Empirical force fields for biologically active divalent metal cations in water

Area of Science:

• Computational Chemistry
• Biophysics
• Materials Science

Background:

• Accurate simulation of metal ions in aqueous solution is crucial for understanding biological and chemical processes.
• Existing ion-water van der Waals (vdW) parameters often fail to capture microscopic solvent effects, limiting simulation accuracy.
• Developing robust parameters is essential for modeling complex systems like metalloproteins.

Purpose of the Study:

• To present a novel strategy for deriving ion-water vdW parameters that implicitly include solvent molecular effects.
• To obtain accurate vdW parameters for 24 divalent metal cations using experimental hydration free energies.
• To validate the derived parameters by reproducing observed hydration free energies, coordination numbers (CNs), and ion-water distances.

Main Methods:

• A numerical procedure linking a coupling parameter in free energy simulations with experimental hydration free energies was employed.
• The strategy incorporated first-shell coordination numbers (CNs) and structural data for divalent cations.
• Van der Waals parameters were derived for 24 divalent metal ions, covering a range of hydration free energies, CNs, and ion-water distances.

Main Results:

• A set of ion-water vdW parameters (MWc) was obtained that simultaneously reproduces experimental hydration free energies, first-shell CNs, and average ion-water distances for all studied dications.
• The MWc parameters accurately captured variations in CN, such as the decrease from Cu2+ (CN=6) to Be2+ (CN=4), and the expansion for Ba2+ (CN=9.5).
• The derived parameters successfully reproduced the observed trend of no change in CN for dications with intermediate hydration free energies.

Conclusions:

• The developed strategy provides accurate ion-water vdW parameters that implicitly account for microscopic solvent effects.
• The MWc parameter set represents a significant advancement for simulating metal ion interactions in aqueous environments.
• These parameters are a foundational step towards more accurate simulations of metalloproteins, though further development including polarizability and charge transfer is needed.