WATER SELF-DIFFUSION MODEL FOR PROTEIN-WATER NMR CROSS-RELAXATION

Analytical motional models are described for the interpretation of NMR cross relaxation between protein protons and water protons, treating bulk water as a self-diffusing continuum. Using intermolecular dipolar relaxation theory, scaling arguments, and numerical calculations, the distance dependence of the cross-relaxation rate constant between the protein proton and the first hydration shell is discussed for planar and spherical surface geometries. It is found that differential effect s between NOESY and ROESY rate constants depend strongly on geometrica l factors and do not directly reflect residence times of individual wa ter molecules on the protein surface. Restriction of water diffusion t o a coarse lattice leads qualitatively to the same NMR relaxation beha vior, but X-ray crystallography would display well-ordered water struc ture, illustrating the complementary views of the two techniques on pr otein hydration.