Experimental support for a novel compount motion model for the time-resolved fluorescence anisotropy decay of TMA-PDH in lipid vesicle bilayers

Abstract

Many structural studies of lipid membrane systems employ fluorescence anisotropy experiments on lipid soluble dyes that have been embedded in the lipid bilayers as optical probes. The conventional models for the interpretation of the anisotropy decay curves have a number of conceptual problems related to the form of the effective potential experienced by the probe molecules due to the interaction with the surrounding lipid environment. Therefore, a new model recently proposed by van der Sijs. (Chem. Phys. Letters 216 (1993) 559). In this paper we test this new compound model in a reanalysis of time-resolved fluorescence anisotropy experiments using TMA-DPH as a fluorescent probe. The orientational order and reorientational dynamics of TMA-DPH in small unilamellar vesicles (SUV) of POPC, DOPC, EGGPC, DLPC, EGGPG, DOPG, SQDG and DGDG was studied. We find that the new model improves the description of the underlying motional processes at short time scales and lacks certain unphysical aspects of previous models, e.g. a population of TMA-DPH probe molecules at and beyond 90° with the local bilayer normal. In contrast with previous models the compound motion model yields a good agreement between our vesicle data and previously published results from oriental lipid bilayers using ESR and AFD.