Amino acid racemisation dating
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For example, a shoe is chiral: you cannot turn a left-foot shoe into a right-foot shoe by turning it round or flipping it over.
On the other hand, an object such as a table-knife is not chiral: if you have it lying on the table so that the blunt edge is on the right and the serrated edge is on the left, then you can produce the mirror-image of this situation by rotating the knife around its long axis. For example, consider the two molecules in the picture to the right.
Uncertainty as to the extent to which modern organisms represent in detail the characteristics of their ancient counterparts introduces additional lack of precision in a fossil age based on amino acid ratios.
Amino acids have been reported from fossils distributed throughout the geologic column (Florkin 1969).
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.
It gives the reader some idea of the difficulties of the method that they were obliged to use the single common foram species N.
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 survival of amino acids in fossils from the Paleozoic era and the trend for the apparent racemization rate constant to decrease with conventional fossil age assignment raise a serious question concerning the accuracy with which radioisotope age data have been used to represent the real-time history of fossils.
The instability of the twenty amino acids which are the building blocks of proteins provides a possible means for determining the ages of fossils.