Dynamic processes in aqueous protein systems. Molecular theory and NMR relaxation

A model theory is presented explaining a series of striking phenomena observed with nuclear magnetic relaxation in protein systems such as solutions or tissue. The frequency and concentration dependences of the nuclear magnetic relaxation times are discussed. It is concluded that the translational diffusion of water molecules along the rugged surfaces of proteins and ‐ to a minor degree ‐ protein backbone fluctuations are crucial processes. The rate limiting factor of macromolecular tumbling is assumed to be given by the free‐water content in a certain analogy to the free‐volume model of Cohen and Turnbull. The two characteristic water mass fractions indicating the saturation of the hydration shells and the onset of protein tumbling can be related to each other on this basis. A closed and relatively simple set of relaxation formulas is presented. The potentially fractal nature of the diffusion of water molecules on the protein surface is discussed.