Tamai Nobutake, Uemura Maiko, 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.
Colloids Surf B Biointerfaces. 2008 Sep 1;65(2):213-9. doi: 10.1016/j.colsurfb.2008.04.004. Epub 2008 Apr 16.
We investigated the phase behavior of cholesterol/diheptadecanoylphosphatidylcholine (C17:0-PC) 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 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 and ca. 490 nm for the liquid-crystalline (L(alpha)) phase. The transition temperatures were determined from the temperature dependence of lambda(max) and endothermic peaks of the DSC thermograms. Both measurements showed that the pre- and main transition disappear around X(ch)=0.05 and 0.30, respectively. The constructed temperature-X(ch) phase diagram resembled a typical phase diagram for a eutectic binary mixture containing a peritectic point. The presence of a peritectic point at X(ch)=0.15 suggested that a complex of cholesterol and C17:0-PC is stoichiometrically formed in the gel phase. Consideration based on the hexagonal lattice model revealed that the compositions of 0.05 and 0.15 correspond to the bilayer states where cholesterol molecules are regularly distributed in different ways. The former is nearly equal to the composition for the membrane occupied entirely with Units (1:18), composed of a cholesterol and 18 surrounding C17:0-PC molecules within the next-next nearest neighbor sites. The latter is represented by a Unit (1:6), including a cholesterol and 6 surrounding C17:0-PC molecules. Further, the disappearance of the main transition at X(ch)=0.30 indicates that the pure L(o) phase can exist in X(ch)>0.30. The eutectic behavior observed in the phase diagram was explainable in terms of phase separation between two different types of regions with different types of regular distributions of cholesterol.
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），研究了胆固醇/二庚酰磷脂酰胆碱（C17:0-PC）二元双层膜的相行为随胆固醇组成（X(ch)）的变化。荧光光谱表明，最大强度处的波长（lambda(max)）根据双层状态而变化：层状凝胶（L'(β)或L(β)）相和液晶有序（L(o)）相时约为440 nm，液晶（L(α)）相时约为490 nm。通过lambda(max)的温度依赖性和DSC热谱图的吸热峰确定转变温度。两种测量结果均表明，预转变和主转变分别在X(ch)=0.05和0.30左右消失。构建的温度-X(ch)相图类似于含有包晶点的共晶二元混合物的典型相图。在X(ch)=0.15处存在包晶点表明，在凝胶相中化学计量地形成了胆固醇和C17:0-PC的复合物。基于六角晶格模型的考虑表明，0.05和0.15的组成对应于胆固醇分子以不同方式规则分布的双层状态。前者几乎等于完全由单元（1:18）占据的膜的组成，该单元由一个胆固醇和在次近邻位点内的18个周围的C17:0-PC分子组成。后者由一个单元（1:6）表示，包括一个胆固醇和6个周围的C17:0-PC分子。此外，在X(ch)=0.30时主转变的消失表明，在X(ch)>0.30时可以存在纯L(o)相。相图中观察到的共晶行为可以用胆固醇具有不同规则分布的两种不同类型区域之间的相分离来解释。
