Interactions of Nonprotic Organic Solvents with [Val5]angiotensin in Water

Intermolecular solvent−solute nuclear Overhauser effects have been used to explore interactions of the organic component of acetonitrile−water, acetone−water, and dimethyl sulfoxide−water mixtures with the peptide hormone [val5]angiotensin. As reported by the NOEs, many cross relaxation terms for interactions of these organic cosolvents are adequately accounted for using a hard spheres interaction model in which encounters of peptide and cosolvent molecules take place by mutual diffusion. However, there are indications of localized solvent−peptide interactions that are not well described by this model. In dimethyl sulfoxide−water at 0 °C, organic solvent near the C-terminal Phe8 residue and the Val3 residue produce strongly enhanced cross-relaxation terms. NOEs for all peptide N−H protons and the protons of the Tyr4 aromatic ring were significantly more positive than expected in 33% acetone−water (v/v) at 0 °C, while those for most side-chain protons were close to predictions of the hard sphere model. All peptide−organic solvent NOEs in 35% acetonitrile water (v/v) at 0 °C are consistent with the hard spheres interaction model.