Comparison of isoleucine epimerization in a model dipeptide and fossil protein

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.