Solvation of Ion Pairs: The Poisson-Langevin Model

Salt bridges play an important role in protein stability and protein-protein interactions.We have computed the electrostatics free energies of analogues of charged amino acid sidechains using two different implicit solvent models, the Poisson-Boltzmann (PB) model and our own Poisson-Langevin (PL) model,and compared their values to results from explicit solvent free energy calculations. A systematic difference is observed between the PB and explicit results, which we attribute to the basic assumption of PB that water density is constant.In contrast, the PL results match remarkably well with the explicit solvent results.We attribute this success to the ability of the PL model to give a realistic picture of the dielectric response of water to the presence of charged molecules.