Comparison of amino acid racemization geochronometry with lithostratigraphy, biostratigraphy, uranium-series coral dating, and magnetostratigraphy in the Atlantic Coastal Plain of the southeastern United States     

L. McCartan  J. P. Owens  B. W. Blackwelder  B. J. Szabo  D. F. Belknap  N. Kriausakul  R. M. Mitterer  J. F. Wehmiller 《Quaternary Research》1982,18(3):337-359

The results of an integrated study comprising litho- and biostratigraphic investigations, uranium-series coral dating, amino acid racemization in molluscs, and paleomagnetic measurements are compared to ascertain relative and absolute ages of Pleistocene deposits of the Atlantic Coastal Plain in North and South Carolina. Four depositional events are inferred for South Carolina and two for North Carolina by all methods. The data suggest that there are four Pleistocene units containing corals that have been dated at about 100,000 yr, 200,000 yr, 450,000 yr, and over 1,000,000 yr. Some conflicts exist between the different methods regarding the correlation of the younger of these depositional events between Charleston and Myrtle Beach. Lack of good uranium-series dates for the younger material at Myrtle Beach makes the correlation with the deposits at Charleston more difficult.  相似文献   

Comparison of isoleucine epimerization in a model dipeptide and fossil protein     

Nivat Kriausakul  Richard M. Mitterer 《Geochimica et cosmochimica acta》1980,44(5):753-758

The dipeptide isoleucyl-glycine in aqueous solution has activation energies for isoleucine epimerization and hydrolysis of 23.1 and 20.9 kcal mol^?1, respectively. The activation energy for epimerization of NH₂-terminal isoleucine is 5–8 kcal mol^?1 less than activation energies reported for epimerization of isoleucine in the free state or in various protein systems. Furthermore, the rate of isoleucine epimerization in the NH₂-terminal position exceeds the rate of hydrolysis at temperatures between 0 and 152°C. Consequently, a high rate of epimerization in fossils should only occur when isoleucine is in the terminal position. Partially epimerized isoleucine will be converted through hydrolysis from the terminal form to the slower epimerizing free isoleucine. Over the course of the total isoleucine epimerization reaction, either in dipeptides or proteins, the activation energy will increase and the reaction rate will decrease as terminal isoleucine is converted to free isoleucine. This process may explain the non-linear kinetics observed for isoleucine epimerization in carbonate fossils.The degree of isoleucine epimerization is low in different molecular weight fractions of a fossil protein from Mercenaria in all fractions greater than 500 mol. wt, where most isoleucine is likely to be in the interior position. In the less than 500 mol. wt peptide fraction, where a considerable portion of the isoleucine is likely to be in the terminal position, the degree of epimerization is significantly greater. These analytical results from fossil protein support the interpretation of isoleucine kinetics obtained by study of dipeptides.  相似文献   

Epimerization of COOH-terminal isoleucine in fossil dipeptides     

Nivat Kriausakul  Richard M. Mitterer 《Geochimica et cosmochimica acta》1983,47(5):963-966

Using procedures employed for protein sequencing to remove NH₂-terminal amino acids, the degree of epimerization of COOH-terminal isoleucine in dipeptides from fossil mollusc shells has been measured directly. The results show that isoleucine in this position is highly epimerized in fossil dipeptides and that the COOH-terminal isoleucine is more highly epimerized than the free amino acid. Hydrolysis of the highly epimerized terminal isoleucine leads to the high alloisoleucine/isoleucine values found in the free amino acid fraction. The results also provide evidence for the formation of diketopiperazines in fossils as a mechanism to account for the high degree of epimerization of COOH-terminal isoleucine.  相似文献   

Comparison of rates and degrees of isoleucine epimerization in dipeptides and tripeptides     

Richard M. Mitterer  Nivat Kriausakul 《Organic Geochemistry》1984,7(1):91-98

The degrees and rates of isoleucine epimerization in heating experiments with pure di- and tripeptides are dependent on: (1) the position of isoleucine in the peptide chain; (2) the formation from dipeptides of diketopiperazines, cyclic intermediates that are in equilbrium with, and hydrolyze to, both the parent and inverted dipeptides as follows: NH₂-terminal?diketopiperazine?COOH-terminal. The results of heating experiments show that isoleucine residues in diketopiperazine are the most highly epimerized form in both di- and tripeptide solutions. NH₂-terminal isoleucine also undergoes relatively rapid epimerization, while the rates of epimerization of COOH-terminal and free isoleucine are much slower. Through hydrolysis reactions, high degrees of isoleucine epimerization are transferred to slower epimerizing species. Consequently, the relative rate of isoleucine epimerization in the various positions differs from the relative degree of epimerization.The relative rates of isoleucine epimerization are: NH₂ ? diketopiperazine ? COOH ≈ interior ≈ free, while the relative degrees of isoleucine epimerization are: diketopiperazine > NH₂ > COOH ? interior. The high degree of epimerization exhibited by COOH-terminal isoleucine in fossil dipeptides is due to prior preferential epimerization in diketopiperazine. The high degree of epimerization of free isoleucine in carbonate fossils is due to the hydrolysis of highly epimerized terminal isoleucine. These results are applicable to peptides both in fossil shells and in aqueous solutions and form the basis for a revised model of isoleucine epimerization in fossil shells.  相似文献