Research on Amino Acid Racemization Age-Dating

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Amino acid racemization dating is known as an unique dating method that cover the generation fields from 500 to 2,000 years and from fifty thousands to hundreds of thousands years which are the determination domain outside of radiocarbon and K-Ar dating methods. However, the racemization reaction is temperature dependent, the method can use only for determining relative ages. In order to determine the absolute ages, age-established another sample excavated at the same place must be required as a standard. As the racemization follows first-order reversible kinetics, the relationship between the rate constant and the reciprocal of absolute temperature can be explained by arrhenius' equation. Here we utilized the temperature-dependent racemization reaction of protein amino acid in radiocarbon-dated fossil shell to estimate the paleoenvironmental temperature. For practical analysis, the shell has been heated at 7 steps from 90 to 170℃ to proceed the racemization artificially to draw arrhenius' equation. After cleaned the shell by ultrasonication, it was dissolved in 3M-HCl, and added 4 times amount of ethanol with subsequent cooling down to -10℃ for 2 hours to precipitate proteins. After centrifugation, the proteins were collected, added 6M-HCl, and heated at 110℃ for 24 hours for hydrolysis. The hydrolyzate was purified by ion-exchange column packed with Dowex 50W-X8. D/L ratios of aspartic acid (Asp) have been determined either by chiral-phase capillary gas chromatography and high-pressure liquid chromatography. The relationship obtained from the D/L ratios of Asp between the rate constant and the reciprocal of the absolute temperature showed well-fitted straight line with correlation coefficient of 0.998. By extraporating the line, the shell (^<14>C age, 5,100±20 BP.) showed the average temperature of 15.0±0.2℃. As a result, the method was found to be available for estimating the paleoenvironmental temperature.