Deficiency of the Coulomb-field approximation in the generalized Born model: An improved formula for Born radii evaluation

The weaknesses of the widely used Coulomb-field (CF) approximation in the generalized Born model are discussed from the standpoint of the Kirkwood model of biopolymer electrostatics. It is shown that the effective Born radii are overestimated by up to a factor of 2 because of the CF approximation. The problem is made worse by the fact that these critical parameters are used not only to model the self-contributions to the free enthalpy of solvation but they also appear in the analytical function for a pairwise contribution. A new simple formula for the effective Born radii evaluation overcoming some limits of the CF approximation is proposed and then tested on a model of a biopolymer with nonspherical symmetry. Some conclusions regarding the pairwise contribution are also formulated.