Amino acid dating

Short description: Paleontological dating technique

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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]

## Contents

* 1 Background
* 1.1 Chemistry
* 1.2 Amino acids and environmental conditions
* 2 Applications
* 3 Methods
* 4 References
* 5 External links
* 5.1 Active laboratories

## Background

### Chemistry

L-isoleucine, an amino acid used in amino acid dating analysis D-alloisoleucine, the epimer of L-isoleucine

Amino acids are a set of organic compounds that are used by living organisms to synthesise proteins. All amino acids (except glycine)[6] have one or more pairs of stereoisomers, isomers which share the same bond order but are organized differently in 3D space. Amino acid stereoisomer pairs that are optically active and mirror images of each other are enantiomers; pairs that are not mirror images are diastereomers or epimers.[7] Biological systems are stereoselective, preferring certain stereoisomers for chemical reactions; living organisms keep all their amino acids in their "left-handed" (L or levo-) forms (a state called homochirality) because they are unable to use the "right-handed" (D or dextro-) forms for protein synthesis.[6] This ratio of D and L forms is unstable, as pairs of stereoisomers spontaenously convert between each other.[7][3]

When an organism becomes unable to keep its amino acids in that unbalanced ratio, such as by dying or shedding tissue, the system will proceed towards chemical equilibrium. Measuring the progress of this interconversion reaction (known as racemization or epimerization respective to the type of stereoisomer pair involved)[8] allows estimation of an organism's time of death, if environmental variables like moisture and temperature are accounted for.[6][2][3][9]

### Amino acids and environmental conditions

Amino acids commonly used for amino acid dating analysis are leucine, aspartic acid, valine, glutamic acid, and diastereoisomer isoleucine.[6]

The properties of the amino acid(s) chosen for analysis influence what kind of dating can be performed. Amino acid interconversion reactions happen at a variety of speeds: aspartic acid racemizes very quickly and hence is used for recent samples where high resolution is important,[6] while valine and leucine take much longer to racemize and are more appropriate for older fossils. Additionally, these reaction rates are sensitive to temperature, to a degree depending on the specific interconversion reaction. The racemization rate of aspartic acid varies with small changes in temperature, while valine's racemization rate is less temperature dependent.[2][6][9]

Besides higher temperatures accelerating interconversion, other environmental variables also impact reaction rates. Wetter environments produce faster reaction rates,[3] and interconversion reactions may be catalyzed by the presence of acids, bases, or metal cations.[10] The chosen host organisms or taxa also introduce bias into age estimates.[11]

Amino acids which are bound within peptides interconvert more slowly than those which are free or are occupying the terminal position of peptide chains. The degree of hydrolysis of peptides increases with fossil age and is considered for the purposes of amino acid dating a non-reversible reaction, unlike the interconversion of amino acid stereoisomers.[2][10]

## 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,[12] conservation paleobiology,[13] taphonomy and time-averaging,[14][15][11]sea level determinations, and thermal history reconstructions.[16][17][18][19]

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 be helpful.[4][9] Verification of radiocarbon and other dating techniques by comparison with amino acid dating is also possible.[20] 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]

Forensic science can use this technique to estimate the age of a cadaver[21] 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.[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.[22]

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).[23][24]

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.[25][6]

