Report

Tyrosinase Reactivity in a Model Complex: An Alternative Hydroxylation Mechanism

Science  24 Jun 2005:
Vol. 308, Issue 5730, pp. 1890-1892
DOI: 10.1126/science.1112081

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Abstract

The binuclear copper enzyme tyrosinase activates O2 to form a μ-η22-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic μ-η22-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymatic active oxidant, is reported to rapidly hydroxylate phenolates at –80°C. Upon phenolate addition at extreme temperature in solution (–120°C), a reactive intermediate consistent with a bis-μ-oxodicopper(III)-phenolate complex, with the O–O bond fully cleaved, is observed experimentally. The subsequent hydroxylation step has the hallmarks of an electrophilic aromatic substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O–O bond cleavage and C–O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O–O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.

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