Matthew Collins of the Department of Biology and Archaeology at the University of York wrote recently to point out that in my short course Timing is Everything: Scientific Dating in Archaeology I had omitted mention of racemization dating. Dr. Collins was kind enough to provide this description for us:
Racemization dating is a process which uses the measurement of the decay rate of carbon protein amino acids to date once-living organic tissue. All living organisms have protein; protein is made up of amino acids. All but one of these amino acids (glycine) have two different chiral forms (mirror images of each other). While an organism lives, their proteins are composed of only 'left-handed' (laevo, or L) amino acids, but once the organism dies the left-handed amino acids slowly turn into right-handed (dextro or D) amino acids. Once formed, the D amino acids themselves slowly turn back to L forms at the same rate.
The ratio between the D and L versions of each amino acid increases with time from 0 (0%D / 100% L) to 1 (50%D / 50%L). This process is called amino acid racemization, and derives from the term racemic mixture, the point at which there is an equal balance of D and L amino acids within an organism. Once an organism has achieved a racemic mixture, even though L amino acids still racemize to D and vice versa, the D/L ratio will not change because the amino acids going in each direction cancel each other out. Racemic mixtures of amino acids have been found in fossils, millions of years old.
Unlike radiocarbon decay which is a physical process independent of temperature or pressure, racimization is a chemical reaction and, like other chemical reactions, occurs faster under hotter conditions. To make matters more complicated different amino acids have different rates of conversion from D to L.
Racemization can be used to date objects between 5,000 and 1,000,000 years old, and was used recently to date the age of sediments at Pakefield, the earliest record of human occupation in northwest Europe.
Racemization dating is a process which uses the measurement of the decay rate of carbon protein amino acids to date once-living organic tissue. All living organisms have protein; protein is made up of amino acids. All but one of these amino acids (glycine) have two different chiral forms (mirror images of each other). While an organism lives, their proteins are composed of only 'left-handed' (laevo, or L) amino acids, but once the organism dies the left-handed amino acids slowly turn into right-handed (dextro or D) amino acids. Once formed, the D amino acids themselves slowly turn back to L forms at the same rate.
The ratio between the D and L versions of each amino acid increases with time from 0 (0%D / 100% L) to 1 (50%D / 50%L). This process is called amino acid racemization, and derives from the term racemic mixture, the point at which there is an equal balance of D and L amino acids within an organism. Once an organism has achieved a racemic mixture, even though L amino acids still racemize to D and vice versa, the D/L ratio will not change because the amino acids going in each direction cancel each other out. Racemic mixtures of amino acids have been found in fossils, millions of years old.
Unlike radiocarbon decay which is a physical process independent of temperature or pressure, racimization is a chemical reaction and, like other chemical reactions, occurs faster under hotter conditions. To make matters more complicated different amino acids have different rates of conversion from D to L.
Racemization can be used to date objects between 5,000 and 1,000,000 years old, and was used recently to date the age of sediments at Pakefield, the earliest record of human occupation in northwest Europe.
- Scientific Dating in Archaeology, resources and definitions of dating techniques
- Timing is Everything, a short course in scientific dating methods
- Racemization Dating, including this definition and more resources
- Amino Acid Racemization, more information on the process from a consortium of laboratories
- Amino Acids, from About's guide to Chemistry, Anne-Marie Helmenstine
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