Dipartimento di Scienza ed Alta Tecnologia , Università degli Studi dell'Insubria , via Valleggio 11 , 22100 Como , Italy.
Dipartimento di Biotecnologie e Scienze della Vita , Università degli Studi dell'Insubria , via J. H. Dunant 3 , 21100 Varese , Italy.
J Phys Chem B. 2019 Oct 24;123(42):8872-8888. doi: 10.1021/acs.jpcb.9b06017. Epub 2019 Oct 15.
In this work, we performed titration simulations of weak linear polyelectrolytes via the Monte Carlo method and the constant pH ensemble aiming to understand how polyelectrolyte concentration, chain rigidity, and the formation of intra- and inter-chain charged hydrogen bonds (c-H-bonds) impact on ionization and conformations of polyacidic species, counterions (CIs) distribution, and system Helmholtz energy. Increasing polyelectrolyte concentration resulted in enhanced acidity for all cases investigated due to the increased screening of chain charges by CIs and, when possible, the formation of interchain c-H-bonds. Our simulations also evidenced that polyelectrolytes able to form c-H-bonds can populate simultaneously two conformational states (aggregated and unfolded) in a range of pH, the transition between the two appearing first order-like. To better understand how properties of two polyelectrolytic chains are modified by their relative distance, we performed window sampling (WS) simulations, which highlighted nontrivial features in the ionization and conformational behaviors. As byproducts of WS simulations, we obtained also the potential of mean force between two chains; from this, it emerges that the reversible work needed to reach a specific interchain distance does not always increase with the pH, especially for c-H-bonds forming semirigid chains brought at short distances.
在这项工作中，我们通过蒙特卡罗方法和恒 pH 系综对弱线性聚电解质进行了滴定模拟，旨在了解聚电解质浓度、链刚性以及形成链内和链间带电氢键 (c-H-bonds) 如何影响聚酸物种的离解和构象、抗衡离子 (CIs) 的分布以及系统亥姆霍兹能量。增加聚电解质浓度会导致所有研究案例的酸度增强，这是由于 CIs 对链电荷的屏蔽增强，并且在可能的情况下形成链间 c-H-bonds。我们的模拟还表明，能够形成 c-H-bonds 的聚电解质可以在一定 pH 范围内同时占据两种构象状态（聚集和展开），这两种状态之间的转变呈一级类似物。为了更好地理解两条聚电解质链的性质如何被它们的相对距离所改变，我们进行了窗口采样 (WS) 模拟，这突出了离解和构象行为中的重要特征。作为 WS 模拟的副产品，我们还获得了两链之间的平均力势；从中可以看出，达到特定链间距离所需的可逆功并不总是随 pH 增加而增加，尤其是对于形成半刚性链并在短距离内被带入的 c-H-bonds。
