Department of Physics, North Dakota State University, Fargo, ND 58105-5566, USA.
Biophys Chem. 2010 Aug;150(1-3):112-8. doi: 10.1016/j.bpc.2010.01.012. Epub 2010 Feb 6.
Nonideal mixing provides the physical basis of domain formation and macroscopic phase separation in lipid bilayers. We present a model for the electrostatic contribution to the nonideality of a two-component acidic-zwitterionic lipid membrane. Our model is based on the mean-field Poisson-Boltzmann approach; it includes a protonation/deprotonation equilibrium applicable to acidic lipids such as phosphatidic acid or phosphatidylglycerol. It also includes an electrostatic model for zwitterionic lipids such as phosphatidylcholine or phosphatidylethanolamine that accounts for the spatial separation of the two headgroup charges and the orientational freedom of the headgroup. Modeling the nonelectrostatic contribution to the free energy using the Bragg-Williams approximation of a binary lattice gas enables us to compute binodal lines that reflect the influence of membrane electrostatics on the nonideal mixing properties of the bilayer. If we neglect the zwitterionic lipid's electrostatic contribution, then increasing pH is predicted to always oppose demixing, stabilizing the membrane due to the repulsion between the charged lipids. If the electrostatic properties of the zwitterionic lipids are accounted for, this opposing tendency weakens and may even reverse. In this case, increasing the fraction of charged lipids through increasing pH would, somewhat unexpectedly, promote domain formation. We discuss the corresponding physical mechanism.
非理想混合为双层脂膜的域形成和宏观相分离提供了物理基础。我们提出了一种用于描述两亲性酸性-两性离子脂质膜非理想性的静电贡献模型。我们的模型基于平均场泊松-玻尔兹曼方法；它包括适用于酸性脂质（如磷脂酸或磷脂酰甘油）的质子化/去质子化平衡。它还包括一种针对两性离子脂质（如磷脂酰胆碱或磷脂酰乙醇胺）的静电模型，该模型考虑了两个头基电荷的空间分离和头基的取向自由度。使用二元格子气体的布拉格-威廉姆斯近似来模拟非静电自由能贡献，使我们能够计算出反映膜静电对双层非理想混合性质影响的双节点线。如果我们忽略两性离子脂质的静电贡献，那么增加 pH 值通常会反对相分离，由于带电脂质之间的排斥作用，从而稳定膜。如果考虑到两性离子脂质的静电特性，这种相反的趋势会减弱，甚至可能逆转。在这种情况下，通过增加 pH 值增加带电脂质的分数，会出人意料地促进域形成。我们讨论了相应的物理机制。
