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Complicating things somewhat is the fact that some amino acids have two asymmetric carbon atoms and can exist in four different forms, known as diastereoisomers.
The racemization of L-isoleucine to D-alloisoleucine is, therefore, of special interest in the amino acid racemization dating system.
Since the amino acids in proteins of living things are of the L-form, but upon death of the plant or animal spontaneously tend to change to mixtures of the L- and D-forms, the extent of this racemization process could possibly serve as a dating method.
All amino acids in proteins (except glycine) are L-amino acids.
These amino acids spontaneously tend to slowly change to the D-form.
Thus, the older a fossil shell or bone, the greater should be the extent of racemization of the amino acids which are contained in the proteins found in the bone or shell.
Hare and Mitterer measured the rate of racemization of L-isoleucine to D-alloisoleucine in modern shell fragments heated in water at high temperatures and extrapolated these data to lower temperatures in order to estimate the rate of racemization of L-isoleucine in fossil shells to obtain what they believed to be an approximate age for these fossil shells.
Amino acids are the "building blocks," or sub-units, of proteins.
About 20 different kinds of amino acids are found in proteins.
Each amino acid has two chemical groups, an amino group and a carboxyl group, which can form chemical bonds with other amino acids.
The amino group of one amino acid can combine with the carboxyl group of a second amino acid to form a "peptide" bond, and its carboxyl group can combine with the amino group of a third amino acid, and the chain can thus be extended indefinitely.
The D-form tends to revert to the L-form, and eventually an equilibrium is obtained, as illustrated here for alanine: Mixture of equal amounts of the L- and D-forms, The process by which an L-amino acid changes into a mixture of the L- and D-forms (or the D-form changes into a mixture of the L- and D-forms) is called racemization.