Hydrogen-1 and carbon-13 nuclear magnetic resonance conformational studies of the His-Pro peptide bond: conformational behavior of TRH.
Cis/trans isomerism of the His-Pro peptide bond provides a convenient model for the effect of a slow conformational change which may have wider biological significance. Above the imidazole pK, His-Pro is conformationally analogous to the (isosteric) peptide Phe-Pro. Protonation of the imidazole sidechain is associated with a large decrease in the cis/trans ratio. Detailed 1H and 13C n.m.r. analysis suggests the importance of electrostatic/hydrogen bonding interactions between the charged imidazolium sidechain and the proline carboxyl as the basis for this effect. In contrast to a previous report, cis/trans isomerism in TRH is shown to be related to titration of the imidazole sidechain, exhibiting a pK of 6.1.
D004151:Dipeptides; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D011487:Protein Conformation; D013973:Thyrotropin-Releasing Hormone
D004151:Dipeptides; C035699:histidylproline; D013973:Thyrotropin-Releasing Hormone