Simulation of Ca2+ and Mg2+ Solvation Using Polarizable Atomic Multipole Potential

The alkaline earth metals calcium and magnesium are critically involved in many biomolecular processes. To understand the hydration thermodynamics of these ions, we have performed molecular dynamics simulations using a polarizable potential. Particle-mesh Ewald for point multipoles has been applied to the calculation of electrostatic interactions. The parameters in this model have been determined from an ab initio quantum mechanical calculation of dimer interactions between ions and water. Two methods for ion solvation free energy calculation, free energy perturbation, and the Bennett acceptance ratio have been compared. Both predict results consistent with other theoretical estimations while the Bennett approach leads to a much smaller statistical error. Based on the Born theory and the ion−oxygen radial distribution functions, we estimate the effective size of the ions in solution, concluding that K+ > Na+ ≅ Ca2+ > Mg2+. There appears to be much stronger perturbation in water structure, dynamics, and di...