Hemerythrin (Hr) is an oxygen-transport protein that includes a binuclear iron active site, which is found in marine invertebrates. We investigated the origin of the magnetic couplings, the nature of the chemical bond, and the mechanism of the dioxygen binding of Hr using an appropriate theoretical method and realistic models. First, we selected the appropriate functional of DFT; spin-polarized Becke's half and half LYP (UB2LYP) showed reasonably good agreement with the experimental values. Secondly, in order to elucidate the origin of the magnetic couplings and understand the nature of the chemical bond, we estimated a range of chemical indices using natural orbitals obtained at the UB2LYP level. The results indicate that the origin of the antiferromagnetic coupling for Hr is the σ- and π-type orbital interactions via weak Fe-μO conjugation (σ- and π-type superexchange interactions) and that the difference in the strength of the dσ-pσ and dπ-pπ interactions of the diiron core in the active site of Hr is responsible for the difference in the magnetic coupling constants between oxygenated and deoxygenated Hr. Finally, using the Mulliken charge-transfer theory, we analyzed the ionization potentials of molecular oxygen and Hr. We concluded that the dioxygen binding in Hr would proceed via a two-step electron-transfer mechanism.
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