THE INTERPRETATION OF FLUORESCENCE ANISOTROPY DECAYS OF PROBE MOLECULES IN MEMBRANE SYSTEMS

Previous analyses of the fluorescence anisotropy decays of 1- rimethyl ammonio)-6-phenyl]-phenyl-1,3,5-hexatriene (TMA-DPH) probe molecules i n lipid vesicles using the Brownian rotational diffusion model yield o rientational distribution functions with a significant population of m olecules with their axes parallel to the bilayer plane. We show that t his unsatisfactory situation arises from the way the model fits the fa st decaying components of the fluorescence anisotropy. A plausible and consistent description of the anisotropy decay is obtained if these c omponents are ascribed to fast, though restricted, local motions of th e probe molecules within a slowly rotating ''cage'' in the lipid struc ture. The ''cage'' may be envisaged as a cavity between the lipid mole cules, so that its position and orientation changes with the internal motions of the hydrocarbon chains. While this approach utilizes five a djustable parameters in the least-squares fits, it has the merit of yi elding unimodal distribution functions with the TMA-DPH molecules pref erentially oriented along the normal to the local bilayer surface.