An investigation of the mechanisms of nitroxide-induced proton relaxation enhancements in biopolymers

An investigation into the modes of interaction and mechanisms of proton relaxation enhancement of the ternary system dimethyl sulfoxide-nitroxide-gramicidin S has been performed. The data are consistent with previous studies of binary solvent-nitroxide systems. Both random collisional complexes involving van der Waals nitroxyl electron-proton contacts and hydrogen-bonded nitroxyl group-amide hydrogen interactions are important: they rationalize how the conformation of the peptide governs the relative proton relaxation rate enhancements and provide a mechanism that allows electron dipole-nuclear dipole relaxation. Although a fully quantitative account of the relative proton relaxation enhancements is too complex, this qualitative interpretation gives further justification for the use of the nitroxide perturbation method for delineating exposed vs. shielded protons in molecules and hence as an alternative approach to solution conformational analysis. © 1984 American Chemical Society.