ISOTOPE EFFECTS IN PEPTIDE GROUP HYDROGEN-EXCHANGE

Kinetic and equilibrium isotope effects in peptide group hydrogen exch ange reactions were evaluated. Unlike many other reactions, kinetic is otope effects in amide hydrogen exchange are small because exchange pa thways are not limited by bond-breaking steps. Rate constants for the acid-catalyzed exchange of peptide group NH, ND, and NT in H2O are ess entially identical, but a solvent isotope effect doubles the rate in D 2O. Rate constants for base-catalyzed exchange in H2O decrease slowly in the order NH>ND>NT. The alkaline rate constant in D2O is very close to that in H2O when account is taken of the glass electrode pH artifa ct and the difference in solvent ionization constant. Small equilibriu m isotope effects lead to an excess equilibrium accumulation of the he avier isotopes by the peptide group. Results obtained are expressed in terms of rate constants for the random coil polypeptide, poly-DL-alan ine, to provide reference rates for protein hydrogen exchange studies as described in Bai et al. [preceding paper in this issue]. (C) 1993 W iley-Liss, Inc.