Absolute Hydration Free Energy Scale for Alkali and Halide Ions Established from Simulations with a Polarizable Force Field (original) (raw)
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A polarizable potential function for the hydration of alkali and halide ions is developed on the basis of the recent SWM4-DP water model [G. Lamoureux, A. D. MacKerell, Jr., and B. Roux, J. Chem. Phys., 119, p. 5185, 2003]. Induced polarization is incorporated using classical Drude oscillators that are treated as auxiliary dynamical degrees of freedom. The ions are represented as polarizable Lennard-Jones centers, whose parameters are optimized to reproduce the binding energies of gas-phase monohydrates and the hydration free energies in the bulk liquid. Systematic exploration of the parameters shows that the monohydrate binding energies can be consistent with a unique hydration free energy scale if the computated hydration free energies incorporate the contribution from the air/water interfacial electrostatic potential (−540 mV for SWM4-DP). The final model, which can satisfyingly reproduce both gas and bulk-phase properties, corresponds to an absolute scale in which the intrinsic hydration free energy of the proton is −247 kcal/mol.
Relative Free Energies for Hydration of Monovalent Ions from QM and QM:MM Simulations
Methods directly evaluating the hydration structure and thermodynamics of physiologically relevant cations (Na + , K + , Cl − , etc.) have wide ranging applications in the fields of inorganic, physical, and biological chemistry. All-atom simulations based on accurate potential energy surfaces appear to offer a viable option for assessing the chemistry of ion solvation. Although MD and free energy simulations of ion solvation with classical force fields have proven their usefulness, a number of challenges still remain. One of them is the difficulty of force field benchmarking and validation against structural and thermodynamic data obtained for a condensed phase. Hybrid quantum mechanical/molecular mechanical (QM/MM) models combined with sampling algorithms have the potential to provide an accurate solvation model and to incorporate the effects from the surrounding, which is often missing in gas-phase ab initio computations. Herein, we report the results from QM/MM free energy simulations of Na + /K + and Cl − /Br − hydration where we simultaneously characterized the relative thermodynamics of ion solvation and changes in the solvation structure. The Flexible Inner Region Ensemble Separator (FIRES) method was used to impose a spatial separation between QM region and the outer sphere of solvent molecules treated with the CHARMM27 force field. FEP calculations based on QM/MM simulations utilizing the CHARMM/deMon2k interface were performed with different basis set combinations for K + /Na + and Cl − /Br − perturbations to establish the dependence of the computed free energies on the basis set level. The dependence of the computed relative free energies on the size of the QM and MM regions is discussed. The current methodology offers an accurate description of structural and thermodynamic aspects of the hydration of alkali and halide ions in neat solvents and can be used to obtain thermodynamic data on ion solvation in condensed phase along with underlying structural properties of the ion−solvent system. Figure 11. Oxygen−ion−oxygen ADF for Cl − and Br − simulations with polarizable force field (Drude), QM, and QM/MM systems. Journal of Chemical Theory and Computation Article dx.doi.org/10.1021/ct400296w | J. Chem. Theory Comput. 2013, 9, 4165−4175
Free energy of ion hydration: Interface susceptibility and scaling with the ion size
The Journal of Chemical Physics, 2015
Free energy of solvation of a spherical ion in a force-field water is studied by numerical simulations. The focus is on the linear solvation susceptibility connecting the linear response solvation free energy to the squared ion charge. Spherical hard-sphere solutes, hard-sphere ions, and Kihara solutes (Lennard-Jones modified hard-sphere core) are studied here. The scaling of the solvation susceptibility with the solute size significantly deviates from the Born equation. Using empirical offset corrections of the solute size (or the position of the first peak of the solute-solvent distribution function) do not improve the agreement with simulations. We advance a new perspective on the problem by deriving an exact relation for the radial susceptibility function of the interface. This function yields an effective cavity radius in the Born equation calculated from the solute-solvent radial distribution function. We find that the perspective of the local response, assuming significant al...