Amino Acid Racemization
Most amino acid molecules possess an assymmetric 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. Life, however, requires certain composition AND
shape from its minions in order to complete a 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).
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 affectations restrict amino acid chronologies to materials with
known environmental histories and/or relative intercomparisons with other dating methods.
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, mainly because of difficulties in
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
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
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. Applications include 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 possible. Human cultural changes and its
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 of 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.
This specificity, or chirality, also has numerous nonbiological applications
such as in solvents, adsorption characteristics, and nanomaterial development.
Its' frequent applications have led to widespread dispersal of the equipment
necessary for chiral determinations, and to development of multiple techniques.
Amino acid racemization analysis consists of sample preparation, isolation of
the amino acid wanted, and measure of its D:L ratio. Sample preparation consists
of 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.