LOCATION AND DYNAMICS OF ALPHA-TOCOPHEROL IN MODEL PHOSPHOLIPID-MEMBRANES WITH DIFFERENT CHARGES

Studies were made on the position and dynamics of the OH-group of alpha-tocopherol in phospholipid membranes. There was no difference in the spin-lattice (T1)relaxation times at the 5a-position of alpha-tocopherol labeled with C-13- or (CF3)-F-19-determined from the nuclear magnetic resonance (NMR) spectra of liposomes positively charged with stearylamine (SA) and negatively charged with dicetylphosphate (DCP). The zeta-potentials of egg yolk phosphatidylcholine (EYPC) liposomes with and without SA or DCP were not affected by incorporation of 20 mol% alpha-tocopherol, though incorporation of 10 mol% ascorbylpalmitate decreased the zeta-potentials of EYPC and EYPC-SA liposomes. The P=O stretching band (1235 cm-1) of the phosphate-group and C=O stretching band (1734 cm-1) of the acyl ester linkage in dimyristoylphosphatidylcholine (DMPC) liposomes, measured by Fourier transform-infrared (FT-IR) spectroscopy, were not changed by incorporation of alpha-tocopherol: These results suggest that no specific interaction occurred between the OH-group of alpha-tocopherol and the polar interfacial region of the bilayer. The dynamic quenching effects of n-(N-oxy-4,4'-dimethyloxazolidine-2-yl)stearic acids (n-NSs) on the intrinsic fluorescence of alpha-tocopherol were in the order 5-NS > 7-NS = 12-NS > 16-NS. Acrylamide, a water-soluble fluorescence quencher with a very low capacity to penetrate through phospholipid bilayers, had very low quenching efficiency. These results indicate that the bulk of the chromanol moiety of alpha-tocopherol is located in a position close to that occupied by the nitroxide group of 5-NS in the membranes and is poorly exposed at the membrane surface. No difference was found in the oxidation rates of alpha-tocopherol induced by water-soluble 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) in gel state dipalmitoylphosphatidylcholine (DPPC) liposomes and liquid crystalline state DMPC liposomes, indicating that the OH-group is not located deep in the hydrophobic region.