DYNAMICS OF THE BILAYER WATER INTERFACE OF PHOSPHOLIPID-VESICLES AND THE EFFECT OF CHOLESTEROL - A PICOSECOND FLUORESCENCE ANISOTROPY STUDY

The motion of the head group of phospholipid molecules in the bilayer structure was investigated by a picosecond fluorescence anisotropy technique using a newly synthesized fluorescent phospholipid, dipalmitoyl-L-alpha-phosphatidyl-(3-p-methoxyphenyl)umbelliferone (DPPU). In this phospholipid, a coumarin derivative is attached covalently to the phosphate moiety. The motion of the acyl chain of the phospholipid was also investigated by the same method using 1-palmitoyl-2-(3-diphenylhexatrienyl)-propanoyl-L-alpha-phosphatidylcholine (DPH(p)PC). From fluorescence anisotropy decay the wobbling diffusion rate (D(w)) of DPPU and DPH(p)PC in DPPC vesicles at 45-degrees-C was calculated to be 2.7 x 10(9) s-1 and 5.1 x 10(7) s-1 using the wobbling-in-cone-model. The range of the motion was calculated as the cone angle (theta-c), which is half of the angle of the cone in which the fluorophore can diffuse. The cone angle of the coumarin skeleton of DPPU in DPPC vesicles at 45-degrees-C was 64-degrees, which was larger than that of the DPH skeleton of DPH(p)PC, 40-degrees. These results indicate that the motion of the head group is much faster and wider than that of the acyl chain. When cholesterol was added to the DPPC vesicles, the range of motion of the acyl chain decreased, but that of the head group increased. These facts show that cholesterol restricts the motion of the acyl chain but enhances that of the head group in the phospholipid bilayer.