Location and dynamics of α-tocopherol in model phospholipid membranes with different charges

Abstract

Studies were made on the position and dynamics of the OH-group of α-tocopherol in phospholipid membranes. There was no difference in the spin-lattice ( T 1) relaxation times at the 5a-position of α-tocopherol labeled with 13C- or C 19F 3-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% α-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 α-tocopherol. These results suggest that no specific interaction occurred between the OH-group of α-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 α-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 α-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 α-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.