Protonation state of the CuS Cu site in nitrous oxide reductase: redox dependence and insight into reactivity.

Spectroscopic and computational methods have been used to determine the protonation state of the edge sulfur ligand in the CuS Cu form of the active site of nitrous oxide reductase (NOR) in its 3CuCu (1-hole) and 2Cu2Cu (2-hole) redox states. The EPR, absorption, and MCD spectra of 1-hole Cu indicate that the unpaired spin in this site is evenly delocalized over Cu, Cu, and Cu. 1-hole Cu is shown to have a μ-thiolate edge ligand from the observation of S-H bending modes in the resonance Raman spectrum at 450 and 492 cm that have significant deuterium isotope shifts (-137 cm) and are not perturbed up to pH 10. 2-hole Cu is characterized with absorption and resonance Raman spectroscopies as having two Cu-S stretching vibrations that profile differently. DFT models of the 1-hole and 2-hole Cu sites are correlated to these spectroscopic features to determine that 2-hole Cu has a μ-sulfide edge ligand at neutral pH. The slow two electron (+1 proton) reduction of NO by 1-hole Cu is discussed and the possibility of a reaction between 2-hole Cu and O is considered.