Alfredo Suzuki responds:
Briefly, amino acids with an asymmetric carbon can exist in two forms: d- (dextrorotary or right-handed) and l- (levorotary or left-handed). l- forms can transform to d- forms and back, randomly (racemizing). Living organisms make and use l- forms. Dead protein has a mixture. The longer it has been dead, the closer the d-/ l- ratio may come to 1/1.
Dating by amino acids (amino acid racemization dating)
The presence of residues (traces) of amino acids in fossils throughout the phanerozoic (geological eras since the Cambrian) has been object of investigation for as long as three decades or more. Among other goals, this study seeks to establish a new technique for dating fossil samples; in other words, a kind of "biological chronometer".
The basic principles underlying the "biological chronometer" of amino acids are as follows:
1. Proteins are organic macromolecules built up by amino acids;
2. Vital amino acids (aproximately 20 in number) are levorotary, except for glycine, which does not contain any asymmetric carbon atom;
3. Proteins outside vital metabolism (i.e., from dead organisms) degrade into their component amino acids as time goes by;
4. Degradation occurs in such a way that the concentration of levorotary amino acids,(l-), decreases with time, while the concentration of dextrorotary amino acids, (d-), increases, until an equilibrium point is reached; This process is known as "racemization."
5. The fraction of l-/d- in the equilibrium state and the rate of conversion of l- amino acids to d- amino acids depends on the amino acid, more precisely, on how many asymmetric carbon atoms a given amino acid contains;
6. One studies then, the speed of racemization of a given amino acid, and seeks to build a function of time, i.e., a functional relation between the rate of conversion of the l- for to the d- form, and time;
7. Finally, in a laboratory procedure, one measures the relative abundances of the two components by standard techniques of chromatography and attempts to assign a time for the observed degree of conversion of the l- form to the racemic mixture.
Difficulties associated with the method
The main drawback of this type of procedure to date fossil samples is that racemization is by no means an easy process to describe mathematically as a function of time. It depends on many environmental factors, and other parameters and variables such as
Amino acid dating, by R.H.Brown, Geoscience Research Institute.