Isoleucine epimerization in peptides and proteins: kinetic factors and application to fossil proteins.

The observed rate of isoleucine epimerization in peptides and proteins is dependent on (in addition to time and temperature) (i) the position of isoleucine in the peptide chain, (ii) the nature of adjacent amino acids, and (iii) the stability of the isoleucine peptide bonds. The relative rate is: NH(2)-terminal > COOH-terminal >> interior >/= free amino acid. The gradual hydrolysis of peptides and proteins to the more slowly epimerizing free amino acid causes a decrease in the apparent first-order rate constant with time. These results explain the isoleucine kinetics observed in fossil shells.