EPR spectra of spin labels in lipid bilayers. II. Rotation of steroid spin probes

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Abstract
The spin label method is used to investigate the nature of molecular motion in a liquid crystalline bilayer. The EPR spectra of different spin labels are simulated by means of perturbation theory. Using the diffusion model with a restoring potential the rotational diffusion coefficients for the different spin labels are evaluated from the linewidth. Rotational diffusion and viscosity are related by a modified Stokes-Einstein-Perrin formula. The correction factors account for the difference between the effective viscosity during rotation and the (translational) viscosity and are interpreted in terms of the geometry and the ordering of the spin probes. The bilayer viscosity η is found to be 3.7 cp ≤ η ≤ 5.7 cp.