Spectroscopic studies of the coupled binuclear non-heme iron active site in the fully reduced hydroxylase component of methane monooxygenase: comparison to deoxy and deoxy-azide hemerythrin

A combination of circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies has been used to probe the geometric and electronic structure of the binuclear Fe(II) active site of the reduced hydroxylase component of methane monooxygenase (MMOH). Excited-state data provide the numbers and energies of d→d transitions which are interpreted in terms of ligand field calculations to estimate the geometry of each iron. Variable-temperature variable-field (VTVH) MCD data are analyzed by using a non-Kramers doublet model to obtain the zero field splitting (ZFS) and g∥ value of the ground state and the excited sublevel energies. These results are further interpreted in terms of a spin Hamiltonian which includes the ZFS of each Fe 2+ combined with the exchange coupling between iron centers