Dynamic structure of lipid bilayers studied by nanosecond fluorescence techniques

Molecular motions in liposomes of dipalmitoylphosphatidylcholine (DPPC) were studied by nanosecond fluorescence techniques. As a fluorescent probe for the hydrocarbon region, 1,6-diphenyl-1,3,5-hexatriene (DPH) was used. Time courses of fluorescence intensity I_T(t) and emission anisotropy r(t) of DPH embedded in DPPC liposomes were measured at various temperatures. The value of the fluorescence lifetime τ obtained from a single exponential decay of I_T(t) was somewhat higher than that in liquid paraffin below the transition temperature T_t and decreased above T_t. Higher values of τ below T_t indicate the almost complete hydrophobic environment. The decay curves of r(t) were separated into two phases: an initial fast decreasing phase of the order of one nanosecond and a second almost constant phase. This indicates that the orientational motion of DPH in the hydrocarbon region is described by a wobbling diffusion restricted by a certain anisotropic potential. The results were analyzed on the model that the wobbling diffusion is confined in a cone with a uniform diffusion constant. Though temperature dependence of the cone angle was sigmoidal, that of the wobbling diffusion constant was like the exponential function. The change in the cone angle at T_t was sharper than that in the wobbling diffusion constant at T_t. Estimated values of the viscosity in the cone were an order of magnitude smaller than the values of "microviscosity" which were estimated from the steady-state emission anisotropy without considering the restrictions on the rotational motion.