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 [val 5 ]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 localiaed 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 dose 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.