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Diagenesis of amino acids in sediments and shells involves many processes including (1) conversion of one amino acid to another, (2) interconversion of amino acid enantiomers from dominantly L- to D, L-configurations, and (3) ultimate destruction. Amino acids, generally in the form of biopolymers such as proteins, enter the lithosphere from the biosphere. Hydrolytic processes begin immediately to release monomeric amino acids, and in some shells amino acid polymers no longer exist after about 12 m.y. As with any other organic compounds, destruction of amino acids occurs with time, but for many amino acids, such as glycine, alanine, valine, leucine, and isoleucine, the rate is very slow. Evidence for conversion of one amino acid into another is provided by observations whe ein certain amino acids tend to disappear with time, while new amino acids are created. For example, pipecolic acid recently found in Pleistocene and Miocene Mercenaria is thought to have been derived from lysine through deamination and internal cyclization. The mechanism for the conversion is not yet known.
Interconversion of amino acid enantiomers also takes place with time, L-amino acids racemize to D, L-amino acids at different rates. In deep-sea sediments from the Atlantic Ocean and Caribbean Sea, valine, glutamic acid, and leucine racemize at slower rates than do phenylalanine, aspartic acid, alanine, and proline. Rates of racemization are temperature dependent; in the lithosphere, racemization of amino acids should be complete in about 15 m.y. at the maximum. Measurement of the extent of racemization of amino acids provides a potentially useful geochronologic tool for the very late Cenozoic Era.
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