The effect of α-tocopherol on the structure and thermotropic phase behaviour of distearoylphosphatidylethanolamine was examined by using synchrotron X-ray diffraction methods. There was evidence that α-tocopherol does not distribute randomly in the dispersed phospholipid but instead phospholipid phases enriched in α-tocopherol are formed. Heating codispersions from lamellar gel phase induced formation of hexagonal-II phase at temperatures below the main transition of the pure phospholipid and which were enriched in α-tocopherol. Codispersions containing 5 or 10 mol% α-tocopherol were induced to form a cubic phase at temperatures above the lamellar to hexagonal-II phase transition. Such phases were not observed in codispersions containing 2.5 or 20 mol% α-tocopherol in which only lamellar and hexagonal-II phases were formed. The space group of the cubic phases were tentatively assigned as Pn3m. Equilibration of codispersions at 4°C results in the formation of lamellar crystalline phases enriched in α-tocopherol and phase separated domains of pure phospholipid. Two lamellar crystalline phases were characterized on the basis of their particular wide-angle X-ray scattering patterns. The lamellar crystalline phases were also distinguished from other lamellar phases of the pure phospholipid by the lamellar repeat. Partitioning of α-tocopherol into phosphatidylethanolamine domains in membranes may introduce instability into the structure. © 2002 Published by Elsevier Science Ireland Ltd.
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