Surfactant and Hydrophobic Derivatives of trans-Stilbenes as Probes of Vesicle and Micelle Solubilization Sites. Studies Using Fluorescence and Photoisomerization as Probes

Article abstract of DOI:10.1021/ja00306a005

An investigation of the fluorescence and photoisomerization of several surfactant trans-stilbene derivatives in aqueous micelles and vesicles is reported.The temperature dependence of Φ_f was determined for several media over the range 5-70 deg C.The results present a picture of similar micellar solubilization sites for the trans-stilbene chromophore in every case; the micelle is "seen" by the stilbene as a moderately viscous fluid medium with slightly higher values for Φ_f and slightly reduced Φ_t->c compared to nonviscous homogeneous solutions.For vesicles considerably more complex behavior is observed.Generally Φ_f is considerably higher than in solution and Φ_t->c is much lower.Several, but not all, of the surfactant and hydrophobic stilbenes show a sharp sensitivity in Φ_f as a function of T when passing through T_c, the vesicle phase transition temperature.Arrhenius plots for the process competing with fluorescence as a function of temperature show relatively high values for E_a and log A, above T_c, consistent with the high temperature phase providing a microenvironment for the stilbene chromophore like a viscous solvent.Below T_c, the variation of Φ_f gives much lower values for E_A and log A which suggest escape from the fluorescence state is "order limited" rather than viscosity dominated.These results suggest that the low-temperature phase of vesicles presents an environment for the stilbene chromophore more like an inclusion complex than a fluid medium.The effect of cholesterol addition on DODAC vesicles containing the stilbenes is to increase fluidity below T_c and to effectively increase viscosity above T_c.