Variable-Temperature, Variable-Field Magnetic Circular Dichroism Studies of Tris-Hydroxy- and μ₃-Oxo-Bridged Trinuclear Cu(II) Complexes:  Evaluation of Proposed Structures of the Native Intermediate of the Multicopper Oxidases

Multicopper oxidases catalyze the 4e- reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 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 O2 reduction. The most characteristic feature of NI is the intense magnetic circular dichroism pseudo-A feature (a pair of temperature-dependent C-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-mu2-hydroxy-bridged and the mu3-oxo-bridged structures, are evaluated through spectroscopic and electronic structure studies on relevant model complexes, TrisOH and mu3O. It is found that the two components of a pseudo-A-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 mu3O 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 mu3-oxo-bridged structure is consistent with the spectroscopic properties of NI. The Cu-O sigma-bonds in the mu3-oxo-bridged structure would provide the thermodynamic driving force for the 4e- reduction of O2 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.