The effect of α-tocopherol (vitamin E) on the phase behaviour of fully-hydrated dioleoyl phosphatidylcholine membranes

We report on the lyotropic phase behaviour of fully-hydrated mixtures of α-tocopherol (α-toc) with the unsaturated phospholipid dioleoyl phosphatidylcholine (DOPC), as studied by synchrotron small-angle x-ray diffraction. Increasing amounts of α-toc progressively swell the layer spacing of the fluid lamellar L_α phase of DOPC, and then induce a transition to an inverse hexagonal H_II phase. Low-resolution electron density profiles show that this increase is largely due to an increased thickness of the bilayer, with little change in the water layer thickness. In the range 30 – 50 mol% α-toc, additional weak low-angle peaks were observed, whose characteristic ratios are in agreement with the presence of swollen inverse bicontinuous cubic phases of spacegroups Im3m / Pn3m. This research has applications both in the biological field and for industrial product development. We show that the effect of α-toc addition in DOPC membranes has some similarities to that of cholesterol by stabilizing inverse curvature structures, which play crucial roles in cell division, membrane trafficking and endocytosis. Concerning industrial applications, the stabilization of inverted hexagonal (H_II) and swollen bicontinuous cubic phases offers the opportunity to develop new delivery systems.