Parasassi T, Giusti A M, Raimondi M, Gratton E
Istituto di Medicina Sperimentale, Consiglio Nazionale delle Richerche, Roma, Italy.
Biophys J. 1995 May;68(5):1895-902. doi: 10.1016/S0006-3495(95)80367-X.
The fluorescence generalized polarization (GP) of 2-dimethylamino-6-lauroylnaphthalene (Laurdan) reveals different effects of cholesterol on the phase behavior of phospholipid bilayers. Phospholipid vesicles composed of gel, liquid-crystalline, and coexisting domains of the two phases have been studied at temperatures from 1 to 65 degrees C, without cholesterol and with cholesterol concentrations of 3-50 mol %. Laurdan GP measurements show the general effect of cholesterol of increasing the molecular dynamics of the gel and of decreasing the molecular dynamics of the liquid-crystalline phase. In the liquid-crystalline phase, the increased order yields Laurdan GP values close to those obtained in the gel phase. At cholesterol concentrations > 15 mol % a phase transition cannot be detected. Using the wavelength dependence of the excitation and emission GP spectra we determine that differences between the two phospholipid phases cannot be detected. In particular, in vesicles composed of coexisting gel and liquid-crystalline phases the GP wavelength dependence characteristic of coexisting domains cannot be observed at cholesterol concentrations > or = 15 mol %. Cholesterol causes the decrease in both the polarity and the dipolar relaxation effects on the neighborhood of the fluorescent naphthalene moiety of Laurdan. Probably because of a cholesterol-induced increase in the bilayer packing, these effects do not occur continuously with the increase of cholesterol concentration in the bilayer. Cholesterol concentrations inducing higher Laurdan GP values have been determined at about 5, 10, 15, 30, and 45 mol % with respect to phospholipids. We propose that the formation of ordered molecular microdomains at critical cholesterol concentrations can explain the occurrence of the observed discontinuities.
2-二甲基氨基-6-月桂酰萘（Laurdan）的荧光广义极化（GP）揭示了胆固醇对磷脂双层相行为的不同影响。在1至65摄氏度的温度范围内，研究了由凝胶相、液晶相以及这两个相共存区域组成的磷脂囊泡，实验分为无胆固醇以及胆固醇浓度为3 - 50摩尔%的情况。Laurdan GP测量结果表明，胆固醇的总体作用是增加凝胶相的分子动力学，并降低液晶相的分子动力学。在液晶相中，分子排列的增加使得Laurdan GP值接近在凝胶相中获得的值。当胆固醇浓度>15摩尔%时，无法检测到相变。利用激发和发射GP光谱的波长依赖性，我们确定无法检测到两个磷脂相之间的差异。特别是，在由凝胶相和液晶相共存组成的囊泡中，当胆固醇浓度≥15摩尔%时，无法观察到共存区域特有的GP波长依赖性。胆固醇导致对Laurdan荧光萘部分周围环境的极性和偶极弛豫效应均降低。可能是由于胆固醇诱导的双层堆积增加，这些效应不会随着双层中胆固醇浓度的增加而持续出现。相对于磷脂，已确定在约5、10、15、30和45摩尔%的胆固醇浓度下会诱导出更高的Laurdan GP值。我们提出，在临界胆固醇浓度下形成有序分子微区可以解释所观察到的不连续性的出现。
