Tamai Nobutake, Uemura Maiko, Takeichi Tetsuya, Goto Masaki, Matsuki Hitoshi, Kaneshina Shoji
Department of Life System, Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima 770-8506, Japan.
Biophys Chem. 2008 Jun;135(1-3):95-101. doi: 10.1016/j.bpc.2008.03.008. Epub 2008 Apr 4.
We investigated the thermotropic phase behavior of the distearoylphosphatidylcholine (DSPC)-cholesterol binary bilayer membrane as a function of the cholesterol composition (X(ch)) by fluorescence spectroscopy using 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and differential scanning calorimetry (DSC). The fluorescence spectra, each of which has a single maximum, showed that the wavelength at the maximum intensity (lambda(max)) changed depending on the bilayer state: ca. 440 nm for the lamellar gel (L(beta)' or L(beta)) and the liquid ordered (L(o)) phases, ca. 470 nm for the ripple gel (P(beta)') phase and ca. 490 nm for the liquid crystalline (L(alpha)) phase, respectively. The transition temperatures were determined from the temperature dependences of the lambda(max) and endothermic peaks of the DSC thermograms. Both measurements showed that the pretransition disappears around X(ch)=0.035. The constructed temperature-X(ch) phase diagram indicated that the phase behavior of the binary bilayer membrane at X(ch)< or =0.15 is similar to that of general liquid-solid equilibrium for a binary system where both components are completely miscible in the liquid phase and completely immiscible in the solid phase. It was also revealed that the diagram has two characteristic points: a congruent melting point at X(ch)=0.08 and a peritectic-like point at X(ch)=0.15. The hexagonal lattice model was used for the interpretation of the phase behavior of the binary bilayer membrane. These characteristic compositions well correspond to the bilayer states in each of which cholesterol molecules are regularly distributed in the hexagonal lattice in a different way. That is, each composition of 0.035, 0.08 and 0.15 is nearly equal to that for the binary bilayer membrane which is entirely occupied with units, each composed of a cholesterol and 30 surrounding DSPC molecules within the next-next-next nearest neighbor sites (Unit (1:30): L(beta)(1:30)), with units, each of a cholesterol and 12 surrounding DSPC molecules within the next nearest sites (Unit (1:12): L(beta)(1:12)) or with units, each of a cholesterol and 6 surrounding DSPC molecules at the nearest neighbor sites (Unit (1:6): L(beta)(1:6)), respectively. Therefore, the eutectic behavior observed in the phase diagram was fully explainable in terms of a kind of phase separation between two different types of regions with different types of regular distributions of cholesterol. Further, the L(o) phase was found in the higher X(ch)-region (X(ch)>0.15). No endothermic peak over the temperature range from 10 to 80 degrees C at X(ch)=0.50 suggested that the single L(o) phase can exist at X(ch)>0.50.
我们通过使用6-丙酰基-2-（二甲基氨基）萘（Prodan）的荧光光谱法和差示扫描量热法（DSC），研究了二硬脂酰磷脂酰胆碱（DSPC）-胆固醇二元双层膜的热致相行为与胆固醇组成（X（ch））的关系。荧光光谱中每个都有一个单一的最大值，表明最大强度处的波长（λ（max））根据双层状态而变化：层状凝胶（L（β）'或L（β））和液晶有序（L（o））相约为440 nm，波纹凝胶（P（β）'）相约为470 nm，液晶（L（α））相约为490 nm。从DSC热谱图的λ（max）和吸热峰的温度依赖性确定转变温度。两种测量都表明，在X（ch）=0.035左右预转变消失。构建的温度-X（ch）相图表明，在X（ch）≤0.15时二元双层膜的相行为类似于二元系统的一般液-固平衡，其中两种组分在液相中完全互溶而在固相中完全不互溶。还发现该相图有两个特征点：X（ch）=0.08处的共熔点和X（ch）=0.15处的类包晶点。六边形晶格模型用于解释二元双层膜的相行为。这些特征组成与双层状态很好地对应，在每种双层状态中胆固醇分子以不同方式规则地分布在六边形晶格中。也就是说，0.035、0.08和0.15的每种组成几乎等于二元双层膜中完全被单元占据的组成，每个单元由一个胆固醇和在次近邻位点内的30个周围的DSPC分子组成（单元（1:30）：L（β）（1:30）），或由一个胆固醇和在近邻位点内的12个周围的DSPC分子组成（单元（1:12）：L（β）（1:12）），或由一个胆固醇和在最近邻位点的6个周围的DSPC分子组成（单元（1:6）：L（β）（1:6））。因此，在相图中观察到的共晶行为可以完全根据胆固醇具有不同类型规则分布的两种不同类型区域之间的一种相分离来解释。此外，在较高的X（ch）区域（X（ch）>0.15）中发现了L（o）相。在X（ch）=0.50时，在10至80℃的温度范围内没有吸热峰，这表明在X（ch）>0.50时可以存在单一的L（o）相。
