Amino acid racemization dating method
They both have exactly the same chemical formula, but one is left-handed, and the other is right-handed. When we make chiral molecules using ordinary chemical processes, we usually produce equal quantities of both enantiomers. However, biological processes produce molecules with a distinct chirality: all the amino acids are "left-handed" (with the exception of glycine, which is not chiral) and all the sugars are "right-handed".So when an organism dies, its amino acids are left-handed.The proteins in each lens nucleus were acid hydrolyzed into free amino acids, which were then derivatized.The levels of D- and L-aspartic acid were analyzed by ion exchange chromatography or reversed-phase HPLC.Researchers turned to Jeffrey Bada and his colleagues at the Scripps Institution of Oceanography in La Jolla, CA, who are considered experts in the field of racemization analysis.By conducting aspartic acid racemization measurements using an HPLC/fluorescence detection method, Bada analyzed the bowheads eye lenses to estimate the whales age at the time of death (2).In this article we shall discuss the principles behind amino acid dating (also known as racemization dating); we shall discuss how it ought to work, and why it often doesn't.An object is said to have chirality if it is not possible to make it into a mirror-image of itself by turning it round.
Racemization is the process in which one enantiomer of a compound, such as an L-amino acid, converts to the other enantiomer.With these measurements, scientists can estimate the rate at which one enantiomer is converted to the other.Currently, these techniques are used to estimate the age of fossils, determine the life span of bowhead whales, and detect evidence of extraterrestrial life.(For it would be strange and anti-scientific to conjecture that the rate of racemization of the shells in the Arctic mud is constant whenever we can check it, but variable when we can't.) Just this was established by Kaufman et. (2008) in their paper Dating late Quaternary planktonic foraminifer Neogloboquadrina pachyderma from the Arctic Ocean by using amino acid racemization, Paleoceanography, 23(3).It gives the reader some idea of the difficulties of the method that they were obliged to use the single common foram species N.
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An older convention, commonly used by biochemists to describe amino acids and sugars, uses the letters D and L to designate absolute configuration (Figure 1).