Comparative CHARMM and AMOEBA Simulations of Lanthanide Hydration Energetics and Experimental Aqueous-Solution Structures

The accurate understanding of metal ion hydration in solutions is a prerequisite for predicting stability, reactivity, and solubility. Herein, additive CHARMM force field parameters were developed to enable molecular dynamics simulations of lanthanide (Ln) speciation in water. Quantitatively similar to the much more resource-intensive polarizable AMOEBA potential, the CHARMM simulations reproduce the experimental hydration free energies and correlations in the first shell (Ln-oxygen distance and hydration number). Comparisons of difference pair-distribution functions obtained from the two simulation approaches with those from high-energy X-ray scattering experiments reveal good agreement of first-coordination sphere correlations for the Lu3+ ion (CHARMM only), but further improvement to both approaches is required to reproduce the broad, non-Gaussian distribution seen from the La3+ experiment. Second-coordination sphere comparisons demonstrate the importance of explicitly including an anion in the simulation. This work describes the usefulness of less resource-intensive additive potentials in some complex chemical systems such as solution environments where multiple interactions have similar energetics. In addition, 3-dimensional descriptions of the La3+ and Lu3+ coordination geometries are extracted from the CHARMM simulations and generally discussed in terms of potential improvements to solute-structure modeling within solution environments.精准理解溶液中金属离子的水合作用，是预测其稳定性、反应活性与溶解性的先决条件。本研究开发了加和型CHARMM力场（CHARMM force field）参数，以实现水溶液中镧系元素（lanthanide, Ln）存在形态的分子动力学模拟（molecular dynamics simulations）。该CHARMM模拟在定量表现上与资源消耗更高的可极化AMOEBA势（AMOEBA potential）高度接近，能够复现实验测得的水合自由能以及第一配位层的相关关联参数（Ln-氧原子距离与配位数）。将两种模拟方法得到的差对对分布函数（pair-distribution function）与高能X射线散射实验结果进行对比后发现，仅采用CHARMM模拟时，Lu³+离子的第一配位层相关参数吻合度良好；但要复现La³+实验中观察到的宽幅非高斯分布，两种模拟方法均需进一步优化。第二配位层的对比分析表明，在模拟中明确引入阴离子具有重要意义。本研究证实了低资源消耗的加和型势函数在部分复杂化学体系中的应用价值，例如在多种相互作用能相近的溶液环境中。此外，本研究从CHARMM模拟中提取了La³+与Lu³+配位几何构型的三维描述，并围绕溶液环境中溶质结构建模的潜在优化方向展开了一般性讨论。