Assessment of protein reorientational diffusion in solution by 13C off-resonance rotating frame spin-lattice relaxation: effect of anisotropic tumbling.

The 13C off-resonance rotating frame spin-lattice relaxation technique is applicable to the study of protein rotational diffusion behavior in a variety of experimental situations. The original formalism of James and co-workers (1978) (J. Amer. Chem. Soc. 100, 3590-3594) was constrained by the assumption of random isotropic reorientational motion. Here we include in the formalism anisotropic tumbling, and present the results of computer simulations illustrating the differences between anisotropic and isotropic reorientational motion for the off-resonance rotating frame spin-lattice relaxation experiment. In addition, we have included chemical shift anisotropy of the peptide carbonyl carbon as an additional relaxation mechanism contribution, to permit high-field nmr protein rotational diffusion measurements.