Wang X, Takahashi H, Hatta I, Quinn P J
Division of Life Sciences, King's College London, Campden Hill, London W8 7AH, UK.
Biochim Biophys Acta. 1999 May 12;1418(2):335-43. doi: 10.1016/s0005-2736(99)00044-9.
The effect of alpha-tocopherol on the thermotropic phase transition behaviour of aqueous dispersions of dimyristoylphosphatidylethanolamine was examined using synchrotron X-ray diffraction methods. The temperature of gel to liquid-crystalline (Lbeta-->Lalpha) phase transition decreases from 49.5 to 44.5 degrees C and temperature range where gel and liquid-crystalline phases coexist increases from 4 to 8 degrees C with increasing concentration of alpha-tocopherol up to 20 mol%. Codispersion of dimyristoylphosphatidylethanolamine containing 2.5 mol% alpha-tocopherol gives similar lamellar diffraction patterns as those of the pure phospholipid both in heating and cooling scans. With 5 mol% alpha-tocopherol in the phospholipid, however, an inverted hexagonal phase is induced which coexists with the lamellar gel phase at temperatures just before transition to liquid-crystalline lamellar phase. The presence of 10 mol% alpha-tocopherol shows a more pronounced inverted hexagonal phase in the lamellar gel phase but, in addition, another non-lamellar phase appears with the lamellar liquid-crystalline phase at higher temperature. This non-lamellar phase coexists with the lamellar liquid-crystalline phase of the pure phospholipid and can be indexed by six diffraction orders to a cubic phase of Pn3m or Pn3 space groups and with a lattice constant of 12.52+/-0.01 nm at 84 degrees C. In mixed aqueous dispersions containing 20 mol% alpha-tocopherol, only inverted hexagonal phase and lamellar phase were observed. The only change seen in the wide-angle scattering region was a transition from sharp symmetrical diffraction peak at 0.43 nm, typical of gel phases, to broad peaks centred at 0.47 nm signifying disordered hydrocarbon chains in all the mixtures examined. Electron density calculations through the lamellar repeat of the gel phase using six orders of reflection indicated no difference in bilayer thickness due to the presence of 10 mol% alpha-tocopherol. The results were interpreted to indicate that alpha-tocopherol is not randomly distributed throughout the phospholipid molecules oriented in bilayer configuration, but it exists either as domains coexisting with gel phase bilayers of pure phospholipid at temperatures lower than Tm or, at higher temperatures, as inverted hexagonal phase consisting of a defined stoichiometry of phospholipid and alpha-tocopherol molecules.
采用同步加速器X射线衍射方法研究了α-生育酚对二肉豆蔻酰磷脂酰乙醇胺水分散体热致相变行为的影响。随着α-生育酚浓度增加至20 mol%，凝胶到液晶（Lβ→Lα）相转变温度从49.5℃降至44.5℃，凝胶相和液晶相共存的温度范围从4℃增加到8℃。含有2.5 mol%α-生育酚的二肉豆蔻酰磷脂酰乙醇胺共分散体在加热和冷却扫描中给出与纯磷脂相似的层状衍射图谱。然而，当磷脂中含有5 mol%α-生育酚时，会诱导出反相六角相，在转变为液晶层状相之前的温度下，它与层状凝胶相共存。含有10 mol%α-生育酚时，在层状凝胶相中显示出更明显的反相六角相，但此外，在较高温度下，另一个非层状相与层状液晶相一起出现。这个非层状相与纯磷脂的层状液晶相共存，在84℃时可通过六个衍射级次标定为Pn3m或Pn3空间群的立方相，晶格常数为12.52±0.01 nm。在含有20 mol%α-生育酚的混合水分散体中，仅观察到反相六角相和层状相。在广角散射区域观察到的唯一变化是，在所有测试混合物中，从凝胶相典型的0.43 nm处尖锐对称衍射峰转变为以0.47 nm为中心的宽峰，这表明烃链无序。通过凝胶相的层状重复利用六个反射级次进行电子密度计算表明，由于存在10 mol%α-生育酚，双层厚度没有差异。结果表明，α-生育酚并非随机分布在以双层构型排列的磷脂分子中，而是在低于Tm的温度下，以与纯磷脂凝胶相双层共存的区域形式存在，或者在较高温度下，以由特定化学计量比的磷脂和α-生育酚分子组成的反相六角相形式存在。
