into free amino acids and the racemization of free amino acids is considerably slower than of amino acids bound in polypeptides. When hydrolysis is conducted at lower temperatures for longer times, the amino acids bound within the peptide chain are exposed for a longer time to the effects actually causing racemization. As a result, we may say that any factor which speeds up hydrolysis, will lower the degree of racemization. After 48 hours at 110° С and in presence of 4M barium hydroxide, all amino acids (whether free or bound in peptide) totally racemized. Therefore the racemization of tryptophan cannot be determined using barium hydroxide promoted protein hydrolysis. High temperature hydrolysis (at 160 °Cfor 45 to 60 minutes, at 170 °Cfor 30-45 minutes and 180 °Cfor 30 minutes) are recommended for those who would like to hydrolyse the protein for short times and to determine the degree of racemization occurring in the polypeptide chain, but do not wish to use enzyme hydrolysis. After developing the protein hydrolysis method with low racemization, a method has been developed to determine the age of fossil bone samples based on amino acid racemization (AAR). Approximately one hundred fossil bone samples of known ages from Hungary were collected and analysed for D- and Lamino acids. As the racemization of amino acids is affected by temperature, pH, metal content of the soil, and the time that passed since death, these factors were eliminated by comparing the estimated age to age determined by the radiocarbon method. Determining the D- and L- amino acid contents in samples of known ages, determining the half life of racemization and plotting the D/L ratio as a function of time, calibration curves were obtained. These curves can be used for the age estimation of samples after determining their D- and L- amino acid content. The D/L ratio for 2 to 3 amino acids was determined for each sample and the mean value of estimated ages based on calibration curves was considered to estimate the ages of the fossil samples. Following this, a method for evaluation of age of wool carpets and textiles was developed based on the age dependent alteration of amino acid composition of proteins. Samples of 23 wool carpets and textiles of known age, obtained from the Hungarian Museum of Industrial Arts and the Hungarian National Museum were analysed for amino acid content. Results were compared with data obtained for contemporary, untreated wool and wool carpets. The cysteic acid content of wool increased with age. The contemporary wool carpet contained 0.31 g of cysteic acid in 100 g of protein. Comparable figures were 1.87 g for an 550-year old carpet and 4.01-4.39 g for 1600-1750-year old wool carpets. Cystine content decreased with age corresponding figures being 7.88, 3.12, 1.19- 0.97, respectively. Relevant contents of methionine were 0.43, 0.21, and 0.20-0 and for tyrosine were 3.07, 2.11 and 0.20-0. Prediction equations were developed for linear regressions between the age of wool and cysteic acid, cystine and tyrosine contents. The 95% confidence intervals of estimates for two samples of unknown age were estimates plus or minus 30 and 38 years. Keywords: Age estimation, amino acid racemization, protein hydrolysis with low racemization, determination of D-amino acids, cysteic acid, cystine, methionine, tyrosine, fossil bone wool carpet, wool cloth, samle preparation. * PANNON Agricultural University Faculty of Animal Science, H-7401 Kaposvár. P.O.Box 16. Guba S. и. 40. Hungary **'Janus Pannonius University Faculty of Natural Sciences, Natural Geography Department H-7624 Pécs, Ifjúság и. 6. 21