The backbone dynamics and overall tumbling of protein
G have been investigated using 15N relaxation.
Comparison of measured R2/R1
relaxation rate ratios with known three-dimensional coordinates
of the protein show that the rotational diffusion tensor
is significantly asymmetric, exhibiting a prolate axial
symmetry. Extensive Monte Carlo simulations have been used
to estimate the uncertainty due to experimental error in
the relaxation rates to be D∥/D⊥
= 1.68 ± 0.08, while the dispersion in the NMR ensemble
leads to a variation of D∥/D⊥
= 1.65 ± 0.03. Incorporation of this tensorial description
into a Lipari–Szabo type analysis of internal motion
has allowed us to accurately describe the local dynamics
of the molecule. This analysis differs from an earlier
study where the overall rotational diffusion was described
by a spherical top. In this previous analysis, exchange
parameters were fitted to many of the residues in the alpha
helix. This was interpreted as reflecting a small motion
of the alpha helix with respect to the beta sheet. We propose
that the differential relaxation properties of this helix
compared to the beta sheet are due to the near-orthogonality
of the NH vectors in the two structural motifs with respect
to the unique axis of the diffusion tensor. Our analysis
shows that when anisotropic rotational diffusion is taken
into account NH vectors in these structural motifs appear
to be equally rigid. This study underlines the importance
of a correct description of the rotational diffusion tensor
if internal motion is to be accurately investigated.
