Equilibria and tyrosinase activity of a dinuclear and its analogous tetranuclear imidazolate-bridged copper(II) complexes

A copper(II) complex, [Cu(apip)(imH)](2+), containing an imidazole ligand, in addition to a discrete tridentate imine (apip = 2-[2-(2-pyridyl)ethylimi no-1-ethyl]pyridine), was prepared. Both mono- and dinuclear species coexis ted in aqueous solution exhibiting a pH-dependent monomer-dimer interconver sion and were monitored by different techniques. Similar data were obtained with the complex [SECu(imH)](+), where SE = 2-salicylidene-aminoethane. Mo reover, these species were isolated as perchlorate salts and characterized by cyclic voltammetry and spectroscopic techniques (U-V-Vis, IR, Raman, and EPR). Experiments by capillary electrophoresis, in addition to parallel EP R spectra at different pH values, permitted to estimate the equilibrium con stant, corresponding to the equation: 2[LCu(imH)](2+) + OH- reversible arro w [LCu(im)CuL](3+) + H2O + imH, K= (8 +/- 3) x 10(7) mol(-1)dm(3), in reaso nable agreement to the value determined by spectrophotometric measurements, K= (0.12 +/- 0.01) x 10(7) mol(-1)dm(3). A further tetranuclear species, [ Cu-4(apip)(4)im(4)](4+), was obtained by a pH-dependent self-assembly proce ss, separated from very alkaline solutions of the corresponding mononuclear complex. The tetranuclear and dinuclear species were observed to behave as good functional models of the tyrosinase enzyme, catalyzing the aerobic ox idation of 2,6-di-tert-butylphenol, in methanolic solution, and that of 3,4 -dihydroxyphenylalanine (L-dopa) in aqueous solution, with a biphasic behav ior in the range of pH 7-11. Applying the Michaelis- Menten approach, the k inetic parameters determined indicated that the tetranuclear species is a b etter catalyst than the dinuclear ones, exhibiting a higher rate constant, k(3), as well as a higher Km value. The hydrophobic cavity in the polynucle ar complex seems to facilitate its interaction with the phenol substrate, s ince the structural parameters determined for both the complexes, di- and t etranuclear species were very similar. (C) 2001 Elsevier Science B.V. All r ights reserved.