Electronic energy transfer in anisotropic systems. Part 5.—Rhodamine–lipid derivatives in model membranes

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Abstract
The electronic energy transfer between molecules located at lipid/water interfaces of a model membrane has been investigated. The donor–donor systems were lipid derivatives of rhodamine B and rhodamine 101 solubilized in vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The systems were studied at low and high dye concentrations, as well as at different temperatures by means of steady-state and time-resolved fluorescence spectroscopy. The time-dependent fluorescence anisotropy, r(t), obtained at low dye concentrations is solely determined by rotational motions of the rhodamine moiety and its orientation. The orientation of the rhodamine chromophore with respect to the normal of the membranes was determined by using linear dichroism (LD) spectroscopy. It is found that the electronic absorption and emission transition dipoles of the dyes are preferentially oriented in the plane of the lipid bilayer. At high dye concentration, r(t) depends on rotational motions of the rhodamine dye as well as energy transfer between adjacent chromophores. An analytical model of energy transfer and a semi-empirical approach of r(t) were used to analyse the experimental results. The models and the analysis of experimental data were tested against simulated data. Our results are compatible with energy transfer among chromophores located in the lipid/water interfaces of a lipid bilayer with their transition dipoles oriented parallel to the membranes, and furthermore undergoing rapid reorientation compared with the rate of energy transfer. The model predicts that the rhodamine B group is located in the water region at ca. 3.5 Å from the lipid/water boundary, while the rhodamine 101 dyes are located in this boundary. Furthermore, the dye concentrations predicted from the analysis of the experiments agree with those calculated from independent data.