Catecholase Activity of a Series of Dicopper(II) Complexes with Variable Cu−OH(phenol) Moieties

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Abstract
The catecholase activity of a series of dicopper(II) complexes containing different numbers of phenol groups coordinated to the metal centers was studied to identify functional as well as structural models for the type III copper enzymes tyrosinase and catechol oxidase. The syntheses and characterization of complexes [Cu2(H2bbppnol)(μ-OAc)(H2O)2]Cl2·2H2O (1) and [Cu2(Hbtppnol)(μ-OAc)](ClO4)2 (2) were previously reported by us (Inorg. Chim. Acta1998, 281, 111−115; Inorg. Chem. Commun. 1999, 2, 334−337), and complex [Cu2(P1−O-)(OAc-)](ClO4)2 (3) was previously reported by Karlin et al. (J. Am. Chem. Soc.1997, 119, 2156−2162). The catalytic activity of the complexes 1 − 3 on the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butyl-o-benzoquinone characteristic absorption band at about 400 nm over time in methanol saturated with O2/aqueous buffer pH 8 solutions at 25 °C. The complexes were able to oxidize 3,5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity. A kinetic treatment of the data based on the Michaelis−Mentèn approach was applied. The [Cu2(H2bbppnol)(μ-OAc)(H2O)2]Cl2·2H2O complex showed the highest catalytic activity of the three complexes as a result of a high turnover rate (kcat = 28 h-1) combined with a moderate substrate−catalyst binding constant (Kass = 1.3 × 103 M-1). A mechanism for the oxidation reaction is proposed, and reactivity differences, kcat/KM of the complexes, were found to be dependent on (ΔE)1,2, the difference in the driving force for the reduction reactions CuII2/CuIICuI and CuIICuI/CuI2.