Nuclear hyperfine coupling of nitrogen in the coordination sphere of the diiron center of methane monooxygenase hydroxylase

Electron spin echo envelope modulation spectroscopy identified two ligand ¹⁴N interactions with the mixed-valence. Fe(II/III) diiron center of methane monooxygenase hydroxylase from Methylococcus capsulatus (Bath). Characteristic features of the spectra obtained at 9 and 10 GHz were analyzed and fit by simulation. One of the nitrogens possessed superhyperfine parameters (A(iso) = 0.8 MHz, r(eff) = 3.2 Å, e²Qq = 1.8 MHz, η = 0.35) consistent with a non-coordinating amino nitrogen of a histidine imidazole ligand to a Fe(III). The second, more strongly interacting nitrogen (A(iso) = 5.0 MHz, r(eff) = 2.2 Å, e²Qq = 3.0 MHz, η = 0.3) corresponds to the N(δ) directly bound to the effective Fe(II). These findings extend the previous electron nuclear double resonance results on the Methylosinus trichosporium hydroxylase (Hendrich, M. P., Fox, B. G., Andersson, K. K., Debrunner, P. G., and Lipscomb, J. D. (1992) J. Biol. Chem. 267, 261-269), which identified the N(δ)-Fe(II) interaction but failed to quantify its magnitude. Measurement of the linear electric field g shift of this mixed-valence species indicated that the site is charge-polarized on to one of the iron atoms, and its symmetry suggests that either charge is shifted away from the Fe-Fe axis (if g(max) is defined by the Fe-Fe axis) or that g(mid) and g(max) are perpendicular to the Fe-Fe axis (charge strongly localized at Fe(III) and axis taken as g(min)).