Comparison of H-2 and C-13 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 ubiquit in as a model system, the field dependence of carbon and deuterium relaxati on parameters has been measured and used to probe the utility of various fo rms of the model-free formalism in revealing the underlying dynamics. We fi nd that both approaches reveal the same overall dynamical features provided that suitable parametrization and model-free spectral densities are employ ed. It is found that the original and extended model-free formalisms yield different descriptions of the methyl group dynamics and that the extended v ersion is more appropriate for the analysis of carbon relaxation. Because o f the inherent differences in the types of information that H-2 and C-13 of fer, deuterium methods appear to provide robust access to methyl symmetry a xis order with the least amount of data, while carbon methods provide more robust access to model-free parameters defining the time scale of methyl ro tation and methyl symmetry axis motion.