Sensitivities to parameterization in the size-modified Poisson-Boltzmann equation.

Area of Science:

• Biophysics
• Computational Biology
• Physical Chemistry

Background:

• The nonlinear Poisson-Boltzmann equation (NLPBE) is a key model for ion distributions around biomolecules.
• Experimental data on counterion numbers around nucleic acids deviate from NLPBE predictions.
• Existing modifications, like the size-modified Poisson-Boltzmann equation (SMPBE), primarily focus on ion size.

Purpose of the Study:

• To investigate the impact of other physical parameters on ion binding to nucleic acids.
• To evaluate the sensitivity of electrostatic binding free energy derivatives to model parameters.
• To determine the implications for parameterizing biomolecular ion distribution models.

Main Methods:

• Analysis of experimental counterion data against modified Poisson-Boltzmann models.
• Systematic variation of parameters including Stern layer thickness and molecular surface definition.
• Calculation of the derivative of electrostatic binding free energy (SK) with respect to salt concentration.

Main Results:

• Stern layer thickness and molecular surface definition alter bound ion numbers as much as ion size.
• These parameters must be simultaneously fit against experimental data.
• The derivative SK is sensitive to these parameters, offering a route for model parameterization.
• Interior dielectric constant affects electrostatic binding free energy (ΔGel) but not ion distribution.

Conclusions:

• Accurate modeling of ion distributions around biomolecules requires simultaneous fitting of multiple parameters beyond ion size.
• Experimental measurement of SK can aid in parameterizing these models.
• Improvements in ion size and Stern layer thickness may not directly enhance predictions of ΔGel.