Tyrosinase Models. Synthesis, Structure, Catechol Oxidase Activity, and Phenol Monooxygenase Activity of a Dinuclear Copper Complex Derived from a Triamino Pentabenzimidazole Ligand

Abstract

The dicopper(II) complex with the ligand N,N,N',N',N"-pentakis[(1-methyl-2-benzimidazolyl)methyl]dipropylenetriamine (LB5) has been synthesized and structurally characterized. The small size and the quality of the single crystal required that data be collected using synchrotron radiation at 276 K. [Cu(2)(LB5)(H(2)O)(2)][ClO(4)](4): platelet shaped, P&onemacr;, a = 11.028 Å, b = 17.915 Å, c = 20.745 Å, alpha = 107.44 degrees, beta = 101.56 degrees, gamma = 104.89 degrees, V = 3603.7 Å(3), Z = 2; number of unique data, I >/= 2sigma(I) = 3447; number of refined parameters = 428; R = 0.12. The ligand binds the two coppers nonsymmetrically; Cu1 is coordinated through five N donors and Cu2 through the remaining three N donors, while two water molecules complete the coordination sphere. Cu1 has distorted TBP geometry, while Cu2 has distorted SP geometry. Voltammetric experiments show quasireversible reductions at the two copper centers, with redox potential higher for the CuN(3) center (0.40 V) and lower for the CuN(5) center (0.17 V). The complex binds azide in the terminal mode at the CuN(3) center with affinity lower than that exhibited by related dinuclear polyaminobenzimidazole complexes where this ligand is bound in the bridging mode. The catechol oxidase activity of [Cu(2)(LB5)](4+) has been examined in comparison with that exhibited by [Cu(2)(L-55)](4+) (L-55 = alpha,alpha'-bis{bis[(1-methyl-2-benzimidazolyl)methyl]amino}-m-xylene) and [Cu(2)(L-66)](4+) (L-66 = alpha,alpha'-bis{bis[2-(1-methyl-2-benzimidazolyl)ethyl]amino}-m-xylene) by studying the catalytic oxidation of 3,5-di-tert-butylcatechol in methanol/aqueous buffer pH 5.1. Kinetic experiments show that [Cu(2)(L-55)](4+) is the most efficient catalyst (rate constant 140 M(-1) s(-1)), followed by [Cu(2)(LB5)](4+) (60 M(-1) s(-1)), in this oxidation, while [Cu(2)(L-66)](4+) undergoes an extremely fast stoichiometric phase followed by a slow and substrate-concentration-independent catalytic phase. The catalytic activity of [Cu(2)(L-66)](4+), however, is strongly promoted by hydrogen peroxide, because this oxidant allows a fast reoxidation of the dicopper(I) complex during turnover. The activity of [Cu(2)(LB5)](4+) is also promoted by hydrogen peroxide, while that of [Cu(2)(L-55)](4+) is little affected. The phenol monooxygenase activity of [Cu(2)(LB5)](2+) has been compared with that of [Cu(2)(L-55)](2+) and [Cu(2)(L-66)](2+) by studying the ortho hydroxylation of methyl 4-hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The LB5 complex is much more selective than the other complexes since its reaction produces only catechol, while the main product obtained with the other complexes is an addition product containing a phenol residue condensed at ring position 2 of the catechol.

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

“…Consequently, we propose that in later stages of the catalytic reaction, the oxidation of DTBCH 2 proceeds by a "classic" mechanism, proposed by Krebs et al [1] and Casella et al, [15,16] involving a stoichiometric reaction between the dicopper(II) species and the substrate, leading to the reduced dicopper(I) species. The oxidation of the second equivalent of the substrate by a peroxo-dicopper(II) adduct, formed upon dioxygen binding to the dicopper(I) intermediate, results in the formation of the second molecule of quinone and water as a by-product (Scheme 1, cycle B).…”

“…The increase of the reaction rates in this case can be explained by a change in the rate-determining step of the reaction, as previously proposed by Casella and co-workers. [15] The change of the catalytic mechanism in the course of the reaction can be explained by proposing that the different mechanistic pathways are directly related to the binding mode of the substrate to the dicopper(II) core. This hypothesis has often been debated in the literature in the past years.…”

“…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.…”

Mimics of copper proteins: structural and functional aspects

“…Dioxygen molecule can bind at these dinuclear copper active centers, and the structure elucidation of both the deoxygenated and the resulting oxygenated protein has been determined by spectral and EPR properties (Solomon et al 1996). Several groups studied model compounds mimicking the most relevant aspects of those species, specially insertion of oxygen atoms in substrates (Sorrell 1989, Monzani et al 1998, binding and activation of dioxygen in copper centers (Tolman 1997, Karlin et al 1997, Kitajima 1992.…”