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

isolation techniques.

   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.

   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.

 

 

 

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