OPLS all-atom model for amines: Resolution of the amine hydration problem

Classical force-field parameters have been developed for amines primarily by fitting to experimental data for pure liquids and to hydrogen-bond strengths from gas-phase ab initio calculations; The resultant parameters were used to calculate relative free energies of hydration for ammonia, methylamine, dimethylamine, and trimethylamine using free energy perturbation calculations in Monte Carlo simulations (MC/FEP). The results including the fact that the most favorable Delta G(hyd) occurs for methylamine are in excellent agreement with the experimental data, in contrast to numerous prior computational reports. The calculations reveal two opposing trends in water: increased contribution from hydrogen-bond acceptance and diminished contribution from hydrogen-bond donation with increasing methylation of the amines. The proper balance of hydrogen-bond strengths, which is achieved with the OPLS-AA force field, is essential for correct ordering of the free energies of hydration. MC simulations for the pure liquids of thirteen additional amines, not included in the original parametrization, then demonstrated the transferability of the force field. These simulations covered aliphatic as well as cyclic and aromatic amines. Furthermore, the appropriateness of the force field for less polar environments was confirmed through MC/FEP calculations of relative free energies of solvation and log P values in chloroform. It is apparent that the prior problems with classical force fields for amines were simply a result of nonoptimal parametrization rather than to a critical omission such as the lack of explicit polarization.