Characterisation of clusters of alpha-tocopherol in gel and fluid phases of dipalmitoylglycerophosphocholine

Abstract

The effect of alpha-tocopherol on the phase behaviour of aqueous dispersions of dipalmitoylglycerophosphocholine has been examined by differential scanning calorimetry, freeze-fracture electron microscopy, and real-time X-ray diffraction methods. The presence of alpha-tocopherol in proportions 2.5, 5, 7.5, and 10 mol/100 mol results in a progressive decrease in the temperature of the gel to liquid-crystalline phase transition from 41.5 degrees C to 36 degrees C and a reduction in transition enthalpy from 35 kJ.mol-1 to 15 kJ.mol-1 phospholipid. The thermal data indicated that the pretransition of the phospholipid is eliminated even in mixtures containing 2.5 mol/100 mol alpha-tocopherol. Real-time X-ray diffraction measurements using synchrotron radiation performed under identical conditions to the thermal studies showed clear transition sequences of L beta-->P beta-->L alpha for all mixtures. The sequence was reversible with hysteresis of 2-3 degrees C on cooling. Low-angle X-ray scattering from mixtures in the gel phase showed three lamellar repeat spacings of 6.35, 7.5, and 8.4 nm. The spacing at 6.35 nm was assigned to pure phospholipid from which alpha-tocopherol has been phase separated into enriched domains giving lamellar repeat spacings of 7.5 nm and 8.4 nm. Low-angle diffraction patterns of mixtures in the fluid phase were characterised by two lamellar repeat spacings. The longer spacing of about 6.6 nm was assigned to pure phospholipid and the shorter spacing at about 6.1 nm to an alpha-tocopherol-enriched phase. Electron microscopy of freeze-fracture replicas of mixtures of phospholipid containing 10 mol/100 mol alpha-tocopherol thermally quenched from 10 degrees C and 60 degrees C, showed evidence of domain structures within the bilayer plane that appeared to be correlated between successive bilayers in multilamellar dispersions. Calculations of the stoichiometry of phospholipid: alpha-tocopherol in the alpha-tocopherol-enriched domains based on enthalpy data and integrated X-ray scattering intensity gave values of 9.6:1 for the fluid phase and 9.2:1 for the gel phase. This was consistent with a clustering of alpha-tocopherol molecules in both gel and liquid-crystal phases of dipalmitoyl-glycerophospholcholine in approximately the same stoichiometry.