Integer-Spin EPR Studies of the Fully Reduced Methane Monooxygenase Hydroxylase Component

The hydroxylase component of methane monooxygenase contains a binuclear iron cluster in which the iron appears to be oxo or R-oxo bridged. Mössbauer and EPR studies have demonstrated antiferromagnetic coupling for the [Fe(III)Fe(III)] and [Fe(II)Fe(III)] states of the cluster. In the [Fe(II)Fe(II)] form the cluster exhibits an intense X-band EPR signal with zero crossing near g = 16, which originates from an electronic system with integer spin. We have studied this signal from 2 to 20 K using a cavity that allows modes where the microwave magnetic field fluctuates either parallel or perpendicular to the static field. We have analyzed the line shapes and the temperature dependence of the spectra with a spin Hamiltonian containing zero-field splitting (D, E) and exchange (J) terms for a pair of S = 2 spins. Two coupling schemes, both involving ferromagnetic coupling, are compatible with the data. In the scheme for strong coupling (|J| ≫ D and D = 1.2 cm^-1), the g = 16 signal results from ΔM = ±1 transitions between the two lowest levels of an S = 4 multiplet. In the scheme for weak coupling (J =-0.75 cm^-1 and D₁= D₂ =-5 cm^-1), the resonance is assigned to Δm_i = 0 transitions. Simulation of the observed spectra has allowed quantitation of the g = 16 signal. This signal represents the majority of the iron in the hydroxylase regardless of the spin-coupling model assumed. Our results demonstrate that integer spin systems with large zero-field splittings are amenable to reliable quantitative analysis.