Ion solvation thermodynamics from simulation with a polarizable force field
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Alan Grossfeild Pengyu Ren Jay W. Ponder. Ion Solvation Thermodynamics from Simulation with a Polarizable Force Field . Gaurav Chopra 07 February 2005 CS 379 A. Ion Solvation : Why do we care?. Ion Solvation: Relative stability of ions as a function of solvent and force field

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Ion solvation thermodynamics from simulation with a polarizable force field l.jpg

Alan Grossfeild Pengyu Ren Jay W. Ponder

Ion Solvation Thermodynamics from Simulation with a Polarizable Force Field

Gaurav Chopra

07 February 2005

CS 379 A


Ion solvation why do we care l.jpg
Ion Solvation : Why do we care?

  • Ion Solvation: Relative stability of ions as a function of solvent and force field

  • Surface & environmental chemistry

  • Study of molecules such as surfactants, colloids and polyelectrolyte

  • Biologically: Structure and function of nucleic acids, proteins and lipid membranes

  • Thermodynamics: Development of continuum solvation models – Interested in Free Energy of Solvation for individual ionic species


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Why Simulate?

  • Motivation: Solvation free energy of salts known experimentally but cannot separate into individual contributions of ions

  • Molecular dynamics used to resolve this using Polarizable Force Field (AMOEBA)

  • Simulations with CHARMM27 and OPLS-AA done for comparison

  • Ions: K+, Na+ and Cl-

  • Solvent: Water (TIP3P model for non-polarizable force field) and Formamide


Molecular model and force field l.jpg
Molecular Model and Force Field

N-body Problem

Non-bonded two body interactions

Inclusion of Polarization: e.g. binding of a charged ligand polarizes receptor part

-by inducing point dipoles

-by changing the magnitude of atomic charges

-by changing the position of atomic charges

3N x 3N Matrix


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Summary of the paper

  • Experiments and standard molecular mechanics force fields (non-polarizable) cannot give correct values for ion solvation free energy for an ion

  • AMOEBA parameters reproduce in vacuo quantum mechanical results, experimental ion-cluster solvation enthalpies, and experimental solvation energies for whole salt

  • Result: Best estimation of ion-solvation free energy for ions using AMOEBA


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  • AMOEBA VdW parameters:

  • High-level QM (Na+, K+)

  • Experimental Cluster Hydration enthalpies combined with solvent parameters using neat-liquid and gas-phase cluster simulation (Cl-)


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Force Field Parameters

  • AMOEBA Force Field

  • Each atom has a permanent partial charge, dipole and quadrupole moment

  • Represents electronic many-body effects

  • Self-consistent dipole polarization procedure

  • Repulsion-dispersion interaction between pairs of non-bonded atoms uses buffered 14-7 potential

  • AMOEBA dipole Polarizabilities of Potassium, sodium and chloride ions is set to 0.78, 0.12 and 4.00 cubic Ang.


Cluster calculations l.jpg
Cluster Calculations

  • Stochastic Molecular dynamics of clusters of 1-6 water molecules with a single chloride ion

  • Velocity Verlet implementation of Langevin dynamics used to integrate equations of motion

Hydration enthalpy of water molecules

n = number of water molecules

<E(n,Cl)> = average potential energy over simulations with n waters and a chloride ion


Molecular dynamics and free energy simulation l.jpg
Molecular Dynamics and Free Energy Simulation

For each value of l energy minimization is performed until RMS gradient per atom is less than 1 kcal/(mol A)

  • AMOEBA took more than 7 days, OPLS-AA and CHARMM27 took less than a day

  • Final structure for l = 1 particle growth simulation used as starting structure for each trajectory in the charging portion

E = potential energy of system

Statistical Uncertainity

N = number of points in time series

s = statistical efficiency


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Ion Solvent Dimers Results

  • Gas-phase behavior gives ion-solvent interaction without statistical sampling

  • High level QM only possible for gas phase unless implicit solvent model used

  • Table 2: Overestimated values

  • Ion-oxygen separation > 2.3 Ang.: less electrostatic attraction than TIP3P water

  • Molecular orbital calculations problematic for chloride



Chloride water clusters results l.jpg
Chloride-Water Clusters Results

Van der Waals parameters for chloride ion compared with enthalpy of formation of chloride-water dimer as molecular orbital calculations is problematic



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Solvent Structure around ions

g(r) = radial distribution function


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To quote Albert Einstein:

The properties of water [and aqueous solutions] are not only strange but perhaps stranger than what we can conceive

Q & A


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