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Phenolic Group of α-Tocopherol Anchors at the Lipid-Water Interface of Fully Saturated Membranes

Abstract:

α-Tocopherol is considered to carry on a very important role as an antioxidant for membranes and lipoproteins and other biological roles as membrane stabilizers and bioactive lipids. Given its essential role, it is very important to fully understand its location in the membrane. In this work, the vertical location of vitamin E in saturated membranes has been studied using biophysical techniques. Small- and wide-angle X-ray diffraction experiments show that α-tocopherol alters the water layer between bilayers in both 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), indicating its proximity to this surface. The quenching of the intrinsic fluorescence of α-tocopherol indicates a low quenching efficiency by acrylamide and a higher quenching by 5-doxyl-PC than by 9- and 16-doxyl-PC. These results suggest that in both DMPC and DPPC membranes, the chromanol ring is not far away from the surface of the membrane but within the bilayer. 1H nuclear Overhauser enhancement spectroscopy magic-angle spinning-nuclear magnetic resonance studies showed that α-tocopherol is localized in a similar manner in DMPC and DPPC membranes, with the chromanol ring embedded in the upper part of the hydrophobic bilayer. Using attenuated total reflection-Fourier transform infrared spectroscopy, it was observed that the tail chain of α-tocopherol lies nearly parallel to the acyl chains of DMPC and DPPC. Taking these results together, it was concluded that in both DMPC and DPPC, the hydroxyl group of the chromanol ring will establish hydrogen bonding with water on the membrane surface, and the main axis of the α-tocopherol molecule will be perpendicular to the bilayer plane.