Fluidity of glycerol skeletal region in phospholipid bilayers: a time-resolved fluorescence depolarization study

The fluidity of glycerol skeletal region in phospholipid bilayer was investigated by the time-resolved fluorescence depolarization technique with L-alpha-dihexadecanoyl-sn-glycero-3-phospho-[N-(4-nitrobenzo-2-oxa-1,3- diazole)]ethanolamine (NBD-PE) as a fluorescent probe. In this probe, the fluorescent moiety, 4-nitrobenz-2-oxa-1,3-diazole (NBD), is attached to a nitrogen atom at the polar head group of phosphatidylethanolamine molecule. When this probe is embedded in a lipid bilayer, the NBD moiety locates near the glycerol skeletal region. The time courses of fluorescence anisotropy of NBD-PE in dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) bilayers were analyzed using a wobbling-in-cone model, in which the molecular motion is characterized by a half cone angle (theta c) and a wobbling diffusion rate (Dw). Values of Dw of NBD moiety in phospholipid bilayers were found to be on the order of 10(7) s-1 at the physiological temperatures, which is almost the same value as that of the hydrocarbon chain in lipid bilayers. This fact indicates that the fluidity in the glycerol skeletal region is similar to that in the hydrocarbon layer.