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Amino acid dating

Determination of the relative or absolute age of materials or objects by measurement of the degree of racemization of the amino acids present. With the exception of glycine, the amino acids found in proteins can exist in two isomeric forms called d- and l-enantiomers. Although the enantiomers of an amino acid rotate plane-polarized light in equal but opposite directions (the d form rotates it to the right and the l form to the left), their other chemical and physical properties are identical. It was discovered by L. Pasteur around 1850 that only l-amino acids are generally found in living organisms, but scientists still have not formulated a convincing reason to explain why life is based on only l-amino acids.

Under conditions of chemical equilibrium, equal amounts of both enantiomers are present (d/l = 1.0); this is called a racemic mixture. Living organisms maintain a state of disequilibrium through a system of enzymes that selectively utilize only the l-enantiomers. Once a protein has been synthesized and isolated from active metabolic processes, the l-amino acids are subject to a racemization reaction that converts them into a racemic mixture. Since racemization is a chemical process, the extent of racemization is dependent not only on the time that has elapsed since the l-amino acids were synthesized but also on the exposure temperature: the higher the temperature, the faster the rate of racemization. The rate of racemization is also different for most of the various amino acids.

A variety of analytical procedures can be used to separate amino acid enantiomers; gas chromatography and high-performance liquid chromatography are the most widely used. Since these techniques have sensitivities in the parts per billion range, only a few hundred milligrams of sample material are normally required. Samples are first hydrolyzed in hydrochloric acid to break down the proteins into free amino acids, which are then isolated by cation-exchange chromatography.

Since the late 1960s, the geochemical and biological significance of amino acid racemization has been extensively investigated. Geochemical uses of amino acid racemization include the dating of fossils or, in the case of known age specimens, the determination of their temperature history. Fossil types such as bones, teeth, and shells have been studied, and racemization has been found to be particularly useful for dating specimens that were difficult to date by other methods. Racemization has also been observed in the metabolically inert tissues of living mammals. Racemization can be studied in certain organisms and used to assess the biological age of a variety of mammalian species; in addition, it may be important in determining the biological lifetime of certain proteins. Fossils have been found to contain both d- and l-amino acids, and the extent of racemization generally increases with geologic age.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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