Amino acid racemization dating michael patrick thornton dating
k where d L/dt is negative since the L-isomer experiences only a net decrease over time until d L/dt becomes zero at equilibrium.The integral solution of Equation (1) is (Bada and Schroeder 1972, Petit 1974) D, together with the accepted value of 1.25 for D/L at equilibrium.Figure 2 illustrates this pattern for isoleucine, which together with aspartic acid has received the greatest attention in amino acid dating. The racemic mixture resulting from the epimerization of isoleucine has been determined to have a ratio of D-alloisoleucine to L-isoleucine in the range 1.25/1 to 1.4/1, with greatest confidence in a figure near 1.25/1.Interaction among diastereomers is called epimerization. For comparison of work done by different investigators 1.25/1 is accepted as a standard value (Kvenvolden 1975, Bada 1981, Wehmiller and Belknap 1982).CHANGE OF D/L RATIO WITH TIME A far more suitable approach to fossil age determination is provided by the fact that all but one (glycine) of the amino acids in proteins are asymmetric and may exist in either a left-handed (L) or a right-handed (D) form, each of which is the mirror image of the other.As an example, Figure 1 illustrates the L- and D-forms of aspartic acid.At the present time there is insufficient knowledge concerning the effective average racemization rate in a fossil as a function of time to justify dependence on D/L ratios for a quantitative determination of age.
Two of these forms are designated as diastereomers, each of which may exist in either an L- or a D-form.
Therefore it has been suggested that the amino acids found in older fossils, such as those from Cambrian sediments, e.g., are recent contaminants rather than actual molecules remaining from the original organisms.
Investigation of this suggestion has identified residual amino acids in Silurian graptolites (400-430 million year putative age) (Florkin 1969).
Before we can evaluate the results of such determinations we will need a relationship between the D/L ratio and time.
QUANTITATIVE RELATIONSHIPS FOR RACEMIZATION RATE Let the symbols D and L now represent the concentrations of the D-form and the L-form, respectively, of an interconverting enantiomer pair such as D- and L-aspartic acid, or an interconverting diastereomer pair such as D-alloisoleucine and L-isoleucine.
It has been well established that shells as old as Jurassic (135-180 million year conventional age) may contain amino acids bound as protein and peptide, and hence residuals from the parent organisms (Akiyama and Wyckoff 1970).