Anisotropic rotation of bacteriorhodopsin in lipid membranes. Comparison of theory with experiment

Rotational diffusion of bacteriorhodopsin in dimyristoyl phosphatidylcholine vesicles has been measured at different temperatures and lipid; protein ratios by the technique of flash-induced transient linear dichroism. The data are used to evaluate critically the theory of anisotropy decay due to protein rotation in the lipid bilayer. The theoretical model assumes that rotation of the protein occurs only around the membrane normal. Under conditions favoring completely monomeric bacteriorhodopsin, namely at molar lipid; protein ratios greater than or approximately 100 and for temperatures sufficiently above the lipid phase transition, it is found that the theoretical model provides an excellent description of the experimental data. Curve-fitting analyses of the experimental decay curves show that the retinal is oriented at an angle of 78 +/- 2 degrees with respect to the membrane normal. Between 25 and 37 degrees C, the protein rotates with a relaxation time of 15 +/- 5 micros in the lipid liquid crystalline phase, corresponding to the membrane viscosity of 3.7 +/- 1.3 P. The curve analysis also provides a sensitive test for the presence of protein aggregates in the lipid bilayer.