19F NMR relaxation of buried tryptophan side chains suggest anisotropic rotational diffusion of the protein RfaH

The recent application of ¹⁹F NMR in the study of biomolecular structure and dynamics has made it a potentially attractive probe to complement traditional ¹⁵N/¹³C labelled probes for backbone and sidechain dynamics, albeit with some complications. The utility of ¹⁵N relaxation rates of rigid backbone amide groups to determine the rotational diffusion tensor of proteins is well established. Here we show that the measured ¹⁹F relaxation rates of two buried and possibly immobile ¹⁹F 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 ¹⁹F 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 ¹⁹F NMR relaxation of rigid tryptophan sidechains, while adding to the expanding literature of ¹⁹F NMR relaxation data sets in biomolecules.