Salt or ion bridges in biological system: A study employing quantum and molecular mechanics

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 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 geometries are in remarkably goodagreement with the molecular mechanics geometries.Several Calculations werealso performed using diffuse fractions. The formate anion binds these modelcations more strongly than does dimethyl phosphate, while the organiccation 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 togive useful insight into the details energies to give useful insight into the details of salt bridges in biological systems.