Solvation thermodynamics of amino acids Assessment of the electrostatic contribution and force-field dependence

The free energies of hydration of the 20 amino acids in their zwitterionic form, their pK _a shifts and the side-chain free energies of transfer have been calculated using the finite difference Poisson–Boltzmann methodology. A comparison of the results obtained with charge and size parameters from some popular force fields used in modelling biomolecules is presented. The force fields considered include recent versions of AMBER, CHARMM, CVFF, GROMOS and OPLS, PARSE and an ab initio-derived charge set. A general agreement between the theoretical predictions, emerging from each of the parameter sets, and experiment is discernible. A critique on the current status of theoretical studies on amino acids in solution is also advanced.