The effect of α‐tocopherol on the thermotropic phase behaviour of dipalmitoylphosphatidylethanolamine

Abstract

The effect of a-tocopherol on the thermotropic phase behaviour of aqueous dispersions of dipalmitoylphosphatidylethanolamine in mixtures containing 0, 2.5, 5, 10 and 20 mol% a-tocopherol was examined using synchrotron X-ray diffraction methods. Dispersions were equilibrated for at least 12 h at 4 8C before measurement. The phospholipid alone undergoes a lamellar gel to liquid-crystalline phase transition at 66 8C during the initial heating scan. When codispersed with a-tocopherol there was evidence of phase separation of a-tocopherol-rich domains from bilayers of pure phospholipid. The a-tocopherol-rich domains were identified from the relationship between scattering intensity of the phase and the proportion of a-tocopherol in the mixture. In initial heating scans a-tocopherol-rich domains were characterized by broad lamellar repeat spacings (5.0 nm at 55 8C) in the small-angle scattering region which first appears at <40 8C and increases in intensity with increasing temperature. In the presence of more than 5 mol% a-tocopherol the wide-angle scattering region showed two scattering profiles consisting of multiple peaks typical of lamellar crystal phases. The lowtemperature crystal phase, designated Lc 1 , was transformed into Lc 2 phase at <50 8C. On further heating the Lc 2 phase was transformed directly into inverted hexagonal phase at <56 8C and coexisted with the lamellar gel phase of the pure phospholipid. The inverted hexagonal phase coexists with the lamellar phase of the pure phospholipid, after transformation into lamellar liquid-crystal phase at 66 8C, at least up to 70 8C. In cooling scans the d-spacing of the inverted hexagonal phase decreases progressively and the scattering intensity weakens on cooling below the liquid-crystal to gel phase transition temperature of the pure phospholipid. There is no evidence of formation of lamellar crystal phases during cooling so that the a-tocopherol either mixes with the lamellar gel phase of the phospholipid without change in the structural parameters of the pure phospholipid or is completely phase separated from the phospholipid. The stoichiometry of phospholipid/a-tocopherol in the a-tocopherol-rich lamellar crystal and inverted hexagonal phases is estimated to be <4 : 1. The structural changes observed in the study are related to the calorimetric transitions reported in independent work on these dispersions.Keywords: phase transition; phospholipid; a-tocopherol; vitamin E; X-ray diffraction.a-Tocopherol is a ubiquitous lipid component of biological membranes. It is believed to have two primary functions in cells: a major role is thought to be as an antioxidant acting to prevent free radical damage to tissues [1±5] and a subsidiary function is to stabilize the structure of membranes [6]. In order to understand the biological functions of a-tocopherol at the molecular level, it is essential to understand the interaction between a-tocopherol and phospholipids that represent the major components of the membrane lipid bilayer matrix.The effect of a-tocopherol on...

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