Racemization fossil bone, amino acid

Bada, J. L. (1985), Amino acid racemization dating of fossil bones, Ann. Rev. Earth Planet. Sci. 13, 241-268. 

Bada, J. L., Shou, M., Effects of various environmental parameters on amino acid racemization rates in fossil bones, Geological Soc. America, Abstracts with Programs, 8, 762-763... 

Bada, J. L., Master, P. M., Hoopes, E., Darling, The Dating of Fossil Bones Using Amino Acid Racemization, In Radiocarbon Dating, pp. 740-756, Berkeley, University of California Press, 1979. 

The amino acid racemization method is based on the measurement of the degree of racemization acquired by a material expressed as the ratio dextro amino acid (D) enantiomeric form/levo amino acid (L) enantiomeric form, which is an indicator of the age of the object. This method has been applied in fossil shells, bones, teeth, wood, plant remains, and coral [68]. 

Although the first demonstration that amino acid racemization took place in fossils used mollusc shells (23), the application of this reaction in dating these materials has been extensively investigated only recently (19,20). Work on Mercenaria (19), Chione (20), and other species (24) has tested the application of racemization dating to fossil mollusc shells from geological contexts and Indian shell middens. These and other studies have shown that there are problems with amino acid racemization dating of carbonaceous fossils which are not encountered with bone. Reversible first-order racemization kinetics which are observed in bone... 

Amino Acid Racemization Dating of Fossil Bone... 

The extent of racemization of several amino acids in a fossil bone can also be used to determine the degree of secondary amino acid contamination (22). Elevated temperature kinetics experiments have shown that the extent of amino acid racemization in a bone should have the... 

Amino acid racemization dating of mollusc shell has not been investigated as extensively as that of fossil bone. Recently several studies have used d/l enantiomeric ratios to resolve stratigraphic relationships (24) and to date geological deposits by the calibration procedure (19,20). Racemization dating has also been applied to midden shell from archaeological sites with limited success (20). 

Bada, J.L., Paleoanthropological applications of amino acids racemization dating of fossil bones and teeth, Anthrop. Anz., 45, 1, 1987. 

Amino Acid Dating Techniques depend on the "rates of hydrolysis reactions of proteins and racemization, epimerization, and decomposition reactions of amino acids [they have] been applied to the age-dating of fossil bone, teeth, and shell. Activation energies range from near 20 kcal per mole for hydrolysis reactions to around 30 kcal per mole for racemization... 

The effect of temperature on the rate of racemization of amino acids in fossils was investigated and the implications of the findings on fossil dating were analyzed313. The high rate of conversion of L-aspartic acid into its D-isomer, observed in uncontaminated bone samples taken from catacombs in Rome (IV century BC) was attributed to collagen decomposition due to the humidity of the catacombs314. 

Racemization rates (Table I) clearly differ among these seven amino acids. To compare results from the four proteins, rate constants were calculated from these data. For casein, D/L ratios were measured at 0, 1, 3, 8 and 24 hours. These results are plotted in Figure 2. The curves have two regions of different racemization rates. Rapid initial rates observable up to about 3 hours are followed by slower rates up to 24 hours. The amino acids apparently have not reached equilibrium by 24 hours of incubation. Theoretically for amino acids having one asymmetric center, the equilibrium D/L ratio is 1.0. This value has been observed in fossil bone protein (see 13) and in dry roasted proteins (10), but not in calcareous marine sediments (17) nor in fossil mollusc shell (18). 

Of the various amino acids, aspartic acid has been the most widely used in fossil bone dating. This amino acid has one of the fastest racemization rates of the stable amino acids (21,22). At 20 C in bone, the half-life for aspartic acid racemization is 15,000 years. Thus, for most mid- or low-latitude sites the racemization rate of aspartic acid is... 

Aspartic acid has been the amino acid most extensively used in racemization dating of fossil bone. The reaction can be written as ... 

Amino acids of L configuration are more abundant in nature the D-enantiomers are usually found in bacterial cell walls, antibiotic compounds, and rare biological molecules. The DL-form (the racemate) are found in geological specimens such as fossil shells and bones, or they are formed by abiotic synthesis. 

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