Characterisation of [Cu4S], the catalytic site in nitrous oxide reductase, by EPR spectroscopyElectronic supplementary information (ESI) available: Listings of the coordinates used for the calculations; comparison of the results from restricted and unrestricted DFT calculations; schematic structure of a model CuZ. See http://www.rsc.org/suppdata/dt/b3/b313913a/

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Abstract
The enzyme nitrous oxide reductase (N2OR) has a unique tetranuclear copper centre [Cu4S], called CuZ, at the catalytic site for the two-electron reduction of N2O to N2. The X- and Q-band EPR spectra have been recorded from two forms of the catalytic site of the enzyme N2OR from Paracoccus pantotrophus, namely, a form prepared anaerobically, CuZ, that undergoes a one-electron redox cycle and CuZ*, prepared aerobically, which cannot be redox cycled. The spectra of both species are axial with that of CuZ showing a rich hyperfine splitting in the g‖-region at X-band. DFT calculations were performed to gain insight into the electronic configuration and ground-state properties of CuZ and to calculate EPR parameters. The results for the oxidation state [Cu+1 3Cu+2 1S]3+ are in good agreement with values obtained from the fitting of experimental spectra, confirming the absolute oxidation state of CuZ. The unpaired spin density in this configuration is delocalised over four copper ions, thus, CuI 20.1%, CuII 9.5%, CuIII 4.8% and CuIV 9.2%, the µ4-sulfide ion and oxygen ligand. The three copper ions carrying the highest spin density plus the sulfide ion lie approximately in the same plane while the fourth copper ion is perpendicular to this plane and carries only 4.8% spin density. It is suggested that the atoms in this plane represent the catalytic core of CuZ, allowing electron redistribution within the plane during interaction with the substrate, N2O.

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