Amino acid dating is a dating technique used to estimate the age of a specimen in paleobiology, archaeology, forensic science, and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.
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All biological tissues contain amino acids. All amino acids except glycine possess an asymmetric carbon atom, which means that the amino acid can have two different configurations, "D" or "L". With a few important exceptions, living organisms keep all their amino acids in the "L" configuration. When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D/L moves slowly toward equilibrium, a process called racemization. Thus, measuring the ratio of D/L of a sample can allow calculations of how long ago the specimen died.
The rate at which racemization proceeds depends upon the type of amino acid, average temperature, humidity, acidity, pH, and characteristics of the enclosing matrix. Also, D/L concentration thresholds appear to occur as sudden decreases in the rate of racemization. These effects restrict amino acid chronologies to materials with known environmental histories and/or relative intercomparisons with other dating methods.
Temperature and humidity histories of microenvironments are being produced at ever increasing rates as technologies advance and technologists accumulate data. These are important to amino acid dating because racemization occurs much faster in warm, wet conditions compared to cold, dry conditions. Temperate to cold region studies are much more common than tropical studies, and the steady cold of the ocean floor or the dry interior of bones and shells have contributed most to the accumulation of racemization dating data.
Strong acidity and mild to strong alkalinity induce greatly increased racemization rates. Generally, they are not assumed to have a great impact in the natural environment, though tephrochronological data may shed new light on this variable.
The enclosing matrix is probably the most difficult variable in amino acid dating. This includes racemization rate variation among species and organs, and is affected by the depth of decomposition, porosity, and catalytic effects of local metals and minerals.
Asparagine (acidified to aspartic acid) racemizes quickly and has frequently been used to date materials from the present back to around 25000 BP. Isoleucine racemizes much more slowly, and has been used to date materials from 5000 to 2 million years of age. Concentration thresholds and less comprehensive environmental histories produce much greater margins of error with older isoleucine measures. Other amino acids are less frequently used for dating, primarily because of difficulties in isolation
Data from the geochronological analysis of amino acid racemization has been building for thirty-five years. Stratigraphy, oceanography, paleogeography, and paleoclimatology have been particularly affected. Their applications include dating correlation, relative dating, sedimentation rate analysis, sediment transport studies, sea level determinations, and thermal history reconstructions.
Paleobiology and archaeology have also been strongly affected. Bone, shell, and sediment studies have contributed much to the paleontological record, including the hominoid. Verification of radiocarbon and other dating techniques by amino acid racemization and vice versa has occurred. The 'filling in' of large probability ranges, such as with radiocarbon reservoir effects, has sometimes been possible. Paleopathology and dietary selection, paleozoogeography and indigineity, taxonomy and taphonomy, and DNA viability studies abound. The differentiation of cooked from uncooked bone, shell, and residue is sometimes possible. Human cultural changes and their effects on local ecologies have been assessed using this technique.
The slight reduction in this repair capability during aging is important to studies of longevity and old age tissue breakdown disorders, and allows the determination of age of living animals.
Amino acid racemization also has a role in tissue and protein degradation studies, particularly useful to developing museum preservation methods. These have produced models of protein adhesive and other biopolymer deteriorations and the concurrent pore system development.
Forensic science can use this technique to estimate the age of a cadaver or an objet d'art to determine authenticity.
Amino acid racemization analysis consists of sample preparation, isolation of the amino acid wanted, and measure of its D:L ratio. Sample preparation entails the identification, raw extraction, and separation of proteins into their constituent amino acids, typically by grinding followed by acid hydrolysis. The amino acid hydrolysate can be combined with a chiral specific fluorescent, separated by chromatography or electrophoresis, and the particular amino acid D:L ratio determined by fluorescence. Or, the particular amino acid can be separated by chromatography or electrophoresis, combined with a metal cation, and the D:L ratio determined by mass spectrometry. Chromatographic and electrophoretic separation of proteins and amino acids is dependent upon molecular size, which generally corresponds to molecular weight, and to a lesser extent upon shape and charge.
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   | Reference List from University of Delaware Research Group - References to Amino Acid Racemization Work |
| | PaleoDNA - PaleoDNA-DNA Extraction Services, DNA Analysis |
| | University of York BIOARCH - Amino acid racemization |
| | Amino Acid Dating - Brown, R. H. --- Amino Acid Dating |
| | The Amino Acid Geochronology Lab - Research Projects @ the Amino Acid Geochron. Lab |
| | New Optical Probes of Chiral Molecules - New Optical Probes of Chiral Molecules |
| | Amino Acid Geochronology Laboratory - Amino Acid Geochronology Laboratory |
| | Fundamentals of sample age determination from its amino acid racemization by Policarp Hortolà - Fundamentals of sample age determination from its amino acid racemization |
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