Karmakar Sanat, Raghunathan V A
Raman Research Institute, Bangalore 560080, India.
Phys Rev E Stat Nonlin Soft Matter Phys. 2005 Jun;71(6 Pt 1):061924. doi: 10.1103/PhysRevE.71.061924. Epub 2005 Jun 29.
We have studied the phase behavior of mixtures of cholesterol with dipalmitoyl phosphatidylcholine (DPPC), dimyristoyl phosphatidylcholine (DMPC), and dilauroyl phosphatidylethanolamine (DLPE), using x-ray diffraction techniques. Phosphatidylcholine (PC)-cholesterol mixtures are found to exhibit a modulated phase for cholesterol concentrations around 15 mol % at temperatures below the chain melting transition. Lowering the relative humidity from 98% to 75% increases the temperature range over which it exists. An electron density map of this phase in DPPC-cholesterol mixtures, calculated from the x-ray diffraction data, shows bilayers with a periodic height modulation, as in the ripple phase observed in many PCs in between the main- and pretransitions. However, these two phases differ in many aspects, such as the dependence of the modulation wavelength on the cholesterol content and thermodynamic stability at reduced humidities. This modulated phase is found to be absent in DLPE-cholesterol mixtures. At higher cholesterol contents the gel phase does not occur in any of these three systems, and the fluid lamellar phase is observed down to the lowest temperature studied (5 degrees C).
我们使用X射线衍射技术研究了胆固醇与二棕榈酰磷脂酰胆碱（DPPC）、二肉豆蔻酰磷脂酰胆碱（DMPC）和二月桂酰磷脂酰乙醇胺（DLPE）混合物的相行为。发现磷脂酰胆碱（PC）-胆固醇混合物在低于链熔化转变温度时，胆固醇浓度约为15摩尔%时会呈现一种调制相。将相对湿度从98%降至75%会增加该相存在的温度范围。根据X射线衍射数据计算得到的DPPC-胆固醇混合物中该相的电子密度图显示，双层具有周期性的高度调制，就像在许多PC的主转变和预转变之间观察到的波纹相一样。然而，这两个相在许多方面存在差异，例如调制波长对胆固醇含量的依赖性以及在降低湿度时的热力学稳定性。发现DLPE-胆固醇混合物中不存在这种调制相。在较高胆固醇含量下，这三个系统中均未出现凝胶相，并且在研究的最低温度（5摄氏度）下观察到流体层状相。
