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}-meth ylbenzene) reacts reversibly with dioxygen at low temperature to form a mu-peroxo adduct. Kinetic studies of O2 binding carried out in acetone in the temperature range from -80 to -55 degrees C yielded the activation parameters DeltaH1(not equal) = 40.4 +/- 2.2 kJ mol(-1), DeltaS1)(not equal) = -41.4 +/- 10.8 J K(-1) mol(-1) and DeltaH(-1)(not equal) = 72.5 +/- 2.4 kJ mol(-1), DeltaS(-1)(not equal) = 46.7 +/- 11.1 J K(-1) mol(-1) for the forward and reverse reaction, respectively, and the binding parameters of O2 DeltaH degrees = -32.2 +/- 2.2 kJ mol(-1) and DeltaS degrees = -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 degrees C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant rho = -1.84. The temperature dependence of the phenol hydroxylation was studied between -84 and -70 degrees 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 DeltaH(not equal)ox = 29.1 +/- 3.0 kJ mol(-1), DeltaS(not equal)ox = -115 +/- 15 J K(-1) mol(-1), and the binding parameters of the phenolate to the mu-peroxo species DeltaH degrees(b) = -8.1 +/- 1.2 kJ mol(-1) and DeltaS degrees(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+.
Palavicini
Sara
S
Department of General Chemistry, University of Pavia, 27100 Pavia, Italy.
Granata
Alessandro
A
Monzani
Enrico
E
Casella
Luigi
L
D040761:Biomimetic Materials / Q000737:chemistry; D002413:Cations, Divalent; D003300:Copper / Q000737:chemistry*; D006900:Hydroxylation; D007700:Kinetics; D014442:Monophenol Monooxygenase / Q000737:chemistry* / Q000378:metabolism*; D009942:Organometallic Compounds / Q000737:chemistry; D010545:Peroxides / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
D002413:Cations, Divalent; D009942:Organometallic Compounds; D010545:Peroxides; D010636:Phenols; D003300:Copper; D014442:Monophenol Monooxygenase