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)ethylimino-1-ethyl]pyridine), was prepared. Both mono- and dinuclear species coexisted in aqueous solution exhibiting a pH-dependent monomer↔dimer interconversion and were monitored by different techniques. Similar data were obtained with the complex [SECu(imH)]+, where SE=2-salicylidene-aminoethane. Moreover, these species were isolated as perchlorate salts and characterized by cyclic voltammetry and spectroscopic techniques (UV–Vis, IR, Raman, and EPR). Experiments by capillary electrophoresis, in addition to parallel EPR spectra at different pH values, permitted to estimate the equilibrium constant, corresponding to the equation: 2[LCu(imH)]2++OH−⇆[LCu(im)CuL]3++H2O+imH, K=(8±3)×107 mol−1 dm3, in reasonable agreement to the value determined by spectrophotometric measurements, K=(0.12±0.01)×107 mol−1 dm3. A further tetranuclear species, [Cu4(apip)4im4]4+, was obtained by a pH-dependent self-assembly process, 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 oxidation of 2,6-di-tert-butylphenol, in methanolic solution, and that of 3,4-dihydroxyphenylalanine (l-dopa) in aqueous solution, with a biphasic behavior in the range of pH 7–11. Applying the Michaelis–Menten approach, the kinetic parameters determined indicated that the tetranuclear species is a better catalyst than the dinuclear ones, exhibiting a higher rate constant, k3, as well as a higher KM value. The hydrophobic cavity in the polynuclear complex seems to facilitate its interaction with the phenol substrate, since the structural parameters determined for both the complexes, di- and tetranuclear species were very similar.