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... Cu within the [Cu302] core. Upon inspection of the crystal structure of the fully reduced form of ascorbate oxidase (11), the three trigonally ligated Cu(I) centers (average Cu Cu distance, 4.5 A) appear geometrically predisposed toward accommodation of ?2 and formation of a [Cu302I cluster. However, no current spectroscopic studies of the metastable oxygen intermediates of multicopper oxidases and their derivatives support the existence of an intensely absorbing oxo-Cu(III) chromophore, and no unusually short Cu-O bond distances such as those observed in 2 are indicated (12, 13, 32 ). In accordance with these studies, however, the facile reaction of three Cu(I) monomers with 02 to form the mixed-valence bis(R. 3-oxo)[Cu(lI)Cu(II) Cu(III)] species 2 does suggest that 02 bond cleavage at trinuclear Cu sites requires full 4e-reduction of 02. In the case of native laccase, the fourth electron is provided by the remote "blue" Cu center, whereas in 2, the extra electron must be obtained at the cost of further oxidation of one of the Cu sites.