Determination of pK values of peptide groups in dipeptides from nuclear magnetic resonance kinetic studies

Abstract

The rate constants of the base catalysed exchange reaction, k_I, of dipeptides RCONHCH₂CO₂^– with various halogen substituents on R were determined form n.m.r. line shape analysis of the methylene group of the glycyl residues. Changes in the values of k_I of the majority of the peptides were correlated successfully with proper substituent constants, determined from the dissociation constants of the corresponding carboxylic acids. By applying the Eigen scheme for proton transfer reactions we could explain the results obtained for all the peptides. Furthermore, it was possible to calculate absolute pK values of peptide groups from a knowledge of their k_I values. Measurements of the chemical shifts of the peptide hydrogens indicate the existence of a correlation between the chemical shifts of the peptide hydrogens and their relative acidity.