Hydration and Charge-Transfer Effects of Alkaline Earth Metal Ions Binding to a Carboxylate Anion, Phosphate Anion, and Guanine Nucleobase

To investigate the ability of alkaline
earth metal ions to tune
ion-mediated DNA adsorption, hydrated Mg2+, Ca2+, Sr2+, and Ba2+ ions bound to a carboxylate
anion, phosphate anion, and guanine nucleobase were modeled using
density functional theory (DFT) and a combined explicit and continuum
solvent model. The large first solvation shell of Ba2+ requires
a larger solute cavity defined by a solvent-accessible surface, which
is used to model all hydrated ions. Alkaline earth metal ions bind
indirectly or directly to each binding site. DFT binding energies
decrease with increasing ion size, which is likely due to ion size
and hydration structure, rather than quantum effects such as charge
transfer. However, charge transfer explains weaker ion binding to
guanine compared to phosphate or carboxylate. Overall, carboxylate
and phosphate anions are expected to compete equally for hydrated
Mg2+, Ca2+, Sr2+, and Ba2+ ions and larger alkaline earth metal ions may induce weaker ion-mediated
adsorption. The ion size and hydration structure of alkaline earth
metal ions may effectively tune ion-mediated adsorption processes,
such as DNA adsorption to functionalized surfaces.

为探究碱土金属离子（alkaline earth metal ions）对离子介导DNA吸附的调控能力，本研究采用密度泛函理论（DFT）结合显式-隐式混合溶剂模型，对结合于羧酸根阴离子、磷酸根阴离子与鸟嘌呤碱基的水合Mg²+、Ca²+、Sr²+及Ba²+进行了建模。钡离子（Ba²+）拥有较大的第一溶剂化壳层，因此需采用溶剂可及表面定义更大的溶质空腔，该建模方案被应用于全部水合离子。碱土金属离子可通过直接或间接方式与各结合位点结合。DFT计算得到的结合能随离子半径增大而降低，该趋势大概率由离子尺寸与水合结构所致，而非电荷转移等量子效应。但相较于磷酸根或羧酸根位点，鸟嘌呤位点的离子结合强度更弱，这一现象可通过电荷转移进行解释。总体而言，羧酸根与磷酸根阴离子对水合Mg²+、Ca²+、Sr²+及Ba²+的结合能力相当；而尺寸更大的碱土金属离子可能会削弱离子介导的吸附过程。综上，碱土金属离子的尺寸与水合结构可有效调控离子介导的吸附过程，例如DNA在功能化表面的吸附。