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 (Fe-B (II; s = 4/2)/N2O2 (-1; s = 1/2)/Fe-b3(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 (Fe-B-O-1-N-1-N-2-O-2-).