Chemical and Biomolecular Engineering Department, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
J Phys Chem B. 2011 Apr 28;115(16):4689-95. doi: 10.1021/jp201085c. Epub 2011 Apr 4.
We present the results of coarse grained molecular dynamics simulation using a charge free model that is able to capture different regions of the morphological phase diagram of charged diblock copolymers. Specifically, we were able to reproduce many phases of the poly(acrylic acid)-(1,4)-polybutadiene (PAA-PBA) diblock copolymer, Ca(2+) and water systems as a function of pH and calcium concentration with short-range LJ type potentials. The morphologies observed range from bilayers to cylinders to spherical micelles. Such polyanionic/cationic amphiphiles in water typically present multiple challenges for molecular simulations, particularly due to the many charge interactions that are long ranged and computationally costly. Further, it is precisely these interactions that are thought to modulate large amphiphile assemblies of interest such as lipid rafts. However, our model is able to reproduce different morphologies due to pH and with or without the addition of Ca(2+) as well as the lateral phase segregation and the domain registration observed in neutral and charged diblock copolymer mixtures. The results suggest that the overall effect of charges is a local structural rearrangement that renormalizes the steric repulsion between the headgroups. This simple model, which is devoid of charges, is able to reproduce the complex phase diagram and can be used to investigate collective phenomena in these charged systems such as domain formation and registration or colocalization of lipid rafts across bilayer leaflets.
我们呈现了使用无电荷模型进行粗粒分子动力学模拟的结果，该模型能够捕捉带电嵌段共聚物形态相图的不同区域。具体来说，我们能够在短程 LJ 型势的作用下，以 pH 和钙离子浓度为函数，重现聚（丙烯酸）-（1,4）-聚丁二烯（PAA-PBA）嵌段共聚物、Ca（2+）和水体系的许多相。观察到的形态范围从双层到圆柱到球形胶束。在水中，这种聚阴离子/阳离子两亲物通常对分子模拟提出了许多挑战，特别是由于许多电荷相互作用是远程的且计算成本高。此外，正是这些相互作用被认为调节了脂质筏等感兴趣的大的两亲物组装体。然而，我们的模型能够由于 pH 值以及是否添加 Ca（2+）以及在中性和带电嵌段共聚物混合物中观察到的横向相分离和域注册来重现不同的形态。结果表明，电荷的总体影响是一种局部结构重排，它重新调整了头基之间的空间排斥。这个没有电荷的简单模型能够重现复杂的相图，并可用于研究这些带电系统中的集体现象，例如域形成和注册或脂质筏在双层叶片中的共定位。
