Characterisation of Clusters of α‐Tocopherol in Gel and Fluid Phases of Dipalmitoylglycerophosphocholine

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Abstract
The effect of α‐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 α‐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°C to 36°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 α‐tocopherol. Real‐time X‐ray diffraction measurements using synchrotron radiation performed under identical conditions to the thermal studies showed clear transition sequences of Lβ→Pβ→Lα for all mixtures. The sequence was reversible with hysteresis of 2–3°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 α‐tocopherol had 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 α‐tocopherol‐enriched phase. Electron microscopy of freeze‐fracture replicas of mixtures of phospholipid containing 10mol/100 mol α‐tocopherol thermally quenched from 10°C and 60°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 phospholipids: α‐tocopherol in the α‐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 α‐tocopherol molecules in both gel and liquid‐crystal phases of dipalmitoyl‐glycerophospholcholine in approximately the same stoichiometry.

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