Amino acid dating
Amino acid dating is a dating technique  used to estimate the age of a specimen in paleobiology, 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 an asymmetric carbon 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 died.
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-allosioleucine / 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 Quaternary.
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 reconstructions.
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 technique.
The slight reduction in this[clarification needed] 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 development.
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.
- Bada, J. L. (1985). "Amino Acid Racemization Dating of Fossil Bones". Annual Review of Earth and Planetary Sciences 13: 241–268. Bibcode:1985AREPS..13..241B. doi:10.1146/annurev.ea.13.050185.001325.
- Canoira, L.; Garc�a-Mart�Nez, M. J.; Llamas, J. F.; Ort�z, J. E.; Torres, T. D. (2003). "Kinetics of amino acid racemization (epimerization) in the dentine of fossil and modern bear teeth". International Journal of Chemical Kinetics 35 (11): 576. doi:10.1002/kin.10153.
- Bada, J.; McDonald, G. D. (1995). "Amino Acid Racemization on Mars: Implications for the Preservation of Biomolecules from an Extinct Martian Biota". Icarus 114: 139–143. Bibcode:1995Icar..114..139B. doi:10.1006/icar.1995.1049. PMID 11539479.
- Johnson, B. J.; Miller, G. H. (1997). "Archaeological Applications of Amino Acid Racemization". Archaeometry 39 (2): 265. doi:10.1111/j.1475-4754.1997.tb00806.x.
- 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.
- Kaufman, D.S. and W.F. Manley (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, D.S., 2000 in Perspectives in Amino Acid and Protein Geochemistry: Oxford University Press, New York, 145-160.
- Penkman, K.E.M., D.S. Kaufman, D. Maddy and M. J. Collins (2008). "Closed-system behaviour of the intra-crystalline fraction of amino acids in mollusc shells". Quaternary Geochronology 3 (1–2): 2–25. doi:10.1016/j.quageo.2007.07.001. PMC 2727006. PMID 19684879.
- Johnson, B.J. and G.I. Miller (1997). "Archaeological Applications of Amino Acid Racemization". Archaeometry 39 (2): 265–287. doi:10.1111/j.1475-4754.1997.tb00806.x.
- Kosnik et al. (2007). "Sediment mixing and stratigraphic disorder revealed by the age-structure of Tellina shells in Great Barrier Reef sediment". Geology 35 (9): 811–814. doi:10.1130/G23722A.1.
- Kowalewski et al. (2000). "Dead delta's former productivity: Two trillion shells at the mouth of the Colorado River". Geology 28 (12): 1059–1062. Bibcode:2000Geo....28.1059K. doi:10.1130/0091-7613(2000)28<1059:DDFPTT>2.0.CO;2.
- Carroll et al. (2003). "Quantitative estimates of time-averaging in terebratulid brachiopod shell accumulations from a modern tropical shelf". Paleobiology 29 (3): 381–402. doi:10.1666/0094-8373(2003)029<0381:QEOTIT>2.0.CO;2.
- Kidwell et al. (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 et al. (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. doi:10.1666/0094-8373-35.4.565.
- McCoy, W.D. (1987). "The precision of amino acid geochronology and paleothermometry". Quaternary Science Reviews 6: 43–54. Bibcode:1987QSRv....6...43M. doi:10.1016/0277-3791(87)90016-3.
- Oches, E.A. et al. (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, G.H. et al. (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.
- Kaufman, D.S. (2003). "Amino acid paleothermometry of Quaternary ostracodes from the Bonneville Basin, Utah". Quaternary Science Reviews 22 (8–9): 899–914. Bibcode:2003QSRv...22..899K. doi:10.1016/S0277-3791(03)00006-4.
- McMenamin, M.A.S. et al. (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. and Ogino H. (1988). Journal of Dental Science 67: 1319–1322.
- Fundamentals of sample age determination from its amino acid racemization by Policarp Hortolà
- Brown 1985 Amino Acid Dating. Origins 12(1):8-25
- Northern Arizona University Amino Acid Geochronology Laboratory
- University of Massachusetts Amino Acid Geochronology Laboratory
- The University of Colorado Amino Acid Geochronology Lab
- University of Delaware Research Group
- University of York BioArCh
- Madrid School of Mines Biomolecular Stratigraphy Laboratory