New Insights into the Nature of Observable Reaction Intermediates in Cytochrome P450 NO Reductase by Using a Combination of Spectroscopy and Quantum Mechanics/Molecular Mechanics Calculations

Cytochrome P450 NO reductase is an unusual member of the cytochrome P450 superfamily. It catalyzes the reduction of nitric oxide to nitrous oxide. The reaction intermediates were studied in detail by a combination of experimental and computational methods. They have been characterized experimentally by UV/Vis, EPR, Mossbauer, and MCD spectroscopy. In conjunction with quantum mechanics/molecular mechanics (QM/MM) calculations, we sought to characterize the resting state and the two detectable intermediates in detail and to elucidate the nature of the key intermediate I of the reaction. Six possible candidates were taken into account for the unknown key intermediate in the computational study, differing in protonation state and electronic structure. Two out of the six candidates could be identified as putative intermediates I with the help of the spectroscopic data: singlet diradicals Fe-III-NHO center dot- and Fe-III-NHOH center dot. In a companion publication (C. Riplinger, F. Neese, ChemPhysChem 2011, 12, 3192) we have used QM/MM models based on these structures and performed a kinetic simulation. The combination of these two studies shows the nature of the key intermediate to be the singlet diradical, Fe-III-NHOH center dot.