The distribution of alpha-tocopherol in mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine

The effect of alpha -tocopherol on the structure and phase behaviour of mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine has been examined by synchrotron X-ray diffraction. Equimolar mixtures of dioleoylphosphatidylethanolamine : dioleoylphosphatidylcholine and dimyristoylphosphatidylcholine : dioleoylphosphatidylethanolamine did not show evidence of phase separation of an inverted hexagonal structure typical of alpha -tocopherol and phosphatidylethanolamine from lamellar phase. Mixed dispersions of dioleoyl derivatives of phosphatidylethanolamine : phosphatidylcholine (3:1) form a typical miscible gel phase at low temperatures but which phase separates into lamellar liquid-crystal and inverted hexagonal phases at temperatures greater than 65 degreesC. The presence of 1, 2 or 5 mol% alpha -tocopherol caused a decrease in the temperature at which the inverted hexagonal phase appears. Phase separation of non-lamellar phase from lamellar gel phase can be detected in the presence of 7.5 and 10 mol% alpha -tocopherol, indicating a limited capacity of the phosphatidylcholine to incorporate alpha -tocopherol into the lamellar domain. A partial phase diagram of the ternary mixture has been constructed from the X-ray scattering data. It was concluded that there is no preferential interaction of alpha -tocopherol with phosphatidylethanolamine in mixed aqueous dispersions containing phosphatidylcholines. (C) 2000 Elsevier Science B.V. All rights reserved.