Amino acid dating

Amino acid dating or racemization 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.[1][2][3][4][5]

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. 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, 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.[6]

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. Also, D/L concentration thresholds appear to occur as sudden decreases in the rate of racemization. This environmental variation restricts amino acid chronologies to materials with known environmental histories and/or relative intercomparisons with other dating methods.

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. 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.[6]

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.

Applications

Amino acid dating has applications in archaeology,[4] stratigraphy, oceanography, paleogeography, paleobiology, and paleoclimatology.[4] These include dating correlation, relative dating, sedimentation rate analysis, sediment transport studies,[7] conservation paleobiology,[8] taphonomy and time-averaging,[9][10][11] sea level determinations, and thermal history reconstructions.[12][13][14][15]

Amino acid dating may be used to date samples too old for radiocarbon dating (which has a maximum range of 40 ka to 0 ka), or too young for potassium-argon dating (which has a range of 40 ka to 150 ka) to to be helpful.[4][6] Verification of radiocarbon and other dating techniques by comparison with amino acid dating is also possible.[16] 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.

Bone, shell, and sediment studies have contributed much to the paleontological record, including that relating to hominoids. Many studies have been undertaken in paleopathology and dietary selection, paleozoogeography and indigeneity, taxonomy and taphonomy, and DNA viability. 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.

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 development. 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.

Forensic science can use this technique to estimate the age of a cadaver[17] or an objet d'art to determine authenticity.

Methods

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. 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.

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.[18]

Reversed 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).[19][20]

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.[21]

References

  1. ^ Bada JL (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.
  2. ^ Canoira L, García-Martínez MJ, Llamas JF, Ortíz JE, Torres TD (2003). "Kinetics of amino acid racemization (epimerization) in the dentine of fossil and modern bear teeth". International Journal of Chemical Kinetics. 35 (11): 576–591. doi:10.1002/kin.10153.
  3. ^ Bada JL, McDonald GD (1995). "Amino acid racemization on Mars: implications for the preservation of biomolecules from an extinct martian biota". Icarus. 114 (1): 139–143. Bibcode:1995Icar..114..139B. doi:10.1006/icar.1995.1049. PMID 11539479.
  4. ^ a b c d e f g h Johnson BJ, Miller GH (1997). "Archaeological Applications of Amino Acid Racemization". Archaeometry. 39 (2): 265–287. doi:10.1111/j.1475-4754.1997.tb00806.x.
  5. ^ Scarponi D, Kaufman D, Bright J, Kowalewski M (October 2008). "Quantifying time-averaging in 4th-order depositional sequences: radiocarbon-calibrated amino-acid racemization dating of Late Quaternary mollusk shells from Po Plain, Italy". Geological Society of America Abstracts with Programs. 40 (6): 502. Archived from the original on 2015-01-22. 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.
  6. ^ a b c "Method". Amino acid geochronology laboratory. Northern Arizona University. Archived from the original on 2 October 2016.
  7. ^ Kosnik MA, 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. Bibcode:2007Geo....35..811K. doi:10.1130/G23722A.1.
  8. ^ Kowalewski M, Serrano GE, Flessa KW, Goodfriend GA (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.
  9. ^ Carroll M, Kowalewski M, Simões MG, Goodfriend GA (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. S2CID 131237779.
  10. ^ 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.
  11. ^ 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. doi:10.1666/0094-8373-35.4.565. S2CID 5839861.
  12. ^ McCoy WD (1987). "The precision of amino acid geochronology and paleothermometry". Quaternary Science Reviews. 6 (1): 43–54. Bibcode:1987QSRv....6...43M. doi:10.1016/0277-3791(87)90016-3.
  13. ^ 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.
  14. ^ 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. S2CID 4312380.
  15. ^ Kaufman DS (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.
  16. ^ 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.
  17. ^ 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. PMID 3170888. S2CID 8664035.
  18. ^ "NEaar: North East Amino Acid Racemization". University of York.
  19. ^ 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.
  20. ^ Kaufman DS (2000). Perspectives in Amino Acid and Protein Geochemistry. New York: Oxford University Press. pp. 145–160.
  21. ^ Penkman KE, Kaufman DS, Maddy D, Collins MJ (February 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.
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