A QM/MM study of nitric oxide reductase-catalysed N2O formation

Nitrous oxide (N2O), with a greenhouse effect 300 times that of CO2, is increasingly eliminated into the atmosphere. Using a hybrid quantum mechanics/molecular mechanics (QM/MM) method, we examined nitric oxide reductase-catalysed N2O formation, which includes two important chemical reactions of N–N bond formation and N–O bond cleavage. The N–N bond formation has no activation barrier, but N–O bond cleavage exhibits an activation barrier of 20.9 kcal·mol−1 at the QM/MM level. We show that the N–O bond cleavage occurs via a hyponitrous intermediate (FeB (II; s = 4/2)/N2O2 (−1; s = 1/2)/ (III; s = −1/2)), with bidentate coordination between Glu211 and a non-heme iron atom. The Glu211 coordination decreases the N–O bond cleavage energy barrier by inhibiting the formation of stable, five-membered ring intermediate (FeB–O1–N1–N2–O2–).