Ubiquitin Backbone Motion Studied via NHN−C'Cα Dipolar−Dipolar and C'−C'Cα/NHN CSA−Dipolar Cross-Correlated Relaxation
Journal of the American Chemical Society
While it is recognized that protein side chains undergo large reorientations, very little is known about the extent and the nature of the motions within the protein backbone. These motions can be studied by chemical shift anisotropy (CSA)−dipolar cross-correlated relaxation rates, the interpretation of which relies on prior knowledge of the CSA principal axis values and directions. Alternatively, dipolar−dipolar cross-correlated relaxation rates can be used. Here we propose a CT-HNCO experiment to measure the sum of the two C'Cα/NCα dipolar−NHN/C'HN dipolar cross-correlated relaxation rates. At the same time the experiment gives access to the cross-correlated relaxation rates between C' CSA and the NHN or C'Cα dipoles. The complete set of three cross-correlated relaxation rates, measured for the protein ubiquitin, is interpreted in terms of the Gaussian axial fluctuation model of motion. This model provides a good framework for the description of the motions of peptide planes in α-helical and turn regions, while a poor fitting of the cross-correlated relaxation data is obtained for β-sheet regions. The cross-correlated relaxation rates for the α-helix peptide planes 23−34 are similar, and their characteristic values suggest the possibility of a concerted motion of the helix or systematic changes in the carbonyl CSA principal axis values and directions.