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

Abstract 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 1 H‐nmr studies of selected linear oligopeptides in H 2 O 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.