Dating methods amino acid racemization

Amino acid racemization (Section 27.2) A method for dating archeological samples based on the rate at which the stereochemistry at the a carbon of amino acid components is randomized. It is useful for samples too old to be reliably dated by decay. 

FIGURE 88 Dating methods. Shortly after the discovery of radioactivity, at the beginning of the twentieth century, it was found that the decay of radioactive elements could be used to keep track of time. Many of the dating techniques developed since then are, therefore, based on radioactive decay phenomena, but others, such as the hydration of obsidian, amino acid racemization, and dendrochronology, are based on other physical, chemical, or biological phenomena. 

Rutter, N. W. and B. Blackwell (1995), Amino acid racemization dating, in Rutter, N. W. and N. R. Catto (eds.), Dating Methods for Quaternary Deposits, Vol. 2, Geological Association of Canada, Geotext, St. John s, Newfoundland, pp. 125-164. 

Attempts to apply thermoluminescence (TL) and electron spin resonance (ESR) data to date soil horizons, and to use ESR to date bone are underway. As yet, there are no agreed-upon criteria on which to evaluate the overall reliability of age inferences based on ESR data (13). It has been suggested (14) that in some circumstances the obsidian hydration method can be used to infer chronological age over in excess of 100,000 years (14). However, obsidian is not a widespread natural resource and some of the hydration rate structures appear to yield problematic results. Amino acid racemization (AAR) values can be used under some conditions to infer accurate age values for bone, but seriously anomalous values can be obtained (15, 16). The conditions under which AAR values can be used to accurately infer age, particularly for bone samples, continue to be investigated. 

There is, however, a drawback to this method of dating. The half-life (see Textbox 14) of the racemization process is greatly affected by temperature it is shorter at higher temperatures and slows down as the temperature decreases. An uncertainty of 2°C in the temperature history of dead remains can lead to an error of about 50% in the age determined. For the racemization of amino acids to be applicable for dating, therefore, it is crucial that the temperature history of the environment where the amino acids (that is, skin or bone) have been deposited should be well known. 

The eponymous reaction reported in 1850 by Adolph Strecker is the oldest known method for the synthesis of a-amino acids [100]. It involves condensation of an aldehyde, ammonia, and hydrogen cyanide, followed by hydrolysis of the intermediate a-aminonitrile 130 (Scheme 10.20). To date it constitutes one of the most direct and convenient methods for the preparation of racemic a-amino acids. In parallel with the development of various other stereoselctive methods discussed above, there have been intense research efforts directed towards the identification of enantioselective variants of the Strecker reaction [19, 21, 22, 25, 28]. 

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