Ionic Solution: What Goes Right and Wrong with Continuum Solvation Modeling.

Area of Science:

• Computational chemistry
• Molecular modeling
• Biophysics

Background:

• Solvent-mediated electrostatic interactions are crucial for molecular systems.
• Ionic interactions are vital, especially for charged molecules like nucleic acids.
• Poisson-Boltzmann surface area (PBSA) models are widely used for electrostatic calculations.

Purpose of the Study:

• To evaluate the accuracy of PBSA continuum models for ionic interactions.
• To compare PBSA results with explicit solvent simulations and experimental data.
• To identify limitations of current PBSA methods in modeling salt-dependent energetics.

Main Methods:

• Simulated molality-dependent chemical potentials of sodium chloride (NaCl) using SPC/E explicit solvent.
• Utilized free-energy simulations in SPC/E as a benchmark.
• Performed PBSA free-energy calculations using snapshots from explicit solvent simulations.
• Compared linear and nonlinear PB methods with explicit solvent and experimental results.

Main Results:

• High-quality explicit solvent simulations of NaCl agreed well with previous studies and experiments.
• Both linear and nonlinear PB methods reproduced NaCl chemical potentials, with nonlinear PB showing better agreement.
• Salt presence nonlinearly increases hydrophobic effects, consistent with theoretical predictions.
• Lack of molality-dependency in PBSA's non-electrostatic terms significantly reduces accuracy for salt-dependent energetics.

Conclusions:

• Nonlinear PB models offer improved accuracy over linear PB for ionic interactions.
• Current PBSA models require enhancements to accurately capture molality-dependent energetics.
• Further development is needed for robust continuum solvation models of solvent-mediated interactions.