Explicitly solvated ligand contribution to continuum solvation models for binding free energies: selectivity of theophylline binding to an RNA aptamer.

To provide more accurate computational estimates of binding free energies in solution from molecular dynamics (MD) simulations, a separate solvation contribution for the binding ligand is determined from a linear response treatment. We use explicit water coordinates for this term and combine with MM-PBSA (molecular mechanics, Poisson-Boltzmann, and surface area contributions) in a new approach (MM-PB/LRA-SA). To assess this method, application to the binding between theophylline and its derivatives to an RNA aptamer was performed and compared with experimental binding affinities. Explicitly solvated MD trajectories were generated with the same parameter set used in the previous work by Gouda et al., who compared the relative binding of these molecules by both the MM-PBSA and thermodynamic integration methods. Substituting the linear response term for the ligand in the MM-PB/LRA-SA approach led to an improvement upon MM-PBSA when compared with experimental and thermodynamic integration results at approximately twice the computational cost. The balance between accuracy and computational expense achieved using this method suggests potential advantages in applying it in the virtual drug-screening process.