Dipolar couplings in multiple alignments suggest α helical motion in ubiquitin

Recently, residual dipolar couplings (RDCs) of backbone N-HN vectors measured in 11 different alignment media were analyzed with respect to structure and dynamics in a model-free way in terms of generalized order parameters and motional anisotropies. The anisotropies in the central α-helix were found to be strikingly uniformly distributed. In this communication, these parameters are further interpreted in terms of physically feasible cooperative reorientational motion of the helix with respect to the core of the protein. The RDCs are compatible with a model in which all N-HN vectors of the α-helix of ubiquitin exhibit correlated anisotropic excursions with amplitudes of 21° and 12° along two orthogonal directions x and y of a coordinate system C, if z represents the helix axis. Such motion contradicts neither NOE data nor molecular force-field calculations. Copyright © 2003 American Chemical Society.