Faculty of Health Sciences, University of Ljubljana, SI-1000 Ljubljana, Slovenia.
Phys Chem Chem Phys. 2012 Aug 14;14(30):10613-21. doi: 10.1039/c2cp40923b. Epub 2012 Jun 29.
Certain lipid monolayers at the air-water interface undergo a second-order transition from a tilted to an untilted liquid-crystalline state of their lipid hydrocarbon chains at sufficiently large lateral pressure. Recent experimental observations demonstrate that in the presence of divalent cations DNA adsorbs onto a zwitterionic lipid monolayer and decreases the tilt transition pressure. Lowering of the tilt transition pressure indicates that the DNA condenses the lipid monolayer laterally. To rationalize this finding we analyze a theoretical model that combines a phenomenological Landau approach with an extension of the Poisson-Boltzmann model to zwitterionic lipids. Based on numerical calculations of the mean-field electrostatic free energy of a zwitterionic lipid monolayer-DNA complex in the presence of divalent cations, we analyze the thermodynamic equilibrium of DNA adsorption. We find that adsorbed DNA induces a 10% reduction of the electrostatic contribution to the lateral pressure exerted by the monolayer. This result implies a small but notable decrease in the tilt transition pressure. Additional mechanisms due to ion-ion correlations and headgroup reorientations are likely to further enhance this decrease.
在足够大的侧向压力下，某些位于气液界面的单层脂质会经历从倾斜的液晶态到非倾斜的液晶态的二级相变。最近的实验观察表明，在二价阳离子存在的情况下，DNA 会吸附到两性离子脂质单层上，并降低倾斜相变压力。倾斜相变压力的降低表明 DNA 使脂质单层横向凝聚。为了使这一发现合理化，我们分析了一个理论模型，该模型将唯象的朗道方法与泊松-玻尔兹曼模型对两性离子脂质的扩展相结合。基于对存在二价阳离子时两性离子脂质单层-DNA 复合物的平均场静电自由能的数值计算，我们分析了 DNA 吸附的热力学平衡。我们发现，吸附的 DNA 会使单层所受侧向压力的静电贡献降低 10%。这一结果意味着倾斜相变压力会有一个微小但显著的降低。由于离子-离子相关性和头基重定向等额外机制，这种降低可能会进一步增强。
