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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
journal contribution
posted on 2005-10-05, 00:00 authored by Jungjoo Yoon, Liviu M. Mirica, T. Daniel P. Stack, Edward I. SolomonMulticopper 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-μ2-hydroxy-bridged and the μ3-oxo-bridged structures, are evaluated through
spectroscopic and electronic structure studies on relevant model complexes, TrisOH and μ3O. 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 μ3O 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 μ3-oxo-bridged structure is consistent with the
spectroscopic properties of NI. The Cu−O σ-bonds in the μ3-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.