Catecholase Activity of a Series of Dicopper(II) Complexes with Variable Cu−OH(phenol) Moieties

Abstract

The catecholase activity of a series of dicopper(II) complexes containing different numbers of phenol groups coordinated to the metal centers was studied to identify functional as well as structural models for the type III copper enzymes tyrosinase and catechol oxidase. The syntheses and characterization of complexes [Cu(2)(H(2)bbppnol)(mu-OAc)(H(2)O)(2)]Cl(2).2H(2)O (1) and [Cu(2)(Hbtppnol)(mu-OAc)](ClO(4))(2) (2) were previously reported by us (Inorg. Chim. Acta 1998, 281, 111-115; Inorg. Chem. Commun. 1999, 2, 334-337), and complex [Cu(2)(P1-O(-))(OAc(-))](ClO(4))(2) (3) was previously reported by Karlin et al. (J. Am. Chem. Soc. 1997, 119, 2156-2162). The catalytic activity of the complexes 1-3 on the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butyl-o-benzoquinone characteristic absorption band at about 400 nm over time in methanol saturated with O(2)/aqueous buffer pH 8 solutions at 25 degrees C. The complexes were able to oxidize 3,5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity. A kinetic treatment of the data based on the Michaelis-Mentèn approach was applied. The [Cu(2)(H(2)bbppnol)(mu-OAc)(H(2)O)(2)]Cl(2) small middle dot2H(2)O complex showed the highest catalytic activity of the three complexes as a result of a high turnover rate (k(cat) = 28 h(-1)) combined with a moderate substrate-catalyst binding constant (K(ass) = 1.3 x 10(3) M(-1)). A mechanism for the oxidation reaction is proposed, and reactivity differences, k(cat)/K(M) of the complexes, were found to be dependent on (DeltaE)(1,2), the difference in the driving force for the reduction reactions Cu(II)(2)/Cu(II)Cu(I) and Cu(II)Cu(I)/Cu(I)(2).

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

“…However, in the course of the catalytic oxidation of DTBCH 2 by I 2 + the concentration of H 2 O 2 stops increasing after a few minutes, indicating that, most likely, a different mechanism is operating at later stages of the reaction. A proposal about H 2 O 2 being consumed in the course of the reaction, as suggested by some authors, [24] does not seem plausible in this case, as the results of kinetic measurements indicate that its presence has virtually no influence on the catalytic cycle, except at unrealistically high concentrations levels, which are never reached during the reaction. Another reason to suggest a different mechanistic pathway operating at later stages of the catalytic oxidation is the inhibiting effect of DTBQ, indicating that the formed quinone does not simply accumulate in the reaction mixture, but evidently also participates in the catalytic process.…”

“…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).…”

“…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.…”

Determination of catechin in green tea using a catechol oxidase biomimetic sensor

“…[1][2][3][4][5][6][7][8][9] The development of functional models of metalloenzymes for catalyst oxidation reactions is a subject of great interest. [10][11][12][13][14][15][16] Neves et al [17][18][19][20][21][22][23][24][25] have extensively synthesized and characterized model complexes to mimic the active site of the different enzymes (e.g., catechol oxidase, peroxidase, galactose oxidase, catalase, purple acid phosphatase). The majority of O 2 -reactive copper models have been based on the binuclear-Cu catechol oxidase.…”

“…10,[16][17][18][19][20] Many biomimetic catalysts of catechol oxidase have been reported in the literature. 10,[16][17][18][19][20] However, very few report their uses in biomimetic sensors. 12,13,26 Biomimetic catechol oxidase catalysts have been shown to be efficient catalysts for the oxidation of phenolic substrates to quinones.…”