Properties on electronic structure in an Fe-porphyrin (FeP) complex with the proximal imidazole (Im) ligand, a model of active moiety of hemeprotein for analyzing bonding- and separating-processes of dioxygen molecule (O-2), were studied by means of spin-polarized density functional theory. It was found that in the ionized model, the bonding stability of O-2 was reduced by one order in energy compared with that of the neutral model, implying existence of the state having a large fluctuation between bonded and separated configurations. We proposed a microscopic scenario on O-2 dissociation phenomenon in terms of spin-crossover and allosteric mechanism. (C) 2015 Elsevier B.V. All rights reserved.