Anisotropic Molecular Rotational Diffusion in 15 N Spin Relaxation Studies of Protein Mobility † (original) (raw)

The backbone dynamics of the uniformly 15 N-labeled N-terminal 63-residue DNA-binding domain of the 434 repressor has been characterized by measurements of the individual 15 N longitudinal relaxation times, T 1 , transverse relaxation times, T 2 , and heteronuclear 15 N{ 1 H}-NOEs at 1 H resonance frequencies of 400 and 750 MHz. The dependence of an apparent spherical top correlation time, τ R , on the orientation of the N-H bond vector with respect to the principal axes of the global diffusion tensor of the protein was used to establish the fact that the degree of anisotropy of the global molecular tumbling amounts to 1.2, which is in good agreement with the values obtained from model calculations of the hydrodynamic properties. A model-free analysis showed that even this small anisotropy leads to the implication of artifactual slow internal motions for at least two residues when the assumption of isotropic global motion is used. Additional residues may actually undergo internal motions on the same time scale as the global rotational diffusion, in which case the model-free approach would, however, be inappropriate for quantifying the correlation times and order parameters. Overall, the experiments with 434(1-63) demonstrate that the assumption of isotropic rotational reorientation may result in artifacts of model-free interpretations of spin relaxation data even for proteins with small deviations from spherical shape. † Financial support was obtained from the Schweizerischer Nationalfonds (Project 31.32033.91). X Abstract published in AdVance ACS Abstracts, May 15, 1997. 1 Abbreviations: NMR, nuclear magnetic resonance; 2D, twodimensional; 434(1-63), N-terminal 63-residue DNA-binding domain of the 434 repressor; 434(1-69), N-terminal 69-residue DNA-binding domain of the 434 repressor; CPMG, Carr-Purcell-Meiboom-Gill; SMF, simple model-free approach: EMF, extended model-free approach; MMF, minimal model-free approach.