PH-Dependence of Protein Stability: Absolute Electrostatic Free Energy Differences between Conformations

Abstract

A method for calculating the absolute electrostatic free energy of a titrating system as a function of pH is proposed, and a concise formula for the free energy is presented. Based on the theory of linked functions, the electrostatic free energy is calculated by integration of the titration curve. The approach uses pH = ∞, at which the system is in the unprotonated state, as the reference pH for the integration. The finite-difference Poisson−Boltzmann method is used for the electrostatic free energy, and the titration curve is obtained by the Monte Carlo approach of Beroza et al.1 The method is applied to the native and denatured state of hen egg-white lysozyme in aqueous solution. A dielectric constant of 20 is assigned to the protein interior, in accord with the work of Antosiewicz et al.2 X-ray structures are used for the native state, and an extended β-structure is used for the unfolded reference state; good agreement with experiment is obtained. Comparison of the results for the extended β-structure ...