Multisite Ion Model in Concentrated Solutions of Divalent Cations ( @entity8723 and XXXX ): Osmotic Pressure Calculations.

@entity141 @entity3282 @entity8723 @entity1025
UNASSIGNED: Accurate force field parameters for ions are essential for meaningful simulation studies of proteins and nucleic acids. Currently accepted models of ions, especially for divalent ions, do not necessarily reproduce the right physiological behavior of @entity141 and @entity1025 ions. Saxena and Sept (J. Chem. Theor. Comput. 2013, 9, 3538-3542) described a model, called the multisite-ion model, where instead of treating the ions as an isolated sphere, the charge was split into multiple sites with partial charge. This model provided accurate inner shell coordination of the ion with biomolecules and predicted better free energies for proteins and nucleic acids. Here, we expand and refine the multisite model to describe the behavior of divalent ions in concentrated @entity8723 and @entity3282 electrolyte solutions, eliminating the unusual ion-ion pairing and clustering of ions which occurred in the original model. We calibrate and improve the parameters of the multisite model by matching the osmotic pressure of concentrated solutions of @entity8723 to the experimental values and then use these parameters to test the behavior of @entity3282 solutions. We find that the concentrated solutions of both divalent ions exhibit the experimentally observed behavior with correct osmotic pressure, the presence of solvent separated ion pairs instead of direct ion pairs, and no aggregation of ions. The improved multisite model for ( @entity1025 and @entity141 ) can be used in classical simulations of biomolecules at physiologically relevant salt concentrations.
