ROTATIONAL DIFFUSION ANISOTROPY OF HUMAN UBIQUITIN FROM N-15 NMR RELAXATION

. Longitudinal and transverse N-15 NMR relaxation times in human ubiqu itin have been measured at 600-MHz H-1 frequency with a reproducibilit y of better than 1%. Two independent measurements of the N-15-{H-1} NO E indicate a random error of ca. 0.01, and no values were larger than the theoretical maximum. The relaxation data are incompatible with iso tropic rotational diffusion but agree well with an axially symmetric r otational diffusion tensor with a diffusion anisotropy, D-parallel to/ D-perpendicular to Of 1.17 There is no statistically significant furth er improvement in the fit between the experimental data and those pred icted by a fully asymmetric diffusion tensor, confirming that the rota tional diffusion tensor of ubiquitin is axially symmetric within exper imental uncertainty. The relative ratio of the principal components of the inertia tensor calculated from the X-ray structure is 1.00:0.90:0 .64, and the axis with the smallest inertia component makes an angle o f 11 degrees with the unique axis of the experimentally determined dif fusion tenser. Hydrodynamic calculations agree well with experimental results, provided half a shell of bound water is included and flexibil ity of the C-terminal residues is accounted for either by omitting the m from the calculations or by using conformations for these residues o btained from a Langevin dynamics simulation.