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PubMed2002

Tuning the activity of catechol oxidase model complexes by geometric changes of the dicopper core.

Dicopper(II) complexes of a series of different pyrazolate-based dinucleating ligands [L1](-)-[L4](-) have been synthesized and characterized structurally and spectroscopically. A major difference between the four complexes is the individual metal-metal separation that is enforced by the chelating side arms of the pyrazolate ligand scaffold: it varies from 3.45 A in 2 x (BF4)4 to 4.53 A in 4 x (ClO4)2. All complexes have been evaluated as model systems for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechole (DTBC) as the test substrate. They were shown to exhibit very different catecholase activities ranging from very efficient to poor catalysts (k(obs) between 2430+/-202 and 22.8+/-1.2 h(-1)), with an order of decreasing activity 2 x (ClO4)4 > 1 x (ClO4)2 > 3 x (ClO4)2 >> 4 x (ClO4)2. A correlation of the catecholase activities with the variation in Cu...Cu distances, as well as other effects resulting from the distinct redox potentials, neighboring groups, and the individual coordination spheres are discussed. Saturation behavior for the rate dependence on substrate concentration was observed in only two cases, that is, for the most active 2 x (ClO4)4 and for the least active 4 x (ClO4)2, whereas a catalytic rate that is almost independent of substrate concentration (within the range studied) was observed for 1 x (ClO4)2 and 3 x (ClO4)2. H2O2 was detected as the product of O2 reduction in the catecholase reaction of the three most active systems. The structures of the adducts of "L3Cu2" and "L4Cu2" with a substrate analogue (tetrachlorocatecholate, TCC) suggest a bidentate substrate coordination to only one of the copper ions for those catalysts that feature short ligand side arms and correspondingly exhibit larger metal-metal separations; this possibly contributes to the lower activity of these systems. TCC binding is supported by several H-bonding interactions to water molecules at the adjacent copper or to ligand-side-arm N-donors; this emphasizes the importance of functional groups in proximity to the bimetallic active site.

Catalysis↗
PubMed2005

Catechol oxidase activity of di-Cu2+-substituted aminopeptidase from Streptomyces griseus.

Streptomyces griseus aminopeptidase exhibits activities toward the hydrolyses of peptides and bis(p-nitrophenyl)phosphate (40 billion fold) and catechol oxidation reported herein with catalytic efficiency (kcat/Km) only about 10 times smaller than that of gypsywort catechol oxidase. The multifunctionality of this enzyme suggests that it is a unique system for further exploration of protein structure and function and a template for design of enzymes of diverse activities.

Aminopeptidases↗
PubMed2005

Characterization and biomimetic study of a hydroxo-bridged dinuclear phenanthroline cupric complex encapsulated in mesoporous silica: models for catechol oxidase.

We report the synthesis and characterization of a hydroxo-bridged dinuclear phenanthroline cupric complex, [(phen)2Cu-OH-Cu(phen)2](ClO4)) (HPC, phen = phenanthroline), dispersed in molecular sieves: MCM-41's and sodium zeolite Y. We employed spectroscopic techniques (FT-IR, UV-visible, EPR, and EXAFS) to characterize and study the catalytic activities of immobilized HPC in the oxidation of 3,5-di-tert-butylcatechol (DTBC) to the corresponding quinone (3,5-di-tert-butylquinone, DTBQ) to mimic the functionality of catechol oxidases. HPC complexes can adsorb only on the outside surface of the Y zeolite due to its smaller pore size. The EXAFS spectrum gives 3.51 A for the Cu...Cu distance in HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous solids, which is comparable to the O...O distance of the two hydroxyl groups of DTBC, and this made a simultaneous coordination of the diol group to the dicupric center possible. The resultant complex then allows the transfer of two electrons from DTBC to the dicupric center leading to the production of DTBQ. The nanochannels of calcined Al-MCM-41 mesoporous solids provide stability, due to confined space and surface charge, which could prevent excessive separation of the dinuclear cupric centers after removal of the hydroxo bridge in the catalytic process. A catalytic reaction scheme is proposed based on the spectroscopic data obtained in the characterization. The study demonstrates that HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous materials could be a viable system for a broad range of catalytic oxidation to mimic natural occurring enzymes.

