Role of Charge Regulation and Fluctuations in the Conformational and Mechanical Properties of Weak Flexible Polyelectrolytes
Role of Charge Regulation and Fluctuations in the Conformational and Mechanical Properties of Weak Flexible Polyelectrolytes
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DOI：
10.3390/polym11121962
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发表时间：
2019-12-01
期刊：
影响因子：
5
通讯作者：
Garces, Josep L.
中科院分区：
文献类型：
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作者：
Blanco, Pablo M.;Madurga, Sergio;Garces, Josep L.
这项工作解决了电荷调节(CR)的作用以及与之相关的弱聚电解质构象和机械性能的波动。由于CR，pH值的变化改变了平均大分子电荷和构象平衡。第二个效应是，对于给定的每个位点的平均电荷，波动可以通过结合位点之间的关联来改变相互作用的强度。我们用蒙特卡罗模拟的方法研究了这两种效应，在恒定pH值下，电荷是一个起伏的量。一旦每个站点的平均费用可用，我们就可以通过向每个站点分配相同的平均费用来关闭波动。然后进行了恒电荷MC模拟。我们使用了一个模型，该模型考虑了线性柔性聚电解质的主要基本方面，即质子结合、角度内旋转、键拉伸和弯曲。空间排斥体积和区分治疗的短期和远程相互作用也包括在内。这个模型可以被认为是一种“极小”，因为它包含了最少的参数，但仍然保留了原子的细节。结果表明，如果涨落被激活，Guche态键几率增加，持续长度减小，从而使聚合物变得更折叠。还分析了存在CR(电荷取决于所施加的力)和不存在CR(电荷固定为零力时的值)时的大分子拉伸。对低力标度行为的分析表明，Pincus指数变得与pH有关。无论有无CR，都观察到了从高pH值(幻影链)的1/2到低pH值(Pincus区)的3/5的转变。最后，对中间力拉伸机制进行了研究。研究发现，CR对力-伸长曲线和持久长度有一定的影响(在这个力区域内，这两个参数变得与力有关)。因此，CR对拉伸曲线的影响主要是由于零力下平均电荷的变化。研究还发现，对于所研究的情况，与静电相互作用相比，空间排斥体积的影响几乎是不相关的。
This work addresses the role of charge regulation (CR) and the associated fluctuations in the conformational and mechanical properties of weak polyelectrolytes. Due to CR, changes in the pH-value modifies the average macromolecular charge and conformational equilibria. A second effect is that, for a given average charge per site, fluctuations can alter the intensity of the interactions by means of correlation between binding sites. We investigate both effects by means of Monte Carlo simulations at constant pH-value, so that the charge is a fluctuating quantity. Once the average charge per site is available, we turn off the fluctuations by assigning the same average charge to every site. A constant charge MC simulation is then performed. We make use of a model which accounts for the main fundamental aspects of a linear flexible polyelectrolyte that is, proton binding, angle internal rotation, bond stretching and bending. Steric excluded volume and differentiated treatment for short-range and long-range interactions are also included. This model can be regarded as a kind of "minimal" in the sense that it contains a minimum number of parameters but still preserving the atomistic detail. It is shown that, if fluctuations are activated, gauche state bond probabilities increase and the persistence length decreases, so that the polymer becomes more folded. Macromolecular stretching is also analyzed in presence of CR (the charge depends on the applied force) and without CR (the charge is fixed to the value at zero force). The analysis of the low force scaling behavior concludes that Pincus exponent becomes pH-dependent. Both, with and without CR, a transition from 1/2 at high pH-values (phantom chain) to 3/5 at low pH-values (Pincus regime) is observed. Finally, the intermediate force stretching regime is investigated. It is found that CR induces a moderate influence in the force-extension curves and persistence length (which in this force regime becomes force-dependent). It is thus concluded that the effect of CR on the stretching curves is mainly due to the changes in the average charge at zero force. It is also found that, for the cases studied, the effect of steric excluded volume is almost irrelevant compared to electrostatic interactions.