Molecular dynamics simulations consisted of a steepest descent minimization, a conjugated gradient minimization, a 200 ps NVT heating from 0 K to 298 K, a 1 ns NpT equilibration and a 25 ns NpT production run; in the minimization, heating and equilibration steps the coordinates of the backbone atoms were restrained with a 20 kcal mol 1 potential. These trajectories were used to extract the data used as input for Equation , and were also used as input for the APBS software to estimate the electrostatic hydration free energy of the amino acids using the linearized Poisson-Boltzmann equation, as described in more detail in SI section 3.1.