Simulations for the second iteration of the RESP fit (extracting multiple configurations of the amino acids) consisted of a steepest descent minimization, a conjugated gradient minimization, a 200 ps NVT heating from 0 K to 298 K, and a 1 ns NpT equilibration; in the minimization, heating and equilibration steps the coordinates of the backbone atoms were restrained with a 20 kcal mol 1 potential. The production step was run in the NpT ensemble, keeping the same restraints on the backbone atoms.