Concordance of Collagen-Based Radiocarbon and Aspartic-Acid Racemization Ages

Abstract

By determining the extent of racemization of aspartic acid in a well-dated bone, it is possible to calculate the in situ first-order rate constant for the interconversion of the L and D enantiomers of aspartic acid. Collagen-based radiocarbon-dated bones are shown to be suitable samples for use in "calibrating" the racemization reaction. Once the aspartic-acid racemization reaction has been "calibrated" for a site, the reaction can be used to date other bones from the deposit. Ages deduced by this method are in good agreement with radiocarbon ages. These results provide evidence that the aspartic-acid racemization reaction is an important chronological tool for dating bones either too old or too small for radiocarbon dating. As an example of the potential application of the technique for dating fossil man, a piece of Rhodesian Man from Broken Hill, Zambia, was analyzed and tentatively assigned an age of about 110,000 years.The amino acids commonly found in living organisms consist mainly of the L-isomers. However, fossil materials have been found to contain D-amino acids, and the proportion of Damino acids to L-amino acids increases with the age of the fossil (1-9). The reaction responsible for this conversion is termed racemization. Each amino acid (with the exception of glycine) undergoes this process, some much faster than others. For example, in bone the racemization half-life (time required for D: L ratio to reach 0.333) at 200 for aspartic acid is about 15,000 years (9); at the same temperature the reaction for isoleucine has a half-life in excess of 100,000 years (8).The amino-acid racemization reaction has important applications in anthropology and geochronology. Recent studies have shown that the reaction can be used to date deepsea sediments (4-6) and fossil bones (7-9). Because of the much slower reaction rate, the racemization reaction can be used to date fossil materials too old for radiocarbon dating. Unfortunately, one limitation of this method is that the racemization reaction is temperature dependent. Thus, in order to date materials using the degree of racemization, some estimate of the temperature history of the region where the fossil was found must be available.Recently it has been shown (9) that by determining the extent of racemization of aspartic acid in a bone which has been dated by radiocarbon it is possible to calculate the in situ first-order rate constant for interconversion of the L-isomers and D-isomers of aspartic acid. Once this "calibration" has been carried out for a site, the racemization reaction can be used to date other bones from the area which are either too old or too small for radiocarbon dating. This "calibration" procedure eliminates the need for evaluating the temperature history of a bone before it can be dated using the amino-acid racemization reaction. The only assumption required, in using this approach, is that the average temperature experienced by the "calibration" sample is representative of the average temperature experienced by other samples from t...

Marked longevity of human lung parenchymal elastic fibers deduced from prevalence of D-aspartate and nuclear weapons-related radiocarbon.

“…Racemization rates depend greatly upon temperature and the R-group of the amino acid, and are additionally influenced (but much less strongly) by pH and by ill-defined "local environmental effects" within the polypeptide (14). Content of D-amino acids in proteins has been correlated with the age of various archaeologic and paleontologic specimens (25)(26)(27)(28)(29)(30)(31)(32)(33). Racemization of aspartic acid occurs rapidly relative to that of the other amino acids.…”

Considerations on the Role of Aspartic Acid Racemization in the Aging Process

“…Once the organism dies, the spontaneous chemical reaction of racemiza-tion converts these ¿-amino acids into their ¿»-enantiomers at rates which are primarly proportional to temperature. Due to the temperature dependence of the racemization reaction {Schroeder and Bada, 1974;Bada and Helfman, 1975) and to the fact that aspartic acid has one of the fastest racemization rates Bada, 1972), we expected to observe small amounts of ¿»-aspartic acid accumulating in structural proteins which are not turned over during the lifetime of a warm-blooded animal.…”

“…The extent of racemization of the biological ¿-amino acids to their D-enantiomers in organic materials has been used to estimate the ages of archaeological and geological deposits {Bada and Protsch, 1973;Bada et al, 1974;Bada and Helfman, 1975;Lee et a l, 1976). We have recently demonstrated the applica bility of this dating method to 'biochronological' problems, i.e., estimating the ages of living mammals and their tissues {Helfman and Bada, 1975.…”

Aspartic acid racemization in tooth enamel from living humans.

“…The extent of racemization of amino acids may be used to estimate the ages of various fossil materials, including deepsea sediments (5)(6)(7), shells (8)(9)(10)(11), fossil bones (2)(3)(4)(12)(13)(14)(15), and coprolites (16). Of the various amino-acid racemization reactions, the one involving aspartic acid has received the most attention.…”