First-Shell Solvation of Ion Pairs:  Correction of Systematic Errors in Implicit Solvent Models

Salt bridges play an important role in protein stability, protein−protein interactions, and protein folding. The electrostatic solvation free energies of the analogues of charged amino acid side chains were calculated using both explicit solvent free energy perturbation methods and implicit solvation models such as Poisson Boltzmann and surface-generalized Born model. A systematic difference between explicit and implicit solvent results was observed, which we attribute to a specific first-shell solvation effect, which we refer to as bridging waters. We present a method for including a single explicit bridging water between the pairs that improves the implicit solvation models significantly.