Stevens Mark J, Rempe Susan L B
Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, NM 87185, USA.
Phys Chem Chem Phys. 2023 Nov 8;25(43):29881-29893. doi: 10.1039/d3cp04200f.
The interactions of carboxylate anions with water and cations are important for a wide variety of systems, both biological and synthetic. To gain insight on properties of the local complexes, we apply density functional theory, to treat the complex electrostatic interactions, and investigate mixtures with varied numbers of carboxylate anions (acetate) and waters binding to monovalent cations, Li, Na and K. The optimal structure with overall lowest free energy contains two acetates and two waters such that the cation is four-fold coordinated, similar to structures found earlier for pure water or pure carboxylate ligands. More generally, the complexes with two acetates have the lowest free energy. In transitioning from the overall optimal state, exchanging an acetate for water has a lower free energy barrier than exchanging water for an acetate. In most cases, the carboxylates are monodentate and in the first solvation shell. As water is added to the system, hydrogen bonding between waters and carboxylate O atoms further stabilizes monodentate structures. These structures, which have strong electrostatic interactions that involve hydrogen bonds of varying strength, are significantly polarized, with ChelpG partial charges that vary substantially as the bonding geometry varies. Overall, these results emphasize the increasing importance of water as a component of binding sites as the number of ligands increases, thus affecting the preferential solvation of specific metal ions and clarifying Hofmeister effects. Finally, structural analysis correlated with free energy analysis supports the idea that binding to more than the preferred number of carboxylates under architectural constraints are a key to ion transport.
羧酸根阴离子与水和阳离子的相互作用对于多种生物和合成体系都很重要。为了深入了解局部配合物的性质，我们应用密度泛函理论来处理复杂的静电相互作用，并研究含有不同数量羧酸根阴离子（乙酸根）和与单价阳离子Li、Na和K结合的水的混合物。具有总体最低自由能的最优结构包含两个乙酸根和两个水，使得阳离子呈四重配位，这与之前在纯水或纯羧酸根配体中发现的结构类似。更一般地说，含有两个乙酸根的配合物具有最低的自由能。从总体最优状态转变时，用一个乙酸根交换一个水的自由能垒比用水交换一个乙酸根的自由能垒更低。在大多数情况下，羧酸根是单齿的且位于第一溶剂化层。随着水加入体系，水与羧酸根O原子之间的氢键进一步稳定了单齿结构。这些结构具有涉及不同强度氢键的强静电相互作用，显著极化，其ChelpG部分电荷随着键合几何结构的变化而有很大变化。总体而言，这些结果强调了随着配体数量增加，水作为结合位点组成部分的重要性日益增加，从而影响特定金属离子的优先溶剂化并阐明霍夫迈斯特效应。最后，与自由能分析相关的结构分析支持了这样一种观点，即在结构限制下与超过优选数量的羧酸根结合是离子运输的关键。
