Dynamic structure of biological membranes as probed by 1,6-diphenyl-1,3,5-hexatriene: A nanosecond fluorescence depolarization study

A fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene, was incorporated in four different biological membranes, the purple membrane of Halobacterium halobium, human erythrocyte membrane, rabbit sarcoplasmic reticulum membrane, and rat liver mitochondrial membrane. Time-resolved fluorescence depolarization of the probe suggested that the rotational Brownian motion of the probe in the membranes was restricted in the angular range. The motion of the rodshaped, lipophilic probe molecule, expected to reflect closely the motion of neighboring lipid hydrocarbon chains, was analyzed in terms of the wobbling-in-cone model in which the major axis of the probe was assumed to wobble freely in a cone of semiangle θ_c with a wobbling diffusion constant D_w. At 35 °C, D_w in the four membranes, in the above order, ranged between 0.048 and 0.15 ns^-1 and θ_c between 31 and 53°. From the rotational rate D_w, the viscosity against the wobbling motion was calculated to be 0.9-0.3 P. When the temperature was raised from 10 to 35 0C, Z)w in all membranes increased approximately 3-fold, corresponding to activation energies of 7-8 kcal/mol, and θ_c increased by about 10°, except for the purple membrane in which the angular range remained narrow. The same characteristic temperature dependence has been found in many model membrane systems that contain unsaturated lecithins, suggesting an important role of unsaturated phospholipids in the dynamic structure of the lipid hydrocarbon chain region of biological membranes at physiological temperatures. Comparison with model systems suggests that proteins and cholesterol act mainly as barriers that narrow the angular range.