Ambiguities in the interpretation of time-resolved fluorescence anisotropy measurements on lipid vesicle systems

Analysis of time-resolved fluorescence anisotropy measurements on DPH and TMA-DPH in POPC vesicles with and without cholesterol in terms of the rotational diffusion model shows two distinct χr2 minima which are statistically equivalent. This is explained by the fact that the anisotropy decay function is given by a sum of three correlation functions which cannot be uniquely separated into individual contributions. The two solutions yield contradictory results for the effect of cholesterol on the probe dynamics. It is shown that the Maier-Saupe potential and the “wobble-in-cone” model do not give an adequate picture of the orientational order and the reorientational dynamics.