Variable-Temperature, Variable-Field Magnetic Circular Dichroism Studies of Tris-Hydroxy- and μ<sub>3</sub>-Oxo-Bridged Trinuclear Cu(II) Complexes:  Evaluation of Proposed Structures of the Native Intermediate of the Multicopper Oxidases

Multicopper oxidases catalyze the 4e<sup>-</sup> reduction of O<sub>2</sub> to H<sub>2</sub>O. Reaction of the fully reduced enzyme with O<sub>2</sub> produces the native intermediate (NI) that consists of four oxidized Cu centers, three of which form a trinuclear cluster site, all bridged by the product of full O<sub>2</sub> reduction. The most characteristic feature of NI is the intense magnetic circular dichroism pseudo-<i>A</i> feature (a pair of temperature-dependent <i>C</i>-terms with opposite signs) associated with O → Cu(II) ligand-to-metal charge transfer (LMCT) that derives from the strong Cu−O bonds in the trinuclear site. In this study, the two most plausible Cu−O structures of the trinuclear site, the tris-μ<sub>2</sub>-hydroxy-bridged and the μ<sub>3</sub>-oxo-bridged structures, are evaluated through spectroscopic and electronic structure studies on relevant model complexes, TrisOH and μ<sub>3</sub>O. It is found that the two components of a pseudo-<i>A</i>-term for TrisOH are associated with LMCT to the same Cu that are coupled by a metal-centered excited-state spin−orbit coupling (SOC), whereas for μ<sub>3</sub>O they are associated with LMCT to different Cu centers that are coupled by oxo-centered excited state SOC. Based on this analysis of the two candidate models, only the μ<sub>3</sub>-oxo-bridged structure is consistent with the spectroscopic properties of NI. The Cu−O σ-bonds in the μ<sub>3</sub>-oxo-bridged structure would provide the thermodynamic driving force for the 4e<sup>-</sup> reduction of O<sub>2</sub> and would allow the facile electron transfer to all Cu centers in the trinuclear cluster that is consistent with its involvement in the catalytic cycle.