Comparison of 2H and 13C NMR relaxation techniques for the study of protein methyl group dynamics in solution

A comparison of H-2- and C-13-based NMR relaxation methods to characterize the dynamics of methyl groups in proteins is presented. Using human ubiquitin as a model system, the field dependence of carbon and deuterium relaxation parameters has been measured and used to probe the utility of various forms of the model-free formalism in revealing the underlying dynamics. We find that both approaches reveal the same overall dynamical features provided that suitable parametrization and model-free spectral densities are employed. It is found that the original and extended model-free formalisms yield different descriptions of the methyl group dynamics and that the extended version is more appropriate for the analysis of carbon relaxation. Because of the inherent differences in the types of information that H-2 and C-13 offer, deuterium methods appear to provide robust access to methyl symmetry axis order with the least amount of data, while carbon methods provide more robust access to model-free parameters defining the time scale of methyl rotation and methyl symmetry axis motion.