Electron nuclear double resonance (ENDOR) spec-troscopy is used to probe the coordination f the mixed valence (Fe(II)*Fe(III)) diiron cluster of the methane monooxygenase hydroxylase component ("OH-) iso-lated from Methylosinus trichosporium OB3b. EN-DOR resonances are observed along the principal axis directions g, = 1.94 and g, = 1.76 from at least nine different protons and two different nitrogens. The ni-trogens are strongly coupled and appear to be directly coordinated to the cluster irons. The ratio of their superhyperfine coupling constants is roughly 4:7, which equals the ratio of the spin expectation values of the Fe(I1) and Fe(II1) in the ground state and suggests that at least one nitrogen is coordinated to each iron of the mixed valence cluster. Moreover, the superhyper
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