The effect of alpha-tocopherol on the thermotropic phase behaviour of dipalmitoylphosphatidylethanolamine. A synchrotron X-ray diffraction study.

The effect of alpha-tocopherol on the thermotropic phase behaviour of aqueous dispersions of dipalmitoylphosphatidylethanolamine in mixtures containing 0, 2.5, 5, 10 and 20 mol% alpha-tocopherol was examined using synchrotron X-ray diffraction methods. Dispersions were equilibrated for at least 12 h at 4 degrees C before measurement. The phospholipid alone undergoes a lamellar gel to liquid-crystalline phase transition at 66 degrees C during the initial heating scan. When codispersed with alpha-tocopherol there was evidence of phase separation of alpha-tocopherol-rich domains from bilayers of pure phospholipid. The alpha-tocopherol-rich domains were identified from the relationship between scattering intensity of the phase and the proportion of alpha-tocopherol in the mixture. In initial heating scans alpha-tocopherol-rich domains were characterized by broad lamellar repeat spacings (5.0 nm at 55 degrees C) in the small-angle scattering region which first appears at approximately 40 degrees C and increases in intensity with increasing temperature. In the presence of more than 5 mol% alpha-tocopherol the wide-angle scattering region showed two scattering profiles consisting of multiple peaks typical of lamellar crystal phases. The low-temperature crystal phase, designated Lc1, was transformed into Lc2 phase at approximately 50 degrees C. On further heating the Lc2 phase was transformed directly into inverted hexagonal phase at approximately 56 degrees C 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 degrees C, at least up to 70 degrees C. 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 alpha-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/alpha-tocopherol in the alpha-tocopherol-rich lamellar crystal and inverted hexagonal phases is estimated to be approximately 4 : 1. The structural changes observed in the study are related to the calorimetric transitions reported in independent work on these dispersions.