AMINO ACID DATING is a dating technique used to estimate the
age of a specimen in paleobiology , molecular paleontology ,
archaeology , forensic science , taphonomy , sedimentary geology and
other fields. This technique relates changes in amino acid molecules
to the time elapsed since they were formed.
All biological tissues contain amino acids . All amino acids except
glycine (the simplest one) are optically active , having a
stereocenter at their α-C atom. This means that the amino acid can
have two different configurations, "D" or "L" which are mirror images
of each other. 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 to L moves from a value near 0 towards an equilibrium value
near 1, a process called racemization . Thus, measuring the ratio of D
to L in a sample enables one to estimate how long ago the specimen
* 1 Factors affecting racemization
* 2 Amino acids used
* 3 Applications
* 4 Procedure
* 5 References
* 6 External links
* 6.1 Active laboratories
FACTORS AFFECTING RACEMIZATION
The rate at which racemization proceeds depends on the type of amino
acid and on the average temperature, humidity, acidity (pH ), and
other 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 for 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. As a rule of
thumb, sites with a mean annual temperature of 30°C have a maximum
range of 200 ka and resolution of about 10 ka; sites at 10°C have a
maximum age range of ~2 m.y., and resolution generally about 20% of
the age; at -10°C the reaction has a maximum age of ~10 m.y., and a
correspondingly coarser resolution.
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.
AMINO ACIDS USED
Conventional racemization analysis tends to report a D-alloisoleucine
/ L-isoleucine (A/I or D/L ratio). This amino acid ratio has the
advantages of being relatively easy to measure and being
chronologically useful through the
Reverse phase HPLC techniques can measure up to 9 amino acids useful
in geochronology over different time scales on a single chromatogram
(aspartic acid , glutamic acid , serine , alanine , arginine ,
tyrosine , valine , phenylalanine , leucine ).
In recent years there have been successful efforts to examine
intra-crystalline amino acids separately as they have been shown to
improve results in some cases.
Data from the geochronological analysis of amino acid racemization
has been building for thirty-five years.
Archeology , stratigraphy ,
oceanography , paleogeography , paleobiology , and paleoclimatology
have been particularly affected. Their applications include dating
correlation, relative dating, sedimentation rate analysis, sediment
transport studies, conservation paleobiology, taphonomy and
time-averaging, sea level determinations, and thermal history
Paleobiology and archaeology have also been strongly affected. Bone,
shell, and sediment studies have contributed much to the
paleontological record, including that relating to hominoids.
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
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 in developing museum
preservation methods. These have produced models of protein adhesive
and other biopolymer deteriorations and the concurrent pore system
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 derivative
hydrolysis product can be combined with a chiral specific fluorescent,
separated by chromatography or electrophoresis , and the particular
amino acid D:L ratio determined by fluorescence. Alternatively, 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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* ^ Bada, J.; McDonald, G. D. (1995). "Amino Acid
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* ^ 2008 quote: The results provide a compelling case for
applicability of amino acid racemization methods as a tool for
evaluating changes in depositional dynamics, sedimentation rates,
time-averaging, temporal resolution of the fossil record, and
taphonomic overprints across sequence stratigraphic cycles.
* ^ http://jan.ucc.nau.edu/~dsk5/AAGL/method/principles.html
* ^ http://jan.ucc.nau.edu/~dsk5/AAGL/method/age.html
* ^ http://www.york.ac.uk/palaeo/services/ne-aar/aar/
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* ^ http://jan.ucc.nau.edu/~dsk5/AAGL/method.html
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