Research on Amino Acid Racemization Age-Dating

氨基酸外消旋测年是一种独特的测年方法，它覆盖了500年到20000年和5万年到数十万年的世代范围，是放射性碳法和K-Ar法之外的确定领域。但是，消旋反应依赖于温度，只能用于测定相对年龄。为了确定绝对年龄，必须以在同一地点出土的另一个年龄确定的样本作为标准。由于外消旋遵循一级可逆动力学，反应速率常数与绝对温度倒数之间的关系可用Arrhenius方程来解释。本文利用放射性碳测年化石壳层中蛋白质氨基酸的温度依赖外消旋反应来估算古环境温度。为了进行实际分析，壳体在90~170℃之间分7步加热，进行人工外消旋，得出阿累尼乌斯方程。超声洗壳后，用3M-HCl溶解，加入4倍量的乙醇，冷却至-10℃，2小时后沉淀蛋白质。离心后，收集蛋白质，加入6M-HCl，在110℃下加热24小时进行水解。水解液经Dowex 50W-X8离子交换柱净化。用手性相毛细管气相色谱和高压液相色谱法测定了天冬氨酸(Asp)的D/L比值。由天冬氨酸的D/L比值得到的速率常数与绝对温度倒数之间的关系呈良好的直线关系，相关系数为0.998。通过向外延伸该线，贝壳(^&lt；14；C年龄，5,100±20 BP。)平均体温15.0±0.2℃。结果表明，该方法可用于古环境温度的估算。

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.