Nuclear magnetic resonance studies of linear, cyclic and polymeric peptides containing His, Ser and Asp residues spaced by ß alanylglycine

Abstract

1H and 13C nuclear magnetic resonance studies have been carried out on the linear, cyclic and polymeric peptides having a sequence of ‐Asp‐ßAla‐Gly‐Ser‐ßAla‐Gly‐His‐ßAla‐Gly‐. From the 1 H spectra measured at 400 MHz, the rotamer populations about side chain Cα‐Cß bond for Asp, Ser and His residues were estimated at pD 5 and 8. For Asp and Ser residues, the rotamers T (trans), G and G' (gauche) had an equivalent population at both pDs, while for His residue the rotamer G' had a higher probability than the others. 13C chemical shift of His and Asp residues measured at a frequency 15 MHz showed a considerable change in the pD ranges 3 ˜ 5 and 5 ˜ 8, respectively, owing to the ionizable side chains. The pK value for His residue, i.e. the midpoint of the chemical shift change, corresponded fairly well to that obtained from ester hydrolytic activity measurements, supporting the previous suggestion that the ester hydrolytic reaction is mainly controlled with the imidazole group of His residue. 13C spin‐lattice relaxation time T1 was measured in order to investigate the molecular motion. It was found that the backbone was composed of relatively movable parts, ‐ßAla‐Gly‐, and less movable parts, Asp‐ Ser and His. Thisphenomenon was especially remarkable at the higher pD. The internal motion in the side chain was also investigated using the relaxation theory for the molecules undergoing the internal rotation. These results of relaxation measurements were compared with the results of conformational studies by circular dichroism measurements. The polymeric peptide composed only of ‐His‐ßAla‐Gly‐ sequence was also measured as a related peptide.