title: Variable-temperature, variable-field magnetic circular dichroism studies of tris-hydroxy- and mu3-oxo-bridged trinuclear Cu(II) complexes: evaluation of proposed structures of the native intermediate of the multicopper oxidases.
abstract: 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.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Jungjoo
authors.initials: J
authors.lastname: Yoon
authors.forename: Liviu M
authors.initials: LM
authors.lastname: Mirica
authors.forename: T Daniel P
authors.initials: TD
authors.lastname: Stack
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
mesh_terms: D002942:Circular Dichroism; D003300:Copper / Q000737:chemistry*; D010088:Oxidoreductases / Q000737:chemistry*; D011487:Protein Conformation; D013696:Temperature
chemical_list: D003300:Copper; D010088:Oxidoreductases; C024748:copper oxidase