Observations on AMBER Force Field Performance under the Conditions of Low pH and High Salt Concentrations

Area of Science:

• Computational chemistry
• Biophysics
• Molecular modeling

Background:

• Molecular dynamics (MD) simulations are crucial for studying biomolecular behavior.
• Simulation reliability depends on time scales and force field accuracy.
• All-atom simulations now reach millisecond timescales, but force field flaws persist.

Purpose of the Study:

• To evaluate the accuracy of the AMBER99SB-ILDN force field under uncommon conditions (acidic pH, high salt concentrations).
• To investigate the impact of extreme conditions on protein refolding simulations.
• To test the physical models of protein, water, and ions in molecular dynamics.

Main Methods:

• Performed microsecond-timescale molecular dynamics simulations of BBL protein refolding.
• Simulations were conducted in aqueous solutions with acidic pH and high salt concentrations (up to 6 M).
• Utilized the AMBER99SB-ILDN force field and TIP3P water model.

Main Results:

• Simulations replicated the experimental trend of salt facilitating refolding.
• Observed formation of nonnative secondary structures (β-sheets in helical regions, α-helices/β-sheets in loop regions).
• Identified potential force field deficiencies affecting loop region dynamics and ion-water interactions at high salt concentrations.

Conclusions:

• The AMBER force field shows limitations under extreme conditions, necessitating further investigation.
• High salt concentrations appear to act via nonspecific electrostatic screening.
• Simulations under extreme conditions are valuable for testing and refining physical models in molecular dynamics.