All biological tissues contain amino acids, and all amino acids except glycine (the simplest one) are optically active, having a stereocenter at their α-carbon atom.[6] Each amino acid can thus have two different configurations (enantiomers), D (dextro-) or L (levo-), which are non-superimposable mirror images of each other. With few exceptions,[citation needed] living organisms keep all their amino acids in the L configuration.[4] However, when an organism dies, its biological processes can no longer maintain this thermodynamically unstable ratio of enantiomers, and the ratio of D to L begins to move towards equilibrium, a process called racemization.[4] Thus, measuring the ratio of D to L amino acids in a sample enables one to estimate how long ago the specimen died.[7]
Factors affecting racemization rate
The rate at which racemization proceeds depends on the type of amino acid and on the average temperature, humidity, acidity, the depth of decomposition, porosity, and catalytic effects of local metals and minerals, the species or tissue being investigated and other characteristics of the enclosing matrix.[citation needed] Also, D/L concentration thresholds appear to occur as sudden decreases in the rate of racemization.[citation needed] This environmental variation restricts amino acid chronologies to materials with known environmental histories and/or relative intercomparisons with other dating methods.[citation needed]
Temperature and humidity histories of microenvironments 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.[citation needed] 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 Ma, and resolution generally about 20% of the age; at -10°C the reaction has a maximum age of ~10 Ma, and a correspondingly coarser resolution.[7]
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.[citation needed]
Amino acid dating may be used to date samples too old for radiocarbon dating (which has a maximum range of 40ka to 0ka), or too young for potassium-argon dating (which has a range of 40ka to 150ka) to be helpful.[4][7] Verification of radiocarbon and other dating techniques by comparison with amino acid dating is also possible.[17] The 'filling in' of large probability ranges, such as those caused by variation in 14C levels throughout the biosphere, has sometimes been possible as well.[citation needed]
Bone, shell, and sediment studies have contributed much to the paleontological record, including that relating to hominoids.[citation needed] Many studies have been undertaken in paleopathology and dietary selection, paleozoogeography and indigeneity, taxonomy and taphonomy, and DNA viability.[citation needed] Human cultural changes and their effects on local ecologies have been assessed using this technique; the differentiation of cooked from uncooked bone, shell, and residue is sometimes possible.[citation needed]
Amino acid racemization also has a role in tissue and protein degradation studies, particularly useful in developing museum preservation methods. These studies have produced models of protein adhesive and other biopolymer deteriorations and the concurrent pore system[definition needed] development.[citation needed] The reduction in bodily repair capability during aging is important to studies of senescence and age-associated disease, and allows the determination of age in living animals.[citation needed]
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[4] or electrophoresis, and the particular amino acid D:L ratio determined by fluorescence.[citation needed] 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.[citation needed]
Conventional racemization analysis tends to report a D-alloisoleucine / L-isoleucine (A/I or D/L ratio).[4] This amino acid ratio has the advantages of being relatively easy to measure and being chronologically useful through the Quaternary.[19]
Amino acid dating relies on the assumption that the fraction of amino acids being studied has been a closed system since its formation, exchanging nothing with its surroundings. Removing contaminants decreases variability in results by ensuring that analysis is performed only on the most representative fraction of amino acids. These cleaning methods may include soaking powdered biomineral samples in bleach prior to measuring D/L ratio, destroying the amino acids in the more porous, open areas while leaving the fraction trapped inside the grains unscathed.[22][6]
1 2 3 "Method". Amino acid geochronology laboratory. Northern Arizona University. Archived from the original on 2 October 2016.
↑ Kosnik MA, etal. (2007). "Sediment mixing and stratigraphic disorder revealed by the age-structure of Tellina shells in Great Barrier Reef sediment". Geology. 35 (9): 811–814. Bibcode:2007Geo....35..811K. doi:10.1130/G23722A.1.
↑ Kidwell SM, Best MM, Kaufman DS (2005). "Taphonomic trade-offs in tropical marine death assemblages: Differential time averaging, shell loss, and probable bias in siliciclastic vs. Carbonate facies". Geology. 33 (9): 729–732. Bibcode:2005Geo....33..729K. doi:10.1130/G21607.1.
↑ Kosnik MA, Hua Q, Kaufman DS, Wüst RA (2009). "Taphonomic bias and time-averaging in tropical molluscan death assemblages: Differential shell half-lives in Great Barrier Reef sediment". Paleobiology. 35 (4): 565–586. Bibcode:2009Pbio...35..565K. doi:10.1666/0094-8373-35.4.565. S2CID5839861.
↑ Oches EA, McCoy WD, Clark PU (1996). "Amino acid estimates of latitudinal temperature gradients and geochronology of loess deposition during the last glaciation, Mississippi Valley, United States". Geological Society of America Bulletin. 108 (7): 892–903. Bibcode:1996GSAB..108..892O. doi:10.1130/0016-7606(1996)108<0892:AAEOLT>2.3.CO;2.
↑ Miller GH, Magee JW, Jull AJ (1997). "Low-latitude glacial cooling in the Southern Hemisphere from amino-acid racemization in emu eggshells". Nature. 385 (6613): 241–244. Bibcode:1997Natur.385..241M. doi:10.1038/385241a0. S2CID4312380.
↑ McMenamin MA, Blunt DJ, Kvenvolden KA, Miller SE, Marcus LF, Pardi RR (1982). "Amino acid geochemistry of fossil bones from the Rancho La Brea Asphalt Deposit, California". Quaternary Research. 18 (2): 174–183. Bibcode:1982QuRes..18..174M. doi:10.1016/0033-5894(82)90068-0.
↑ Ogino T, Ogino H (October 1988). "Application to forensic odontology of aspartic acid racemization in unerupted and supernumerary teeth". Journal of Dental Research. 67 (10): 1319–1322. doi:10.1177/00220345880670101501. PMID3170888. S2CID8664035.
↑ Kaufman DS, Manley WG (1998). "A new procedure for determining dl amino acid ratios in fossils using reverse phase liquid chromatography". Quaternary Science Reviews. 17 (11): 987–1000. Bibcode:1998QSRv...17..987K. doi:10.1016/S0277-3791(97)00086-3.
↑ Kaufman DS (2000). Perspectives in Amino Acid and Protein Geochemistry. New York: Oxford University Press. pp.145–160.
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