amino-acid racemization

Used in archaeological dating, a rotation process converting L-isomer molecular structures in the amino acids of living tissue to D-isomers after death. These two structures, one the mirror image of the other, are distinguishable by their chemical behaviour. The concentration ratio of L-isomers to D-isomers in material to be dated may be calibrated against bone samples that have been radiocarbon dated, but the method is still experimental.

The rate at which racemization proceeds depends upon the amino acid involved and the material in which it survives. In fossil bone, isoleucine is the amino acid examined for dating samples over a million years old, and aspartic acid for samples thought to be 0.1–0.3 million years old. In wood, proline and hydroxyproline are examined for these time periods.

The French chemist and microbiologist Louis Pasteur first observed the existence of L-isomers in living tissue in 1849.