Ancient Ostrich Eggs: Dating materials by amino acid racemization

c. Michelle M. Francl, 2004

Full module available as PDF from the author at mfrancl@brynmawr.edu

keywords: chemical kinetics • biochemistry • archeology • proteins

Primary Literature Reference: This module is based on Science 1990, 248, 60-64. "Dating Pleistocene Archeological Sites by Protein Diagenesis in Ostrich Eggshell " by A.S. Brooks, P.E. Hare, J.E. Kokis, G.H. Miller, R.D. Ernst, F. Wendorf.

Background:

How many millions of years ago were these sediments deposited? Were the colors in this folio applied when it was printed, or sometime later? Is this a modern hippo bone, or a fossil of some similar, earlier species? Just how old is it? The determination of the age of an artifact or sample is a crucial question in many fields, including geology, paleontology, archaeology, anthropology and history. Context often provides a clue to the age of an object. For example, the style of decoration of a vase or the types of pigments applied to a fresco can be used to narrow the time period in which a particular artifact was produced. How deeply something is buried is yet another source of information about an itemÕs age. Provenance, or the written record of an objectÕs history, is another source from which the age of an object can be determined. These approaches obviously cannot be applied to find an age for every object or specimen. Chronometric methods, such as the familiar C-14 dating, can be brought to bear when the relative dates derived from contextual information are inadequate, or when forgery is suspected.

Amino acids are nearly always found in the L-form in natural products, such as proteins. Uncatalyzed epimerization at the chiral carbon is, not surprisingly, an exceedingly slow process. If proteins can be extracted from a biological matrix, such as an ostrich shell, the amount of epimerization can be used to determine the time elapsed since the protein was synthesized. Since the process is far slower than the decay of C-14, longer elapsed times can be measured.

Sample Critical Reading Questions:

• In an early paper describing this method [Science, 1970,170, 730-732], there is a note that at equilibrium a small excess of D-alloisoleucine is present (at 140^o,C they report the ratio of alloisoleucine to isoleucine is 1.25), but that for most amino acids the equilibrium constant between the L and D forms is 1. Why should that be? Why do you suppose it is different for isoleucine?

Sample Problem:

• An early version of this method [Bada et al. Science, 1970,170, 730-732] ignored the reverse reaction in computing the A/I ratio. Derive an expression for the A/I ratio as a function of time assuming you can ignore the reverse reaction. A note in the early paper suggests that this approximation is good until the racemization exceeds 15%. Compare your expression with the one you derived in question 8. Do you agree with Bada et al.? How does the approximation change the age range that can be dated using this technique? Could the authors of the paper on ostrich eggs have used the approximate approach and still obtained accurate dates? Support your answer with examples.

Culture of Chemistry Feature:

How old is a whale?

Humans are among the longest-living mammals, with a life-span on the order of 100 years. Human ages can be verified by consulting birth and census records. Whales are also apparently very long-lived, but discovering the age of a whale is a somewhat more daunting task and the life-spans of most whale species have not been established. The ages of some whales, such as blue whales and fin whales can be determined by counting the layers of ear wax in their inner ears. These whales appear to live between 80 and 100 years, similar to humans. Bowhead whales, which live north of the Artic circle, are still hunted in limited numbers by the Inuit. Since the early 1980s, several clearly ancient harpoon heads have been found embedded in modern bowheads. The types of points recovered had not been used since the late 19th century, suggesting the whales were more than 100 years old. Inuit oral histories also supported a long life span for these whales, as multiple generations of hunters described encountering the same whale. The degree of racemization of aspartic acid in the lens of the eye has been used to find the ages in 20 different species, including humans. Ages were obtained for 48 different whales, one which had an apparent age of 211 years [Can. J. Zool. 1999, 77, 571Š580], making the bowheads the longest living mammals known!