Specificity of interproton nuclear Overhauser effects in gramicidin-S dissolved in deuterated ethylene glycol

The 250-MHz high-resolution proton magnetic resonance spectra of gramicidin-S in solution in deuterated methanol, deuterated ethylene glycol, and binary mixtures of these solvents have been recorded. Starting from previously published partial assignments for deuterated methanol solution, the solvent transition yields partial assignments in deuterated ethylene glycol solution. In the latter the rotational correlation time for the peptide backbone, tauc, is calculated to be 14 ns at 25 degrees C. The long tauc leads to proton spin relaxation behavior that mimics that of moderate-sized proteins in water, and yields negative nuclear Overhauser effects, which have been measured for the protons of the phenylalanine ring. The results suggest that there is rapid and efficient spin-diffusion within closely-connected "islands" of protons, and less efficient spin-diffusion between islands. The results are compatible with the accepted solution conformation of gramicidin-S.