Fundamentals of sample age determination from its amino acid racemization

Policarp Hortolà

Principles

It is given the name of isomers (from the Greek isos, same, and meros, part) to the organic compounds which, being identical in its composition (i.e., in its molecular formula), have different physical and/or chemical properties. It is termed stereochemical or structural isomery to that which is given in the compounds that have the same composition but differ in its spatial configuration; that is to say, that their atoms are bound together in different form. Most of their physical and chemical properties are identical.

The stereochemical isomery appears in all the compounds capable of existing in two forms whose structures are, one another, no superimposable images, like in a mirror (specular images); these compounds, which can exist in forms that are one another like either the right and the left hands, are termed chiral compounds (from the Greek cheir, cheiros, hand). The phenomenon of the stereoisomery -also named chirality- appears in all the compounds that possess any carbon atom which be bound to four different atoms o atom groups.

It is termed racemization (from the Latin racemus, raceme) to the chemical process in which a form of a chiral compound becomes its specular image. That is to say, to the transformation of a L compound in D, o vice versa.

Most of the natural amino acids have two structural isomers: the dextrogyrous form (D) and the levogyrous one (L); e.g., alanine, proline, aspartic acid, etc. Three of the proteinaceous amino acids have furthermore two diastereomers or allo forms (L-allo and D-allo): hydroxylysine, threonine, and isoleucine. When racemization takes place in a compound with more than one asymmetric carbon (i.e., with more than one structural isomer), this racemization reaction is termed epimerization (from the Greek epi, upon, at the top).

Generally, the biological reactions are stereospecific: the natural compounds, to can be biologically actives, must have a defined spatial configuration, so the organic molecules of the living beings tend to be chiral ones. Thus, the amino acids are for the most part of L configuration, while, for example, the simple sugars of natural origin are generally D forms.

Though in the proteins of a living being the amino acids be present solely in the levogyrous structural form (L), upon dying the organism will be begun a slow racemization reaction. If we symbolize the considered amino acid with AA, the amount of molecules of each isomer with m and n, and the constants that define the racemization speed with kL and kD, the reaction may be described as:

Thus, since the proportion D-amino acids/L-amino acids increases with the age of the specimen, by knowing, of an amino acid, its racemization rate and the amount L and D forms present in a sample, we will be able to determine the chronometric age of this one.

Techniques

After the proper precesses of obtaining and preparating the sample for its amino acid racemization dating (AAR), several analytical techniques may be used to determine its enantiomeric rate (D-AA/L-AA). The most usual ones are:
- Ion-exchange liquid chromatography.
- High performance liquid chromatography.
- Gas chromatography.
- Capillary electrophoresis.

Applications

AAR may be applicated to many and varied fields, as the following ones:
- Anthropology.
- Archaeology.
- Paleobiology.
- Oceanography.
- Stratigraphy.
- Forensic Medicine.

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