Membrane Biophysics Platform and Institute of Chemical Biology, Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
Phys Chem Chem Phys. 2011 Feb 28;13(8):3033-8. doi: 10.1039/c0cp01783c. Epub 2010 Dec 7.
Over a range of hydration, unsaturated diacylglycerol/phosphatidylcholine mixtures adopt an inverse micellar cubic phase, of crystallographic space group Fd3m. In this study hydrated DOPC:DOG mixtures with a molar ratio close to 1 : 2 were examined as a function of hydrostatic pressure, using synchrotron X-ray diffraction. The small-angle diffraction pattern at atmospheric pressure was used to calculate 2-D sections through the electron density map. Pressure initially has very little effect on the structure of the Fd3m cubic phase, in contrast to its effect on hydrated inverse bicontinuous cubic phases. At close to 2 kbar, a sharp transition occurs from the Fd3m phase to a pair of coexisting phases, an inverse hexagonal H(II) phase plus an (ordered) lamellar phase. Upon increasing the pressure to 3 kbar, a further sharp transition occurs from the H(II) phase to a (fluid) lamellar phase, in coexistence with the ordered lamellar phase. These transitions are fully reversible, but show hysteresis. Remarkably, the lattice parameter of the Fd3m phase is practically independent of pressure. These results show that these two lipids are miscible at low pressure, adopting a single lyotropic phase (Fd3m); they then become immiscible above a critical pressure, phase separating into DOPC-rich and DOG-rich phases.
在一定的水合范围内，不饱和二酰基甘油/磷脂酰胆碱混合物采用反胶束立方相，具有晶状空间群 Fd3m。在这项研究中，使用同步加速器 X 射线衍射，研究了摩尔比接近 1:2 的水合 DOPC:DOG 混合物作为静水压力的函数。在大气压下的小角度衍射模式用于通过电子密度图计算二维截面。与水合反连续立方相相比，压力对 Fd3m 立方相的结构几乎没有影响。在接近 2kbar 时，从 Fd3m 相到一对共存相（反向六方 H(II)相和（有序）层状相）发生急剧转变。当压力增加到 3kbar 时，从 H(II)相到（流体）层状相发生进一步的急剧转变，与有序层状相共存。这些转变是完全可逆的，但存在滞后现象。值得注意的是，Fd3m 相的晶格参数几乎与压力无关。这些结果表明，这两种脂质在低压下是混溶的，采用单一溶致相（Fd3m）；然后在临界压力以上变得不相溶，分离成富含 DOPC 和富含 DOG 的相。
