After death bones, along with the rest of the body, start to decompose. In bone this involves breakdown and change of the protein component — principally the collagen. Like other proteins, collagen is built up from amino acid units, and it is these that are separated and broken down (this decomposition of collagen gives rise to a dating method, nitrogen dating). But besides actual breakdown, the amino acids that remain intact are subject to another change. In life, all amino acids have a particular orientation to their molecular structure (this version is called the L-isomer). After death the amino acids re-align to a mirror-image of this molecular structure (the D-isomer). This reaction is called racemization (or epimerization) and it occurs at a slow, relatively uniform rate. Measurement of the proportion of the D-isomer to L-isomer should therefore provide a dating method. Unfortunately, racemization is also dependent on temperature, groundwater and the degree of breakdown of the collagen molecules. This makes reliable dates difficult to obtain.
The Macmillan dictionary of archaeology, Ruth D. Whitehouse, 1983Copied