Polarizable and Non-Polarizable Force Field Representations of Ferric Cation and Validations

The AMOEBA polarizable force field of ferric ion was optimized and applied to study the hydration of ferric ion and its complexation with porphine in the aqueous phase. The nonpolarizable force field was also optimized for comparison. The AMOEBA force field was found to give a more accurate hydration free energy than the nonpolarizable force field with respect to experimental data, and correctly predict the most stable electronic state of hydrated Fe³⁺, which is the sextet state, and of the Fe^(III)-Por complex, which is the quartet state, consistent with the literature that was carried out using the DFT method. The explicit inclusion of charge transfer between Fe³⁺ and ligand was found to be important in order to obtain a precise picture of polarization energy and van der Waals energy, which otherwise deviate from the corresponding energy components derived from ab initio calculations. The successful application of the AMOEBA force field in the characterization of aquo Fe^(III)-Por complexes suggests that its use may be extended to the study of the dynamics of metalloenzyme containing highly charged metal ions in the condensed phase with reliable treatment of the interactions between metal atom and protein.