Interaction of alcohols with [val5]angiotensin in alcohol-water mixtures

Intermolecular solvent-solute NOE experiments have been used to probe interactions of various alcohols with the peptide hormone [val(5)]angiotensin II at 0 degrees C. It is found that these NOEs are detectable but dependent on the kind of alcohol present and the conformation of the peptide. Solvent-solute NOEs in 100% methanol and 89% methanol-water are basically those predicted by a hard sphere model for intermolecular spin dipole interactions. NOEs at the peptide backbone (N-H, C alpha-H) protons in 25% methanol-water and 36% ethylene glycol-water mixtures indicate that alcohol interactions near these groups are also adequately described by this model. However, in 35% ethanol-water, interactions of alcohol methyl protons with the peptide result in unexpectedly negative NOEs, probably signaling that peptide-alcohol interactions in this solvent take place on a significantly slower time scale than that defined by mutual diffusion of these species. Some side chain-alcohol interactions result in NOEs up to 8 times larger than expected. Possible reasons for these enhanced effects are discussed.