Density Functional Study on the Electronic Structures of Model Peroxidase Compounds I and II

The electronic structures of [Fe(Por)(Im)O]1+ and [Fe(Por)(Im)O] (model compounds I and II, respectively) have been studied on the basis of density functional theory or DFT (Por = porphine, Im = imidazole). The a2u π-cation radical state (4A2u) was determined to be the ground state of compound I with total spin equal to 3/2, while the a1u π-cation state (4A1u) was found to be 0.15 eV higher in energy than the 4A2u state. Since, in both states, the spins were localized to the porphyrin ring (S = 1/2) and the Fe−O center (S = 1), the magnetic coupling interaction between the two spin sites was examined by using a broken symmetry method. The calculated J value revealed very weak magnetic coupling for the A2u state, which corresponded to the experimental data. The calculated J value revealed strong antiferromagnetic coupling for the A1u state. The calculated Mössbauer spectrum parameters (quadrupole splitting and asymmetry) were similar for both the A1u and A2u states, and both agreed well with experimental values. On the other hand, the calculated hyperfine coupling constants for the nitrogen and the proton of the porphyrin ring were different in the two states. Although the experimental coupling constant values of the pyrrole nitrogen atoms were intermediate between the calculated values for the A2u and A1u states, the experimental values for the meso protons were closer to the values calculated for the A2u state. These results suggest that the electronic structure of compound I is closer to the A2u state than to the A1u state. However, these results also suggest that there is the possibility that the electronic structure of compound I is an admixture of the A1u state and the A2u state. The electronic structure of compound II was calculated and compared with the electronic structure of compound I. The energetics of the redox reaction between the two compounds is discussed.