Hydroxylation of phenolic compounds by a peroxodicopper(II) complex: Further insight into the mechanism of tyrosinase

The dicopper(I) complex [Cu2(MeL66)]2+ (where MeL66 is the hexadentate ligand 3,5-bis-{bis-[2-(1-methyl-1H-benzimidazol-2-yl)-ethyl]-amino}-methylbenzene) reacts reversibly with dioxygen at low temperature to form a -peroxo adduct. Kinetic studies of O2 binding carried out in acetone in the temperature range from -80 to -55 °C yielded the activation parameters H1 = 40.4 ± 2.2 kJ mol-1, S1 = -41.4 ± 10.8 J K-1 mol-1 and H-1 = 72.5 ± 2.4 kJ mol-1, S-1 = 46.7 ± 11.1 J K-1 mol-1 for the forward and reverse reaction, respectively, and the binding parameters of O2 H° = -32.2 ± 2.2 kJ mol-1 and S° = -88.1 ± 10.7 J K-1 mol-1. The hydroxylation of a series of p-substituted phenolate salts by [Cu2(MeL66)O2]2+ studied in acetone at -55 °C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant =-1.84. The temperature dependence of the phenol hydroxylation was studied between -84 and -70 °C for a range of sodium p-cyanophenolate concentrations. The rate plots were hyperbolic and enabled to derive the activation parameters for the monophenolase reaction Hox = 29.1  3.0 kJ mol-1, Sox = -115  15 J K-1 mol-1 and the binding parameters of the phenolate to the -peroxo species H°b = -8.1  1.2 kJ mol-1 and S°b = -8.9  6.2 J K-1 mol-1. Thus, the complete set of kinetic and thermodynamic parameters for the two separate steps of O2 binding and phenol hydroxylation have been obtained for [Cu2(MeL66)]2+.