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

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.