## References

1. ↑ "Amino Acid Racemization Dating of Fossil Bones". Annual Review of Earth and Planetary Sciences 13: 241–268. 1985. doi:10.1146/annurev.ea.13.050185.001325. Bibcode: 1985AREPS..13..241B.
2. ↑ 2.0 2.1 2.2 2.3 "Kinetics of amino acid racemization (epimerization) in the dentine of fossil and modern bear teeth". International Journal of Chemical Kinetics 35 (11): 576–591. 2003. doi:10.1002/kin.10153.
3. ↑ 3.0 3.1 3.2 3.3 "Amino acid racemization on Mars: implications for the preservation of biomolecules from an extinct martian biota". Icarus 114 (1): 139–143. 1995. doi:10.1006/icar.1995.1049. PMID 11539479. Bibcode: 1995Icar..114..139B.
4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 "Archaeological Applications of Amino Acid Racemization". Archaeometry 39 (2): 265–287. 1997. doi:10.1111/j.1475-4754.1997.tb00806.x. Bibcode: 1997Archa..39..265J.
5. ↑ "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. October 2008. http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_147285.htm. "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. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Murray-Wallace, Colin V.; Hidayat, Rahmadi; Biribo, Naomi (22 October 2024). "APPLICATION OF AMINO ACID RACEMIZATION TO THE DATING OF FORAMINIFERS AND THE IDENTIFICATION OF REMANIÉ FOSSILS AND REWORKED SEDIMENTS". Journal of Foraminiferal Research 54 (4): 375–393. doi:10.61551/gsjfr.54.4.375. Bibcode: 2024JForR..54..375M. https://pubs.geoscienceworld.org/cushmanfoundation/jfr/article/54/4/375/649344/APPLICATION-OF-AMINO-ACID-RACEMIZATION-TO-THE. Retrieved 25 August 2025.
7. ↑ 7.0 7.1 Penkman, Kirsty (12 August 2016). Encyclopedia of Geoarchaeology. Encyclopedia of Earth Sciences Series. Dordrecht Springer. pp. 14-15. https://link.springer.com/rwe/10.1007/978-1-4020-4409-0_38.
8. ↑ "The precision of amino acid geochronology and paleothermometry". Quaternary Science Reviews 6 (1): 43–54. 1987. doi:10.1016/0277-3791(87)90016-3. Bibcode: 1987QSRv....6...43M.
9. ↑ 9.0 9.1 9.2 "Method". Northern Arizona University. http://nau.edu/CEFNS/NatSci/SESES/AAGL/Method/.
10. ↑ 10.0 10.1 Baum, Rocky; Smith, Grant G. (November 1986). "Systematic pH study on the acid- and base-catalyzed racemization of free amino acids to determine the six constants, one for each of the three ionic species". Journal of the American Chemical Society (108): 7325-7327. doi:10.1021/ja00283a030.
11. ↑ 11.0 11.1 "Taphonomic bias and time-averaging in tropical molluscan death assemblages: Differential shell half-lives in Great Barrier Reef sediment". Paleobiology 35 (4): 565–586. 2009. doi:10.1666/0094-8373-35.4.565. Bibcode: 2009Pbio...35..565K.
12. ↑ "Sediment mixing and stratigraphic disorder revealed by the age-structure of Tellina shells in Great Barrier Reef sediment". Geology 35 (9): 811–814. 2007. doi:10.1130/G23722A.1. Bibcode: 2007Geo....35..811K.
13. ↑ "Dead delta's former productivity: Two trillion shells at the mouth of the Colorado River". Geology 28 (12): 1059–1062. 2000. doi:10.1130/0091-7613(2000)28<1059:DDFPTT>2.0.CO;2. Bibcode: 2000Geo....28.1059K.
14. ↑ "Quantitative estimates of time-averaging in terebratulid brachiopod shell accumulations from a modern tropical shelf". Paleobiology 29 (3): 381–402. 2003. doi:10.1666/0094-8373(2003)029<0381:QEOTIT>2.0.CO;2. Bibcode: 2003Pbio...29..381C.
15. ↑ "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. 2005. doi:10.1130/G21607.1. Bibcode: 2005Geo....33..729K.
16. ↑ "The precision of amino acid geochronology and paleothermometry". Quaternary Science Reviews 6 (1): 43–54. 1987. doi:10.1016/0277-3791(87)90016-3. Bibcode: 1987QSRv....6...43M.
17. ↑ "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. 1996. doi:10.1130/0016-7606(1996)108<0892:AAEOLT>2.3.CO;2. Bibcode: 1996GSAB..108..892O.
18. ↑ "Low-latitude glacial cooling in the Southern Hemisphere from amino-acid racemization in emu eggshells". Nature 385 (6613): 241–244. 1997. doi:10.1038/385241a0. Bibcode: 1997Natur.385..241M.
19. ↑ "Amino acid paleothermometry of Quaternary ostracodes from the Bonneville Basin, Utah". Quaternary Science Reviews 22 (8–9): 899–914. 2003. doi:10.1016/S0277-3791(03)00006-4. Bibcode: 2003QSRv...22..899K.
20. ↑ "Amino acid geochemistry of fossil bones from the Rancho La Brea Asphalt Deposit, California". Quaternary Research 18 (2): 174–183. 1982. doi:10.1016/0033-5894(82)90068-0. Bibcode: 1982QuRes..18..174M.
21. ↑ "Application to forensic odontology of aspartic acid racemization in unerupted and supernumerary teeth". Journal of Dental Research 67 (10): 1319–1322. October 1988. doi:10.1177/00220345880670101501. PMID 3170888.
22. ↑ "NEaar: North East Amino Acid Racemization". University of York. http://www.york.ac.uk/palaeo/services/ne-aar/aar/.
23. ↑ "A new procedure for determining dl amino acid ratios in fossils using reverse phase liquid chromatography". Quaternary Science Reviews 17 (11): 987–1000. 1998. doi:10.1016/S0277-3791(97)00086-3. Bibcode: 1998QSRv...17..987K.
24. ↑ Perspectives in Amino Acid and Protein Geochemistry. New York: Oxford University Press. 2000. pp. 145–160.
25. ↑ "Closed-system behaviour of the intra-crystalline fraction of amino acids in mollusc shells". Quaternary Geochronology 3 (1–2): 2–25. February 2008. doi:10.1016/j.quageo.2007.07.001. PMID 19684879. Bibcode: 2008QuGeo...3....2P.

## External links

| The Wikibook Historical Geology has a page on the topic of: Amino acid dating

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### Active laboratories

* 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

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