Ripple phases induced by alpha-tocopherol in saturated diacylphosphatidylcholines

The effect of alpha-tocopherol on the structure and phase behavior of dilau royl-, dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines was examined using X-ray diffraction and freeze-fracture electron microscopic m ethods. A ripple phase was observed in all of the mixtures at temperatures well below the pretransition temperature of the corresponding pure phosphol ipid, Freeze-fracture studies indicated that with proportion of alpha-tocop herol less than 5 mol% a ripple phase with large periodicity (50-150 nm) pr edominated and with about 10 mol% alpha-tocopherol a ripple phase of period icity about 16 nm was formed. With more than 10 mol% alpha-tocopherol plana r bilayers tended to be formed. Partial phase diagrams of mixed aqueous dis persions of saturated phosphatidylcholines and alpha-tocopherol over temper ature ranges about the gel to liquid-crystal phase boundary have been const ructed. alpha-Tocopherol-enriched domains form ripple phases that coexist w ith regions of lamellar gel phase of the pure phospholipid in mixtures cont aining less than 10 mol% alpha-tocopherol. The presence of increasing amoun ts of alpha-tocopherol in the phospholipid causes an increase in the propor tion of ripple phase at the expense of pure phospholipid bilayer indicating that the alpha-tocopherol-enriched domains might possess a defined stoichi ometry of the two constituents. (C) 2000 Academic Press.