Ripple Phases Induced by α-Tocopherol in Saturated Diacylphosphatidylcholines

Abstract

The effect of α-tocopherol on the structure and phase behavior of dilauroyl-, dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines was examined using X-ray diffraction and freeze-fracture electron microscopic methods. A ripple phase was observed in all of the mixtures at temperatures well below the pretransition temperature of the corresponding pure phospholipid. Freeze-fracture studies indicated that with proportion of α-tocopherol less than 5 mol% a ripple phase with large periodicity (50–150 nm) predominated and with about 10 mol% α-tocopherol a ripple phase of periodicity about 16 nm was formed. With more than 10 mol% α-tocopherol planar bilayers tended to be formed. Partial phase diagrams of mixed aqueous dispersions of saturated phosphatidylcholines and α-tocopherol over temperature ranges about the gel to liquid-crystal phase boundary have been constructed. α-Tocopherol-enriched domains form ripple phases that coexist with regions of lamellar gel phase of the pure phospholipid in mixtures containing less than 10 mol% α-tocopherol. The presence of increasing amounts of α-tocopherol in the phospholipid causes an increase in the proportion of ripple phase at the expense of pure phospholipid bilayer indicating that the α-tocopherol-enriched domains might possess a defined stoichiometry of the two constituents.