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The amino acids combine with each other like the links of a chain to form a long protein chain.
Proteins contain from 50 to several hundred amino acids.
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.
Racemization is complete when equal amounts of the L- and D-forms are obtained.
In other work, the rate of racemization of aspartic acid, instead of isoleucine, was used.
About 20 different kinds of amino acids are found in proteins.
The study with bone indicated that an uncertainty of ±2° would yield an age with an error of ±50%.
Additional uncertainties are introduced by the possible contamination of the fossil with free amino acids from the environment, and the possibility of racemization during the acid hydrolysis of the protein in the fossil.
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.
All amino acids in proteins (except glycine) are L-amino acids.