Oxygenation of Phenols to Catechols by A (μ-η²:η²-Peroxo)dicopper(II) Complex:  Mechanistic Insight into the Phenolase Activity of Tyrosinase

Abstract

Tyrosinase is a copper monooxygenase that catalyzes oxygenation of phenols to catechols (phenolase activity) and the subsequent two-electron oxidation of catechols to the corresponding o-quinones (catecholase activity). 1 Chemical and spectroscopic studies have indicated that the enzyme has a dinuclear copper active site nearly identical to that found in hemocyanin, 1,2 where a side-on type (µ-η 2 :η 2 ) peroxo species 3 is generated by the reaction of the reduced dicopper(I) form and O 2 . 1 As a pioneering work by Karlin and co-workers in Cu/O 2 chemistry, aromatic ligand hydroxylation in a dinuclear Cu(I) complex by O 2 was first reported in early 1980s. 4 The mechanistic studies have indicated that the aromatic ligand hydroxylation reaction involves an electrophilic attack on the arene ring by a (µ-η 2 :η 2 -peroxo)-dicopper(II) intermediate. 5 After their finding, several examples of the aromatic ligand hydroxylation have been reported using similar type of m-xylyl dinucleating ligands. 6 With respect to the intermolecular reactions between phenols and the peroxo intermediate, however, most of the reactions so far reported afford a C-C coupling dimer as a major product. 7 Casella and co-workers have recently reported the first synthetic (µ-η 2 :η 2 -peroxo)dicopper-(II) complex which can react with an exogenous phenolate to yield the corresponding catechol. 8,9 Unfortunately, the low yield of the product (20% based on the dicopper complex) has precluded the kinetic and mechanistic investigation on the reaction between the peroxo intermediate and the phenolate. 8 As such, the mechanism for the catechol formation via intermolecular reactions between the peroxo intermediate and phenol derivatives has yet to be clarified. 10 We report herein that efficient conversion of phenol derivatives to the corresponding catechols is achieved for the first time by intermolecular reactions of a (µ-η 2 :η 2 -peroxo)dicopper(II) complex, supported by tridentate ligand L Py2Bz (N,N-bis[2-(2-pyridyl)-ethyl]-R,R-dideuteriobenzylamine), 11 with lithium salts of phenols. The mechanistic studies on the catechol formation have been performed to provide valuable mechanistic insight into the phenolase activity of the enzyme.Treatment of the copper(I) complex, [Cu I (L Py2Bz )](PF 6 ), with dioxygen in anhydrous acetone at -94°C afforded a brown color solution which exhibited a strong absorption band at 364 nm ( ) 26400 M -1 cm -1 ) together with a small one at 530 nm (1500 M -1 cm -1 ) and a resonance Raman band at 737 cm -1 that shifted to 697 cm -1 upon 18 O-substitution. 12,13 The frozen acetone solution of the intermediate was ESR silent at 77 K, and a Cu:O 2 ) 2:1 stoichiometry was obtained for formation of the intermediate by manometry. These results unambiguously indicate that the oxygenated intermediate is a (µ-η 2 :η 2 -peroxo)dicopper(II) complex as suggested previously by Karlin et al. 11 This compound is quite stable (no self-decomposition) at the low-temperature enabling us to examine the reaction with external subst...

Crystal structure of Manduca sexta prophenoloxidase provides insights into the mechanism of type 3 copper enzymes

“…By using model and enzymatic systems, the mechanism of the mono-oxygenase activity was demonstrated to be electrophilic aromatic substitution (44). Deprotonation of the phenol is an essential step for coordination of the phenolic oxygen to one of the copper atoms and subsequent ohydroxylation of phenols to catechols (44,45). The flexible His-54 in the Streptomyces tyrosinase may act as a base for deprotonation of monophenolic substrates (36).…”

The phenylthiourea is a competitive inhibitor of the enzymatic oxidation of DOPA by phenoloxidase

“…The key enzyme of melanization is phenoloxidase ([PO] or tyrosinase; monophenol, ortho-dihydroxyphenylalanine: oxygen oxidoreductase, EC 1.14.18.1) which contains two copper atoms at its active site 5 . This enzyme has three forms: "met" (with Cu(II)-Cu(II) in the active site), "deoxy"(Cu(I)-Cu(I)), and "oxy"(Cu(II)-O 2 -Cu(II)) 6,7 . Structural models for the active site of these three forms have been proposed [8][9][10][11][12][13][14] .…”

Theoretical Studies on ortho‐Oxidation of Phenols with Dioxygen Mediated by Dicopper Complex: Hints for a Catalyst with the Phenolase Activity of Tyrosinase

“…[9][10][11][12][13][14][15][16] An excellent biomimetic system has been reported by Itoh et al, who used the designed (m-h 2 :h 2 peroxo)dicopper(II) complex 1 to achieve an efficient ortho-oxidation reaction of the lithium salt of phenols to give 1,2-diphenols (Scheme 2). [17,18] The complex 1 has extremely high reactivity (the reaction proceeds at À94 8C) and chemoselectivity (the corresponding o-quinones and coupled dimers are not DEDICATED CLUSTER FULL PAPERS formed). [19] Therefore, this system has been regarded not only as a good model for enzymatic reactions, but also, from the viewpoint of synthetic chemistry, as a promising starting point for the development of new bio-inspired catalysts.…”

“…We highlighted the C À O bond forming step, as it is considered to be rate-determining, and therefore may be the key to the high chemoselectivity. [17] We also addressed the role of the two copper ions in the oxidation.…”