Simulation Protocol for Thermodynamic Integration Simulations. The solvated systems were subjected to 2000 steps of energy minimization. The systems were then heated from 100 to 300 K over a period of 100 ps at constant volume, followed by 100 ps of density equilibration, 100 ps of constant pressure and temperature (NPT) equilibration, and were finally subjected to NPT production runs of 250 ps under 1 bar, 300 K with periodic boundary conditions. Constant temperature and pressure were maintained with the the Langevin thermostat and Berendsen barostat, respectively . Collision frequencies for temperature coupling were 2 ps -1 ; the pressure relaxation time was set to 2 ps. A cutoff of 12.0 Å was used for nonbonded interactions. Long-range electrostatics beyond the real space cutoff were computed with the particle-mesh Ewald (PME) scheme . The time step was set to 1 fs to keep the simulations stable during alchemical transformations (decoupling). Bonds to hydrogen were not constrained during TI simulations.