Department of Physics , Carnegie Mellon University , Pittsburgh , Pennsylvania 15213 , United States.
Laboratory of Computational Biology , National Heart, Lung, and Blood Institute, National Institutes of Health , Bethesda , Maryland 20892 , United States.
J Phys Chem B. 2019 Mar 28;123(12):2697-2709. doi: 10.1021/acs.jpcb.8b12010. Epub 2019 Mar 18.
In addition to obtaining the highly precise volumes of lipids in lipid bilayers, it has been desirable to obtain the volumes of parts of each lipid, such as the methylenes and terminal methyls on the hydrocarbon chains and the head group. Obtaining such component volumes from experiment and from simulations is re-examined, first by distinguishing methods based on apparent versus partial molar volumes. Although somewhat different, both these methods give results that are counterintuitive and that differ from results obtained by a more local method that can only be applied to simulations. These comparisons reveal differences in the average methylene component volume that result in larger differences in the head group component volumes. Literature experimental volume data for unsaturated phosphocholines and for alkanes have been used and new data have been acquired for saturated phosphocholines. Data and simulations cover extended ranges of temperature to assess both the temperature and chain length dependence of the component volumes. A new method to refine the determination of component volumes is proposed that uses experimental data for different chain lengths at temperatures guided by the temperature dependence determined in simulations. These refinements enable more precise comparisons of the component volumes of different lipids and alkanes in different phases. Finally, the notion of free volume is extended to components using the Lennard-Jones radii to estimate the excluded volume of each component. This analysis reveals that head group free volumes are relatively independent of thermodynamic phase, whereas both the methylene and methyl free volumes increase dramatically when bilayers transition from gel to fluid.
除了获取脂质双层中脂质的高度精确体积外，还需要获取每个脂质部分的体积，例如烃链上的亚甲基和末端甲基以及头基。通过区分基于表观摩尔体积和偏摩尔体积的方法，重新检查了从实验和模拟中获取这些组分体积的方法。虽然这两种方法略有不同，但它们都给出了与更局部方法获得的结果相反的结果，更局部的方法只能应用于模拟。这些比较揭示了平均亚甲基组分体积的差异，导致头基组分体积的差异更大。使用了未饱和磷脂酰胆碱和烷烃的文献实验体积数据，并为饱和磷脂酰胆碱获得了新数据。数据和模拟涵盖了较宽的温度范围，以评估组分体积的温度和链长依赖性。提出了一种新的方法来改进组分体积的确定，该方法使用不同链长在模拟确定的温度依赖性指导下的温度下的实验数据。这些改进使得能够更精确地比较不同相中和不同烷烃中的不同脂质的组分体积。最后，使用 Lennard-Jones 半径将自由体积的概念扩展到组分，以估计每个组分的排斥体积。这种分析表明，头基自由体积相对独立于热力学相，而当双层从凝胶相转变为流动相时，亚甲基和甲基自由体积都会急剧增加。
