Residual motion of hemoglobin-bound spin labels and protein dynamics: viscosity dependence of the rotational correlation times.

The residual motion of spin labels bound to cysteine beta 93 and to lysines of methemoglobin has been studied by electron paramagnetic resonance spectroscopy. To separate the influences of the solvent and the protein environment of the label fluctuations, the correlation times, tau, were analyzed as a function of temperature for fixed solvent viscosities, eta. Results show that over a wide range of viscosity the dependence of tau on eta may be empirically described by a power law tau approximately eta kappa. The exponent kappa depends strongly on the location of the label on the protein surface. If one regards the spin labels as artificial amino acid side chains, characteristic values of correlation times and amplitudes of the rotational motion at the surface can be given. For eta = 1 cP and T = 297 K the correlation time of the labels bound to lysines is found to be tau = 9.10(-10) S and the rotational diffusion is nearly isotropic. The spin label bound to cysteine beta 93 occupies a protein pocket, its rotational motion is therefore restricted. The correlation time of the label motion within a limited motion cone of semi angle theta = 30 degrees +/- 3 degrees is found to be tau = 1.3.10(-9) S for eta = 1 cP and T = 297 K.