THE INFLUENCE OF METHYL ROTOR DYNAMICS ON HYDROGEN RELAXATION NETWORKS - DERIVATION OF SPECTRAL DENSITIES IN MODEL-FREE FORM

Nuclear magnetic resonance spectral densities corresponding to three-site jump and continuous diffusion models for methyl rotation have been derived for methyl-non-methyl and methyl-methyl dipolar interactions. These spectral densities have been cast in model-free form such that they may be used to make direct contact with experimentally accessible measurements. It is shown that the calculation of an order parameter S2 and an average [r-6] required for the spectral density functions, can be efficiently performed for any arbitrary geometry and could easily be included in a comprehensive relaxation matrix-based refinement of molecular models. Comparisons are presented which detail the range of variation in these parameters as a function of geometry for the two models considered. Simulations of the nuclear Overhauser effect for a protein using a full relaxation matrix treatment emphasize the need for the rigorous treatment of methyl rotor dynamics, as opposed to the commonly used rigid rotor or pseudoatom approximations.