Comparative kinetic studies on tyrosinase-like catalytic activity of dinuclear imidazole-containing copper(II) complexes

Comparative kinetic studies on the catalytic activity of some new dinuclear copper(II) complexes in the oxidation of phenolic and catechol substrates are reported. These complexes, showing a tri- or tetradentate imine ligand containing an imidazole group, or additionally an imidazolate- or phenolate bridge between the copper ions, were synthesized and characterized by UV-Vis, IR, and EPR spectroscopy, and molar conductivity. Magnetic susceptibility measurements in the solid state, in the temperature range 2-300 K, indicated an anti-ferromagnetic coupling of the copper(II) centers for all the species studied. These compounds exhibited in aqueous solution an equilibrium between the dinuclear and the corresponding mononuclear species, very dependent of the pH. Their catalytic activity in the oxidation of 2,6-di-tert-butylphenol, and of 3,4-dihydroxyphenylalanine (L-dopa) was monitored in methanol solution, following the corresponding quinone formation, at 418 run (epsilon = 5.48 x 10(4) Mol(-1) l cm(-1)), or at 475 run (epsilon = 3.60 x 10(3) Mol(-1) l cm(-1)), respectively. Differing in some structural features, their tyrosinase-like catalytic activity was verified to be influenced by several factors, including hydrophobic character, and steric hindrance of the ligands, accessibility of the oxidant to the catalytic center, and mainly by the stability of the dinuclear species in solution. (C) 2003 Elsevier Science B.V. All rights reserved.