Inverted hexagonal and cubic phases induced by alpha-tocopherol in fully hydrated dispersions of...

The effect of alpha-tocopherol on the thermotropic phase behaviour and structure of aqueous dispersions of 1,2-di-lauryl-sn-glycero-3-phosphoethanolamine was examined by synchrotron X-ray diffraction. The pure phospholipid exhibited a lamellar gel to liquid-crystal phase transition at 30 degrees C on heating at 3 degrees C min(-1) between 10 degrees C and 90 degrees C. The transition was reversible with a temperature hysteresis of 0.3 degrees C on cooling. At temperatures less than 10 degrees C only lamellar gel phase of the pure phospholipid was seen in co-dispersions of up to 20 mol % alpha-tocopherol. The presence of 2.5 mol % alpha-tocopherol caused the appearance of inverted hexagonal phase at temperatures just below the main phase transition temperature that co-existed with the lamellar gel phase. The intensity of scattering from the hexagonal-II phase increased with increasing proportion of alpha-tocopherol in the mixture and in proportions greater than 10 mol % it persisted at temperatures above the main transition and co-existed with the lamellar liquid-crystal phase of the pure phospholipid. At higher temperatures all co-dispersions containing up to 15 mol % alpha-tocopherol showed the presence of cubic phases. These phases indexed a Pn3m or Pn3 space grouping. When the proportion of alpha-tocopherol was increased to 20 mol % the only non-lamellar phase observed was inverted hexagonal phase. This phase co-existed with lamellar gel and liquid-crystal phases of the pure phospholipid, but was the only phase present at temperatures >60 degrees C. The X-ray diffraction data were used to construct a partial phase diagram of the lipid mixture in excess water between 10 degrees and 90 degrees C and up to 20 mol % alpha-tocopherol in phospholipid.