Saturation transfer electron paramagnetic resonance on membrane-bound proteins. I-rotational diffusion of rhodopsin in the visual receptor membrane

We have applied the technique of saturation transfer electron paramagnetic resonance to the study of spin labeled membrane-bound bovine rhodopsin. Based on the comparison with theoretical and experimental spectra corresponding to isotropic slow motion, the present data leads to a rotational correlation time for the membrane-bound rhodopsin molecule of 20 μsec at 20°C. Bleaching does not appear to influence the motion of the protein while addition of glutaraldehyde (5%) stops its rotation completely. These results are in good agreement with what is known about the motion of the membrane-bound rhodopsin, establishing the applicability of the saturation transfer technique to the study of slow anisotropic motions of membrane-bound proteins.