Zaraiskaya T, Jeffrey K R
Department of Physics, University of Guelph, Ontario, Canada.
Biophys J. 2005 Jun;88(6):4017-31. doi: 10.1529/biophysj.104.054601. Epub 2005 Mar 11.
Molecular dynamics simulations were performed on a two-component lipid bilayer system in the liquid crystalline phase at constant pressure and constant temperature. The lipid bilayers were composed of a mixture of neutral galactosylceramide (GalCer) and charged dipalmitoylphosphatidylglycerol (DPPG) lipid molecules. Two lipid bilayer systems were prepared with GalCer:DPPG ratio 9:1 (10%-DPPG system) and 3:1 (25%-DPPG system). The 10%-DPPG system represents a collapsed state lipid bilayer, with a narrow water space between the bilayers, and the 25%-DPPG system represents an expanded state with a fluid space of approximately 10 nm. The number of lipid molecules used in each simulation was 1024, and the length of the production run simulation was 10 ns. The simulations were validated by comparing the results with experimental data for several important aspects of the bilayer structure and dynamics. Deuterium order parameters obtained from (2)H NMR experiments for DPPG chains are in a very good agreement with those obtained from molecular dynamics simulations. The surface area per GalCer lipid molecule was estimated to be 0.608 +/- 0.011 nm(2). From the simulated electron density profiles, the bilayer thickness defined as the distance between the phosphorus peaks across the bilayer was calculated to be 4.21 nm. Both simulation systems revealed a tendency for cooperative bilayer undulations, as expected in the liquid crystalline phase. The interaction of water with the GalCer and DPPG oxygen atoms results in a strong water ordering in a spherical hydration shell and the formation of hydrogen bonds (H-bonds). Each GalCer lipid molecule makes 8.6 +/- 0.1 H-bonds with the surrounding water, whereas each DPPG lipid molecule makes 8.3 +/- 0.1 H-bonds. The number of water molecules per GalCer or DPPG in the hydration shell was estimated to be 10-11 from an analysis of the radial distribution functions. The formation of the intermolecular hydrogen bonds was observed between hydroxyl groups from the opposing GalCer sugar headgroups, giving an energy of adhesion in the range between -1.0 and -3.4 erg/cm(2). We suggest that this value is the contribution of the hydrogen-bond component to the net adhesion energy between GalCer bilayers in the liquid crystalline phase.
在恒压恒温条件下，对处于液晶相的双组分脂质双层系统进行了分子动力学模拟。脂质双层由中性半乳糖神经酰胺（GalCer）和带电的二棕榈酰磷脂酰甘油（DPPG）脂质分子混合物组成。制备了两种脂质双层系统，GalCer与DPPG的比例分别为9:1（10%-DPPG系统）和3:1（25%-DPPG系统）。10%-DPPG系统代表一种塌陷状态的脂质双层，双层之间的水空间狭窄，而25%-DPPG系统代表一种扩展状态，具有约10 nm的流体空间。每次模拟中使用的脂质分子数量为1024，生产运行模拟的时长为10 ns。通过将结果与双层结构和动力学的几个重要方面的实验数据进行比较，对模拟进行了验证。从DPPG链的（2）H NMR实验获得的氘序参数与从分子动力学模拟获得的参数非常吻合。每个GalCer脂质分子的表面积估计为0.608±0.011 nm²。根据模拟的电子密度分布，将双层厚度定义为横跨双层的磷峰之间的距离，计算得出为4.21 nm。正如在液晶相中所预期的那样，两个模拟系统都显示出协同双层波动的趋势。水与GalCer和DPPG氧原子的相互作用导致在球形水合壳中形成强烈的水有序排列并形成氢键（H键）。每个GalCer脂质分子与周围的水形成8.6±0.1个H键，而每个DPPG脂质分子形成8.3±0.1个H键。通过对径向分布函数的分析，估计水合壳中每个GalCer或DPPG的水分子数量为10 - 11个。在相对的GalCer糖头基的羟基之间观察到分子间氢键的形成，其粘附能在-1.0至-3.4 erg/cm²范围内。我们认为该值是氢键成分对液晶相中GalCer双层之间净粘附能的贡献。
