Long-range magnetic non-equivalence of methylene protons and pK's of amide bonds in peptides

Abstract

This research aims at showing that deciphering NMR patterns of peptide anisochronous methylene protons (A and B) coupled to an adjacent NH or CH(X) hydrogen may bring novel structural and dynamical information, using for this purpose four l-Histidine-containing peptides and the NMR patterns of Glycyl or β-Alanyl residues or of the Histidyl side-chain, or else of a grafted N-butyl chain. Investigations were carried out along the following lines: ionization constants of functional end groups and speciation curves of protonated species; specific anisochronies and coupling constants assigned to each state of protonation of each compound; base-catalyzed amide proton exchange rate and peptide pK's from DNMR of anisochronous protons. Vicinal coupling constants JAX and JBX were found (i) equal along the peptide backbone, suggesting either free rotation about the N–C bond, or, more probably, a torsion angle Φ=90°, and (ii) unequal in the Histidyl side-chain, suggesting an Imidazole-carboxylate interaction preventing free rotation. Peptide pK's span a wide range of values, from 13.63 to 17.48, depending on the parent residue and the characteristics of functional end groups.