Catechol oxidase activity of dicopper complexes with N-donor ligands☆

Catecholase Activity of a Copper(II) Complex with a Macrocyclic Ligand: Unraveling Catalytic Mechanisms

“…The protonated N À H moiety acts as a donor of an intermolecular bifurcated hydrogen bond with two perchlorate oxygen atoms as acceptors; the angles at the hydrogen atom add up to 359(4)8 (Table 1). Due to the inversion center in the ligand, the N21-N22 ring and its symmetry related N21'-N22' ring are arranged in a parallel fashion; the centroid-centroid distance is 4.4089 (18) and the parallel distance between the two planes of the aromatic rings is only 3.211 , indicating the presence of p stacking. The Figure 1.…”

“…The plot of the initial reaction rates versus catechol concentration indicates a substrate sat- Formation of H 2 O 2 and its influence on the reaction: The reduction of dioxygen to dihydrogen peroxide upon catechol oxidation by copper(II) complexes has been previously established only in a few cases. [18,[20][21][22][23][24] We have studied the formation of H 2 O 2 in the course of the catalytic reaction for two different catechol to complex ratios (10:1 and 50:1, [I 2 + ] = 2 10 À5 m). In both cases, dihydrogen peroxide was formed at the initial stage of the reaction, and its formation was found to practically stop after a few minutes, although the oxidation of DTBCH 2 still continued ( Figure 9).…”

“…In addition, two other crystallographically characterized dicopper(II) complexes with essentially dinucleating ligands, for which the reduction mode of dioxygen to dihydrogen peroxide has been definitely established, also possess a large metal-metal separation (3.7 and 7.8 ). [18] It thus appears that the mechanism of catechol oxidation by the model compounds is very intricate, which evidently explains often contradictory literature reports on the catalytic behavior of copper(II) complexes. It is also very interesting to note that some authors have recently reported the formation of dihydrogen peroxide, [44] as well as semiquinone radicals, [45] during the catalytic oxidation of DOPA by the structurally related enzyme tyrosinase in haemolymph of some insects.…”

“…In particular, investigations on the substrate coordination to the copper centers, [2][3][4] the dioxygen activation and the subsequent reactivity of the peroxodicopper species, [5][6][7][8][9] and the role of a thioether linkage in a close proximity of the dicopper center, [10,11] found in catechol oxidase from Ipomoea batatas [1] and some hemocyanins [12,13] and tyrosinases, [14] have received a significant interest from the scientific community. Although there have been numerous reports on model complexes of catechol oxidase, detailed mechanistic studies on the catechol oxidation mechanism are unfortunately quite scarce, [15][16][17][18][19] and only in a few cases the mode of dioxygen reduction, for example, to dihydrogen peroxide or water, has been definitely established. [15,16,18,[20][21][22][23][24] The mechanism of catechol oxidation by the natural enzyme, as proposed by Krebs and co-authors, [1] can be briefly outlined as follows.…”

“…Although there have been numerous reports on model complexes of catechol oxidase, detailed mechanistic studies on the catechol oxidation mechanism are unfortunately quite scarce, [15][16][17][18][19] and only in a few cases the mode of dioxygen reduction, for example, to dihydrogen peroxide or water, has been definitely established. [15,16,18,[20][21][22][23][24] The mechanism of catechol oxidation by the natural enzyme, as proposed by Krebs and co-authors, [1] can be briefly outlined as follows. The native met (m-hydroxo dicopper(II)) form of the enzyme reacts stoichiometrically with one equivalent of the substrate, generating the corresponding quinone and the reduced deoxy (dicopper(I)) form.…”