Protonation state of the Cu₄S₂ Cu_Z 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 Cu₄S₂ Cu_Z form of the active site of nitrous oxide reductase (N₂OR) in its 3Cu^ICu^II (1-hole) and 2Cu^I2Cu^II (2-hole) redox states. The EPR, absorption, and MCD spectra of 1-hole Cu_Z indicate that the unpaired spin in this site is evenly delocalized over Cu_I, Cu_II, and Cu_IV. 1-hole Cu_Z is shown to have a μ₂-thiolate edge ligand from the observation of S-H bending modes in the resonance Raman spectrum at 450 and 492 cm^-1 that have significant deuterium isotope shifts (-137 cm^-1) and are not perturbed up to pH 10. 2-hole Cu_Z 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_Z sites are correlated to these spectroscopic features to determine that 2-hole Cu_Z has a μ₂-sulfide edge ligand at neutral pH. The slow two electron (+1 proton) reduction of N₂O by 1-hole Cu_Z is discussed and the possibility of a reaction between 2-hole Cu_Z and O₂ is considered.