Racemization dating definition
Four of the amino acids that make up proteins isoleucine, threonine, hydroxyproline, and hydroxylysine have two asymmetric carbon atoms which produce four structural possibilities for the same chemical composition.Two of these forms are designated as diastereomers, each of which may exist in either an L- or a D-form.Small amounts of D-amino acids are found in organisms, but most of the amino acids in organisms are constituents of protein molecules, and all amino acids (except glycine which is symmetric and has no D- and L-configuration) in the protein of living organisms are in the L-configuration (Kvenvolden 1975).
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.
In spite of these complications, fossils of similar characteristics, and which have experienced similar conditions of preservation, can be placed in a relative age sequence on the basis of D/L ratios.
Due to the strong dependency of racemization rates on temperature, water concentration, and alkalinity, uncertainties regarding conditions of preservation can leave amino-acid-based age relationships among even similar fossils open to question.
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.