Dynamic structure of lipid bilayers studied by nanosecond fluorescence techniques.

Molecular motions in liposomes of dipalmitoyl-phosphatidylcholine (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 IT(t) and emission anisotropy r(t) of DPH embedded in DPPC liposomes were measured at various temperatures. The value of the fluorescence lifetime tau obtained froma single exponential decay of IT(t) was somewhat higher than that in liquid paraffin below the transition temperature Tt and decreased above Tt. Higher values of tau below Tt 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 Tt was sharper than that in the wobbling diffusion constant at Tt. 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.