Use of amide 1H-nmr titration shifts for studies of polypeptide conformation

This paper shows that backbone amide proton titration shifts in polypeptide chains are a very sensitive manifestation of intramolecular hydrogen bonding between carboxylate groups and backbone amide protons. The population of specific hydrogen-bonded structures in the ensemble of species that constitutes the conformation of a flexible nonglobular linear peptide can be determined from the extent of the titration shifts. As an illustration, an investigation of the molecular conformation of the linear peptide H-Gly-Gly-L -Glu-L -Ala-OH is described. The proposed use of amide proton titration shifts for investigating polypeptide conformation is based on 360-MHz ¹H-nmr studies of selected linear oligopeptides in H₂O solutions. It was found that only a very limited number of amide protons in a polypeptide chain show sizable intrinsic intration shifts arising from through-bond interactions with ionizable groups. These are the amide proton of the C-terminal amino acid residue, the amide protons of Asp and the residues following Asp, and possibly the amide proton of the residue next to the N-terminus. Since the intrinsic titration shifts are upfield, the downfield titration shifts arising from conformation-dependent through-space interactions, in particular hydrogen bonding between the amide protons and carboxylate groups, can readily be identified.