Deerfield D W, Nicholas H B, Hiskey R G, Pedersen L G
Department of Chemistry, University of North Carolina, Chapel Hill 27599.
Proteins. 1989;6(2):168-92. doi: 10.1002/prot.340060207.
Equilibrium geometries and binding energies of model "salt" or "ion" bridge systems have been computed by ab initio quantum chemistry techniques (GAUSSIAN82) and by empirical force field techniques (AMBER2.0). Formate and dimethyl phosphate served as anions in the model compounds while interacting with several organic cations, including methyl ammonium, methyl guanidinium, and divalent metal ion (either Mg2+ or Ca2+) without and with an additional chloride; and a divalent metal ion (either Mg2+ or Ca2+), chloride, and four water molecules of hydration about the metal ion. The majority of the quantum chemical computations were performed using a split-valence basis set. For the model compounds studied we find that the ab initio optimized geometries are in remarkably good agreement with the molecular mechanics geometries. Several calculations were also performed using diffuse fractions. The formate anion binds these model cations more strongly than does dimethyl phosphate, while the organic cation methyl ammonium binds model anions more strongly than does methyl guanidinium. Finally, in model compounds including organic anions, Mg2+ or Ca2+ and four molecules of water, and a chloride anion, we find that the equilibrium structure of the magnesium complex involves a solvent separated ion pair (the magnesium ion is six coordinate), whereas the calcium ion complex remains seven coordinate. Molecular mechanics overestimates binding energies, but the estimates may be close enough to actual binding energies to give useful insight into the details of salt bridges in biological systems.
已通过从头算量子化学技术（GAUSSIAN82）和经验力场技术（AMBER2.0）计算了模型“盐”或“离子”桥系统的平衡几何结构和结合能。在模型化合物中，甲酸根和磷酸二甲酯作为阴离子，与几种有机阳离子相互作用，包括甲基铵、甲基胍以及不含和含有额外氯离子的二价金属离子（Mg2+或Ca2+）；以及一个二价金属离子（Mg2+或Ca2+）、氯离子和围绕金属离子的四个水化水分子。大多数量子化学计算使用分裂价基组进行。对于所研究的模型化合物，我们发现从头算优化的几何结构与分子力学几何结构非常吻合。还使用弥散分数进行了几次计算。甲酸根阴离子比磷酸二甲酯更强烈地结合这些模型阳离子，而有机阳离子甲基铵比甲基胍更强烈地结合模型阴离子。最后，在包括有机阴离子、Mg2+或Ca2+、四个水分子和一个氯离子的模型化合物中，我们发现镁配合物的平衡结构涉及溶剂分隔离子对（镁离子为六配位），而钙离子配合物仍为七配位。分子力学高估了结合能，但估计值可能与实际结合能足够接近，从而有助于深入了解生物系统中盐桥的细节。
