F NMR relaxation of buried tryptophan side chains suggest anisotropic rotational diffusion of the protein RfaH.

The recent application of F NMR in the study of biomolecular structure and dynamics has made it a potentially attractive probe to complement traditional N/C labelled probes for backbone and sidechain dynamics, albeit with some complications. The utility of N relaxation rates of rigid backbone amide groups to determine the rotational diffusion tensor of proteins is well established. Here we show that the measured F relaxation rates of two buried and possibly immobile F labelled tryptophan sidechains for the multidomain protein RfaH, in its closed conformation, are in reasonable agreement with the calculated values, only when anisotropic rotational diffusion of the protein is considered. While the sparsity of F relaxation data from a limited number of probes precludes the experimental determination of the rotational diffusion tensor here, these results demonstrate the influence of rotational diffusion anisotropy of proteins on F NMR relaxation of rigid tryptophan sidechains, while adding to the expanding literature of F NMR relaxation data sets in biomolecules.