Department of Chemical Engineering, National Cheng Kung University , Tainan 701, Taiwan.
Langmuir. 2014 Jan 14;30(1):55-62. doi: 10.1021/la403676w. Epub 2013 Dec 24.
Molecular dynamics simulation is applied to investigate the bilayer properties of a novel catanionic vesicle composed of an ion pair amphiphile, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), with cholesterol. Structural properties, such as molecular organization, orientation, and conformation, were analyzed from the resulting trajectory. Simulation results showed that cholesterol could induce both condensing and disordering effects on the rigid HTMA-DS bilayer. The condensing effect of cholesterol was ascribed to the maximizing contact between cholesterol ring and the neighboring hydrocarbon chains. Thus, the inserted cholesterol ring restrained the neighboring hydrocarbon chain segments from motion and increased the order of the neighboring hydrocarbon chains. However, the presence of cholesterol would increase the distance between head groups of HTMA-DS and induce a shift of DS(-) head groups toward the inside of the bilayer. This led to the protrusion of the HTMA(+) head groups and conformational disorder in the front segments of HTMA(+) hydrocarbon chains. In addition, the cholesterol-induced void in the hydrophobic core of the HTMA-DS bilayer increased the motion freedom of the terminal segments of the hydrocarbon chains. The cholesterol-induced space in the polar region and void in the nonpolar region of the bilayer led to a conformational disorder. With high cholesterol contents, the conformational disorder effect would overwhelm the condensing effect, resulting in the apparent disordering effect on the rigid HTMA-DS bilayer.
采用分子动力学模拟方法研究了由离子对两亲分子十六烷基三甲基溴化铵-十二烷基硫酸钠（HTMA-DS）与胆固醇组成的新型混合囊泡的双层性质。从所得轨迹中分析了结构性质，如分子组织、取向和构象。模拟结果表明，胆固醇对刚性 HTMA-DS 双层具有凝结和去有序作用。胆固醇的凝结作用归因于胆固醇环与相邻烃链之间的最大接触。因此，插入的胆固醇环限制了相邻烃链段的运动并增加了相邻烃链的有序性。然而，胆固醇的存在会增加 HTMA-DS 头基之间的距离，并导致 DS(-)头基向双层内部迁移。这导致 HTMA(+)头基的突出和 HTMA(+)烃链前段的构象无序。此外，胆固醇诱导的 HTMA-DS 双层疏水区中的空隙增加了烃链末端段的运动自由度。胆固醇诱导的双层极性区的空间和非极性区的空隙导致构象无序。随着胆固醇含量的增加，构象无序效应会超过凝结效应，导致刚性 HTMA-DS 双层的明显去有序效应。
