FREE-ENERGY CALCULATIONS INVOLVING NH4+ IN WATER

The analysis of the hydration of NH4+ and the estimation of relative or absolute free energies of hydration by means of Monte Carlo computer simulations using different 1-6-12 potential functions is reported. Two electrostatic representations of NH4+ (used respectively by W.L. Jorgensen and P.A. Kollman) in conjunction with two common water models (TIP3P) and TIP4P) are considered. A change in relative hydration free energies of 1.7 kcal/mol is found when the NH4+ models are mutated into each other in either TIP3P or TIP4P. The NH4+ --> Na+ mutation in both solvent models leads to similar but overestimated relative hydration energies of about -28.7 kcal/mol. Similarly, the NH4+ annihilation significantly overestimates the absolute free energy of hydration.