The nonlocal dielectric approach can significantly enhance the classical Poisson dielectric model by including polarization correlations among water molecules. In this paper, a nonlocal dielectric model for protein in ionic solvent is proposed and analyzed, alongside a new efficient numerical algorithm and program package for solving the model. In particular, by using solution splitting and reformulation techniques, it is shown that the solution of the nonlocal dielectric model is unique, and can be found from solving two well-defined partial differential systems and one Poisson-like boundary value problem. Consequently, the singular and computational difficulties caused by Dirac delta distributions and convolution terms are overcome. Furthermore, a nonlocal linearized Poisson-Boltzmann equation with uniform ionic size effect is proposed and numerically tested on three protein molecules with up to 6062 atoms by using a fast finite element solver from the FEniCS project. A nonlocal point charge Born model with a known analytical solution is also tested to validate the new algorithm and program package. Numerical results demonstrate the high performance of the program package, and confirm one advantage of the new algorithm in retaining a high order of accuracy of finite element approximations.
Introduction.Calculation of electrostatic potential energy for proteins in ionic solvent is a fundamental task in the simulation study of the structure and biological function of proteins, catalytic activity, and ligand association [17,25,31]. One commonly used mathematical model for estimating the electrostatic potential function in an ionic solvent is the Poisson-Boltzmann equation (PBE) [2,13,16,22,23,27,37]. But, due to the polarization correlations among water molecules and ionic size effect [21], the PBE model may not work well in some important bioengineering applications (such as ion channel studies and rational drug design). To reflect the polarization correlations among water molecules, several nonlocal dielectric models have been developed for a wide range of dielectric materials and dipolar liquids in the last thirty years [4,5,6,7,8,10,19,20,28,30]. Recent progress in the development of fast numerical algorithms has sharply reduced the complexity of solving a nonlocal dielectric model [15,33,35,36], making it possible for a nonlocal dielectric model to be applied to large scale biomolecular simulations.However, the study of a nonlocal model has been limited to the case of pure water solvent so far due to modeling and algorithmic complications. Most significantly, none of the current ionic models incorporate nonlocal dielectric effects. As the first step toward the direction of changing this situation, in this paper, we propose a nonlocal dielectric model for protein in ionic solvent (see (2.2)) by assuming that