T. Phil Pitner

Solvent exposure of specific nuclei of angiotensin II determined by NMR solvent saturation method

CONFORMATIONAL analysis of peptides in solution by nuclear magnetic resonance (NMR) spectroscopy generally involves determination of the relative exposure to solvent of specific NH hydrogens1–4. The four methods used so far are based on: (1) rates of NH proton exchange with labile hydrogens of the solvent5–6; (2) temperature dependence of chemical shifts of NH resonances7–8; (3) dependence of NH chemical shifts on the composition of a suitable solvent mixture3,9–11, and (4) degree of resonance broadening when a paramagnetic substance is added12. Each method has limitations. Proton exchange rates reflect not only exposure to solvent, but also proximity to functional groups of the peptide which catalyse exchange. Factors which determine the temperature dependence of NH chemical shifts are as yet poorly understood. Changes in solvent composition can alter the conformation of the peptide11. Paramagnetic ions may associate preferentially with the solvent or with specific sites on the peptide.

Mechanism of the intramolecular 1H nuclear Overhauser effect in peptides and depsipeptides.

Abstract 1 H NMR double resonance studies of valinomycin in (CD 3 ) 2 SO were conducted at 90 MHz (FT-mode) and 250 MHz (correlation mode) to determine the mechanism of intramolecular nuclear Overhauser effects (NOE). These studies set specific constraints on any model for the conformation of valinomycin in (CD 3 ) 2 SO and illustrate that NOE experiments of this type hold considerable promise for conformational studies of peptides, proteins and other biomolecules. The NOEs are positive at the lower frequency and negative at the higher frequency. Consideration of the theoretical dependence of the NOE on the proton-proton internuclear correlation time and on the resonance frequency indicates that these results are explained by a predominantly dipolar relaxation mechanism.