Epimerization of COOH-terminal isoleucine in fossil dipeptides |
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Authors: | Nivat Kriausakul Richard M Mitterer |
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Institution: | Geosciences Program, University of Texas at Dallas, P.O. Box 688, Richardson, Texas 75080 USA |
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Abstract: | Using procedures employed for protein sequencing to remove NH2-terminal amino acids, the degree of epimerization of COOH-terminal isoleucine in dipeptides from fossil mollusc shells has been measured directly. The results show that isoleucine in this position is highly epimerized in fossil dipeptides and that the COOH-terminal isoleucine is more highly epimerized than the free amino acid. Hydrolysis of the highly epimerized terminal isoleucine leads to the high alloisoleucine/isoleucine values found in the free amino acid fraction. The results also provide evidence for the formation of diketopiperazines in fossils as a mechanism to account for the high degree of epimerization of COOH-terminal isoleucine. |
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