Biomimetics↗
PubMed2001

Synthesis, crystal structure, spectral studies, and catechol oxidase activity of trigonal bipyramidal Cu(II) complexes derived from a tetradentate diamide bisbenzimidazole ligand.

A new benzimidazole-based diamide ligand-N,N'-bis(glycine-2- benzimidazolyl)hexanediamide (GBHA)-has been synthesized and utilized to prepare Cu(II) complexes of general composition [Cu(GBHA)X]X, where X is an exogenous anionic ligand (X = Cl(-), NO(3)(-), SCN(-)). The X-ray structure of one of the complexes, [Cu(GBHA)Cl]Cl.H(2)O.CH(3)OH, has been obtained. The compound crystallizes in the monoclinic space group C2/c with unit cell dimensions a = 26.464(3) A, b = 10.2210(8) A, c = 20.444(2) A, alpha = 90 degrees, beta = 106.554(7) degrees, gamma = 90 degrees, V= 5300.7(9) A(3), and Z = 8. To the best of our knowledge, the [Cu(GBHA)Cl]Cl.H(2)O.CH(3)OH complex is the first structurally characterized mononuclear trigonal bipyramidal copper(II) bisbenzimidazole diamide complex having coordinated amide carbonyl oxygen. The coordination geometry around the Cu(II) ion is distorted trigonal bipyramidal (tau = 0.59). Two carbonyl oxygen atoms and a chlorine atom form the equatorial plane, while the two benzimidazole imine nitrogen atoms occupy the axial positions. The geometry of the Cu(II) center in the solid state is not preserved in DMSO solution, changing to square pyramidal, as suggested by the low-temperature EPR data g( parallel) > g( perpendicular) > 2.0023. All the complexes display a quasi-reversible redox wave due to the Cu(II)/Cu(I) reduction process. E(1/2) values shift anodically from Cl(-) < NO(3)(-) < SCN(-), indicating that the bound Cl(-) ion stabilizes the Cu(II) ion while the N-bonded SCN(-) ion destabilizes the Cu(II) state in the complex. When calculated against NHE, the redox potentials turn out to be quite positive as compared to other copper(II) benzimidazole bound complexes (Nakao, Y.; Onoda, M.; Sakurai, T.; Nakahara, A.; Kinoshita, L.; Ooi, S. Inorg. Chim. Acta 1988, 151, 55. Addison, A. W.; Hendricks, H. M. J.; Reedijk, J.; Thompson, L. K. Inorg. Chem. 1981, 20 (1), 103. Sivagnanam, U.; Palaniandavar, M. J. Chem. Soc., Dalton Trans. 1994, 2277. Palaniandavar, M.; Pandiyan, T.; Laxminarayan, M.; Manohar, H. J. Chem. Soc., Dalton Trans. 1995, 457. Sakurai, T.; Oi, H.; Nakahara, A. Inorg. Chim. Acta 1984, 92, 131). It is therefore concluded that binding of amide carbonyl oxygen destabilizes the Cu(II) state. The complex [Cu(II)(GBHA)(NO(3))](NO(3)) could be successfully reduced by the addition of dihydroxybenzenes to the corresponding [Cu(I)(GBHA)](NO(3)). (1)H NMR of the reduced complex shows slightly broadened and shifted (1)H signals. The reduction of the Cu(II) complex presumably occurs with the corresponding 2e(-) oxidation of the quinol to quinone. Such a conversion is reminiscent of the functioning of a copper-containing catechol oxidase from sweet potatoes and the met form of the enzyme tyrosinase.

Amides↗
PubMed2003

Structural and functional models for the dinuclear copper active site in catechol oxidases. Synthesis, X-ray crystal structures, magnetic and spectroscopic properties of mu-methoxo-bridged dinuclear copper(II) complexes with N-substituted sulfonamide ligands.

