Spectroscopic definition of electronic structure contributions to oxygen and nitrous oxide activation by mononuclear, binuclear, and tetranuclear copper sites in biology

Copper proteins play important roles in O₂ and N₂O activation in biology. Spectroscopic techniques and density functional calculations were combined to study the structure/function correlations of biological copper active sites and related model complexes involved in O₂/N ₂O activation to gain insight into their electronic structure contributions to reactivity. Three major systems were studied. (1) Side-on and end-on disulfide Cu₂(S₂) complexes in comparison with their peroxide Cu₂(O₂) analogues in the context of coupled binuclear copper proteins. The side-on Cu_II-disulfide/peroxide bonding interactions involve ligand π*_σ donation and σ* backbonding while the end-on Cu II-disulfide/peroxide complexes only have π* _σ donor interactions. Their different metal-ligand bonding interactions result in differences in ligand activation. Frontier molecular orbital theory was then applied to the developed electronic structure descriptions of Cu _n-O₂ species to gain insight into their reactivity in electrophilic and CuII-superoxide complexes in the context of non-coupled binuclear copper sites in peptidylglycine α-hydroxylating monooxygenase (PHM) and dopamine β-monooxygenase (DβM), and the PHM protein. The reaction. The side-on CuII-superoxide complex has a covalently delocalized ground state and can not be described as antiferromagnetic coupled system as previously considered. These model studies were then extended to define the electronic and geometric structure of the non-coupled binuclear copper sites and the putative CuII_M(His)₂(Met)-OOH intermediate in PHM. The CuII_M-OOH species was also 4-sulfide bridged tetranuclear Cu_Z center in N₂O reductase. The resting Cu_Z center is an S = ½ system with 1CuII/3CuI configuration. The single spin of the cluster is partially delocalized with Cu_I being dominantly oxidized. The bridging sulfide forms an superexchange pathway between Cu _I and Cu_II. Further reduction of the resting form gives the all-reduced 4CuI form of Cu_Z, which is active for N₂O reduction. The developed electronic structure description provides a strategy for this Cu_Z center to overcome the reaction barrier of N₂O reduction by a simultaneous 2e− transfer in a μ-1,3 bridged binding mode. Together these studies provide insight into electronic structure contributions to O₂ and N₂O activation by biological copper sites.