Inverted hexagonal and cubic phases induced by α-tocopherol in fully hydrated dispersions of dilauroylphosphatidylethanolamine

Abstract

The effect of α-tocopherol on the thermotropic phase behaviour and structure of aqueous dispersions of 1,2-dilauoryl- 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°C on heating at 3°C min −1 between 10°C and 90°C. The transition was reversible with a temperature hysteresis of 0.3°C on cooling. At temperatures less than 10°C only lamellar gel phase of the pure phospholipid was seen in co-dispersions of up to 20 mol % α-tocopherol. The presence of 2.5 mol % α-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 α-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 % α-tocopherol showed the presence of cubic phases. These phases indexed a Pn3m or Pn3 space grouping. When the proportion of α-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°C. The X-ray diffraction data were used to construct a partial phase diagram of the lipid mixture in excess water between 10° and 90°C and up to 20 mol % α-tocopherol in phospholipid.