Two new mu-methoxo-bridged dinuclear copper(II) complexes with a N-substituted sulfonamide, [Cu(mu-OMe)(L)(NH(3))](2) (1) and [Cu(mu-OMe)(L)(DMSO)](2) (2) [HL, N-2-(4-methylbenzothiazole)benzenesulfonamide], have been prepared and characterized by single-crystal X-ray difraction analyses. Compound 1 crystallizes in the monoclinic space group C(2)/c with a=22.0678(18), b=7.9134(7), c=21.1186(18)A, beta=113.788(4) degrees and Z=8. Compound 2 crystallizes in the monoclinic space group C(2)/c with a=18.0900(10), b=9.5720(10), c=24.2620(10) A, beta=98.7120(10) degrees and Z=8. In both complexes the copper atoms have square-planar environments bridged by two oxygen atoms from methoxide groups. Magnetic susceptibility measurements indicate a very strong antiferromagnetic coupling between the copper(II) ions in both complexes (2J<-1000 cm(-1)). Electronic Paramagnetic Resonance (EPR) spectra of the two complexes both in solid and in solution are silent. 13C NMR spectra of the complexes in solid state have been studied. The complexes have been evaluated as model systems for the catechol oxidase enzyme using 3,5-di-tert-butylcatechol as the test substrate. Complex 2 is slightly more active than complex 1.

Binding Sites↗
PubMed1975

Changes in Catechol Oxidase and Permeability to Water in Seed Coats of Pisum elatius during Seed Development and Maturation.

In the developing seed coat of Pisum elatius, o-dihydroxyphenols are present in appreciable amounts at all stages of development. However, catechol oxidase activity rises sharply during the later stages of development, shows a further abrupt rise during dehydration of the seed coat, and then decreases. It is suggested that a tanning reaction is induced by the contact of enzyme with its substrate while cell membranes are ruptured, and that this reaction renders the seed coats impermeable. The entire chain of events does not occur in Pisum sativum which has permeable seed coats.

Journal Article↗
PubMed1996

Structural and Functional Models for the Dinuclear Copper Active Site in Catechol Oxidases: Syntheses, X-ray Crystal Structures, Magnetic and Spectral Properties, and X-ray Absorption Spectroscopic Studies in Solid State and in Solution.

Two novel tridentate dinucleating ligands containing benzimidazole were prepared, 1,3-bis(2-benzimidazolyl)-2-propanol (Hbbp, 1) and 1,5-bis(2-benzimidazolyl)-3-pentanol (Hbbpen, 2). Their complexing properties toward copper were studied in order to obtain structural and functional models for catechol oxidases. Syntheses and crystal structures of dinuclear Cu(II) complexes derived from these ligands are reported. [Cu(2)bbp(2)](ClO(4))(2).2MeOH, 3, crystallizes in the triclinic space group P&onemacr; with the following unit cell parameters: a = 7.702(3) Å, b = 10.973(6) Å, c = 12.396(6) Å, alpha = 100.59(4) degrees, beta = 99.02(4) degrees, gamma = 98.90(4) degrees, V = 998.7(8) Å(3), and Z = 1. [Cu(2)bbpen(2)](ClO(4))(2).3MeOH, 4, crystallizes in the orthorhombic space group Pccn, with the following unit cell parameters: a = 17.478(9) Å, b = 18.795(8) Å, c = 13.888(6) Å, V = 4562.2(4) Å(3), and Z = 4. Magnetic susceptibility measurements in the temperature ranges 4.6-459 K (3) and 4.6-425 K (4) indicate an antiferromagnetic coupling between the Cu(II) centers of both complexes. In order to determine the structures of the complexes in solution, XAS spectra (EXAFS and XANES) were recorded in the solid state and in solution. The interpretation of these data, including multiple scattering calculations, together with UV-vis titrations, shows that the complexes have the same structure in the crystalline state as well as in methanolic solution. Complex 4 is able to oxidize 3,5-di-tert-butylcatechol (3,5-DTBC) to the quinone (catecholase activity). This reaction was also studied by XAS and UV-vis spectroscopy. These measurements reveal the reduction of Cu(II) to Cu(I) accompanied by a decrease of the coordination number.

Journal Article↗
PubMed2004

Mechanistic insight into the catechol oxidase activity by a biomimetic dinuclear copper complex.

