Racemization of Amino Acids

A biological dating method used on human and animal bone and carbonate shells. Within living organisms amino acids are molecularly structured à L-isomer configuration, but at death they restructure to D-isomer configuration through a process known as Racemization. This restructuring takes place relatively slowly, from a zero point at the time of death, D/L ratio gradually increases so that equilibrium is reached after 1.5 X 105 - 2 X 106 years. The main problem with the technique is that the rate of racemization varies according to temperature and groundwater over long periods à uneven rates of racemization à difficulty in turning D/L ratios into absolute ages; but data about a particular area could be helpful in this forecast.

Methods of estimating the age of rocks:
1. Relative dating techniques: date specimens in relation to one another; e.g. stratigraphy used to establish the succession of fossils.
2. Absolute (or chronometric) techniques: give an absolute estimate of the age and fall into two main groups:
- Existence of something that develops at a seasonally varying rate, as in dendrochronology and varve dating.
- Some measurable change that occurs at a known rate, as in chemical dating, radioactive (or radiometric) dating.

Determination of the relative or absolute age of materials or objects by measurement of the degree of racemization of the amino acids (D/L ratio): With the exception of glycine, an a.a. that can exist in two isomeric forms (D- and L-); they rotate plane-polarized light in equal but opposite directions (D form rotates to right and L form to left), their other chemical and physical properties are identical. L. Pasteur (1850) found that only L-amino acids are generally found in living organisms, but scientists still couldn't explain why life is based on only the L-amino acids.

Under conditions of chemical equilibrium, equal amounts of D- and L- forms are present (D/L = 1.0) à a racemic mixture. Living organisms maintain a state of disequilibrium through a system of enzymes that selectively utilize only the L-form. Once a protein is synthesized and isolated, L-amino acids undergoes racemization to be converted into a racemic mixture. Since racemization is a chemical process, the extent of racemization is dependent not only on the time that has elapsed since the L-amino acids were synthesized but also on the exposure temperature (the higher the temperature, the faster the rate of racemization) and the type of amino acids.

Analytical methods for separation of a.a. isomers include HPLC and GC which require only few mg. of sample material. Samples are first hydrolyzed in HCl to break down proteins and release it’s constituent amino acids.

Uses of amino acid racemization (since late 1960s):
Dating of fossils e.g. bones, teeth, and shells; particularly for specimens that were difficult to date by other methods.
Racemization of the metabolically inert tissues of living mammals.
Biological age of a variety of mammalian species.

Extent of racemization generally increases with geologic age.

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