Simulation of Ca2+ and Mg2+ solvation using polarizable atomic multipole potential

Area of Science:

• Biomolecular chemistry
• Computational chemistry
• Physical chemistry

Background:

• Alkaline earth metals, calcium (Ca2+) and magnesium (Mg2+), play crucial roles in biological systems.
• Understanding their hydration thermodynamics is essential for comprehending biomolecular processes.

Purpose of the Study:

• To investigate the hydration thermodynamics of Ca2+ and Mg2+ ions.
• To compare simulation methods for ion solvation free energy calculations.
• To analyze the impact of these ions on water structure, dynamics, and dipole moment.

Main Methods:

• Molecular dynamics (MD) simulations using a polarizable potential.
• Particle-mesh Ewald (PME) method for electrostatic interactions.
• Ab initio quantum mechanics for parameterization.
• Free energy perturbation (FEP) and Bennett acceptance ratio (BAR) for solvation free energy.

Main Results:

• The Bennett acceptance ratio method yielded smaller statistical errors for solvation free energy calculations.
• Effective ion sizes in solution were estimated: K+ > Na+ ≈ Ca2+ > Mg2+.
• Divalent cations (Ca2+, Mg2+) induced stronger perturbations in water structure, dynamics, and dipole moment than monovalent ions (K+, Na+).
• Average coordination numbers were 7.3 for Ca2+ and 6 for Mg2+.
• Water molecule lifetimes in the Mg2+ solvation shell (hundreds of picoseconds) differed significantly from Ca2+, K+, and Na+ (few picoseconds).

Conclusions:

• MD simulations with polarizable potentials provide insights into ion hydration thermodynamics.
• The Bennett acceptance ratio method is a statistically robust approach for calculating ion solvation free energies.
• Divalent alkaline earth metal ions exhibit distinct hydration behaviors and significantly influence surrounding water compared to monovalent ions.