AMINO ACID RACEMIZATION DATING

Abstract

Amino acid racemization dating is a technique that is based on chemical reactions. Amino acids occur in two chiral forms (they are not superposable on their mirror images). Amino acids are optically active, which means that a plane of polarized light will be rotated while passing through the substance. The chiral forms are labeled l (lævorotatory: turning a plane of polarized light to the left) and d (dextrorotatory: turning a plane of polarized light to the right). The symmetries are similar to left and right hands and, informally, l-amino acids are called ‘left handed’ and d-amino acids ‘right handed’. Nearly all living organisms exclusively produce l-amino acids; therefore, in living tissues the ratio of d-amino acids over l-amino acids (=d/l ratio) is zero. After the death of the organism, l-amino acids convert into d-amino acids by a reversible process called racemization. For some amino acids, the racemization process takes place over geological time frames. As a result, the d/l ratio can be used for dating until equilibrium is achieved (usually d/l ∼1). In ideal circumstances, AAR dating may be applicable to samples as old as 1 million years.