The biomimetic catalytic oxidation of 3,5-di- tert-butylcatechol by the dicopper(II) complex of the ligand alpha,alpha'-bis(bis[1-(1'-methyl-2'-benzimidazolyl)methyl]amino)- m-xylene in the presence of dioxygen has been investigated as a function of temperature and pH in a mixed aqueous/organic solvent. The catalytic cycle occurs in two steps, the first step being faster than the second step. In the first step, one molecule of catechol is oxidized by the dicopper(II) complex, and the copper(II) centers are reduced. From the pH dependence, it is deduced that the active species of the process is the monohydroxo form of the dinuclear complex. In the second step, the second molecule of catechol is oxidized by the dicopper(I)-dioxygen complex formed upon oxygenation of the reduced complex. In both cases, catechol oxidation is an inner-sphere electron transfer process involving binding of the catechol to the active species. The binary catechol-dicopper(II) complex formed in the first step could be characterized at very low temperature (-90 degrees C), where substrate oxidation is blocked. On the contrary, the ternary complex of dicopper(I)-O(2)-catechol relevant to the second step does not accumulate in solution and could not be characterized, even at low temperature. The investigation of the biphasic kinetics of the catalytic reaction over a range of temperatures allowed the thermodynamic (Delta H degrees and Delta S degrees ) and activation parameters (Delta H( not equal) and Delta S( not equal)) connected with the key steps of the catecholase process to be obtained.

Biomimetic Materials↗
PubMed1998

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

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.

Journal Article↗
PubMed2005

Less symmetrical dicopper(II) complexes as catechol oxidase models--an adjacent thioether group increases catecholase activity.

Three new unsymmetrical compartmental dinucleating ligands, 4-bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[{2-(1-piperidyl)ethyl}aminomethyl]phenol (HL1), 4-bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[{2-(morpholin-4-yl)ethyl}aminomethyl]phenol (HL2), and 4-bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[{2-(thiomorpholin-4-yl)ethyl}aminomethyl]phenol (HL3), have been synthesized in order to model the active site of type 3 copper proteins. The dicopper(II) complexes of these ligands give first hints about the influence of a thioether group close to the metal site. The bromophenol-based ligands have one piperazine arm and one other bidentate arm in positions 2 and 6 of the phenolic ring, respectively. With each ligand a dinuclear copper(II) complex was prepared and structurally characterized. The copper ions were found to have square pyramidal environments and a mixture of endogenous phenoxo and exogenous acetate bridging. The influence of a heteroatom in one arm of the ligand on catecholase activity and speciation in solution was studied by UV/Vis spectroscopy, ESI-MS experiments and, DFT calculations.

Catechol Oxidase↗
PubMed1972

The expression of catechol oxidase activity during the hydroxylation of p-coumaric acid by spinach-beet phenolase.

1. The conditions under which oxygen consumption in excess of that required for the hydroxylation of p-coumaric acid to caffeic acid, catalysed by spinach-beet phenolase, can be suppressed, have been examined. 2. With dimethyltetrahydropteridine as electron donor, oxygen uptake was exactly equivalent to the caffeic acid produced, provided that p-coumaric acid was in excess, but with excess of reductant, oxygen uptake caused by the further oxidation of caffeic acid was also observed. 3. With equal concentrations of ascorbate and p-coumaric acid, equivalent oxygen uptake and caffeic acid production was found only in the first stages of the reaction, whereas with NADH substituted for ascorbate, oxygen uptake was in excess throughout. 4. When ascorbate was used, the period of the reaction over which this equivalence was found was decreased at high reaction rates and not observed at all with aged enzyme preparations; equivalence was restored by adding bovine serum albumin to these aged preparations. 5. Equivalence between oxygen consumption and caffeic acid production was observed with NADH, if small quantities of dimethyltetrahydropteridine were also added. 6. It is concluded that hydroxylation proceeds without the concomitant production of caffeic acid only if the enzyme is stabilized for hydroxylation by p-coumaric acid and the reductant, and is protected from attack by o-quinones.

Ascorbic Acid↗