Amino acid dating
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Amino acid dating is a technique used to estimate age in a wide variety of situations. This technique relates changes in amino acid molecules to the time elapsed since they were formed. This technique is used in paleobiology, archaeology, forensic science, and numerous other fields.
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[edit] Racemization chemistry
Most amino acid molecules possess an asymmetric carbon atom or two which can occupy either of two positions like a toggle switch and still be tightly bound to its neighbors. These positions are characterized as D (right), or L (left), according to which way the molecule bends plane polarized light. This is a common phenomenon throughout biochemistry. Left to themselves, over time the ratio of D/L molecules will roughly even out. This process of conversion is known as racemization.
[edit] Amino acids in living organisms
Life requires a certain composition AND shape of amino acid molecules in order to complete their function. Living organisms on earth keep their amino acids in the L position, with a notable exception found in certain bacterial cell walls, and their sugars in the D position. When the organism dies, control ceases, and the ratio of D/L moves slowly toward equilibrium (racemic). Thus, measuring the ratio of D/L of a sample can allow calculations of how long ago the specimen died.
[edit] Factors affecting racemization
The rate at which racemization proceeds depends upon the type of amino acid, average temperature, humidity, acidity, alkalinity, and 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.
[edit] Amino acids used
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 techniques.
[edit] Applications
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 expression of non-racemic amino acids is only known to occur through the life process therefore they have been searched for in meteorites and lunar samples, and will be sought on Mars, thus contributing to studies on extraterrestrial life and the origins of life. Other extreme environment studies concern racemization repair mechanisms in extreme cold dormant states and hydrothermal vent populations. 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. Food adulteration and harsh processing affect its normal racemization ratio, as can bacterial contamination. This can also affect its nutritional value, taste, and aroma. Likewise, many drugs require D or L specificity for effective activity.
[edit] Non-biological applications
This specificity, or chirality, also has numerous nonbiological applications such as in solvent adsorption characteristics and nanomaterial development. Its' widespread applications have led to widespread dispersal of the equipment necessary for chiral determinations, and to development of multiple techniques.
[edit] Procedure
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.
[edit] External links
- Fundamentals of sample age determination from its amino acid racemization by Policarp Hortolà
- Jeffrey L. Bada
- Amino Acid Geochronology Laboratory
- Amino Acid Dating
- The Amino Acid Geochronology Lab
- Racemization by Kozue Takahashi
- New Optical Probes of Chiral Molecules
- Reference List from University of Delaware Research Group
- PaleoDNA
- University of York BIOARCH
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