The interaction of substituted catechols with some binuclear copper(II) compounds

Antioxidation Activity Enhancement by Intramolecular Hydrogen Bond and Non-Browning Mechanism of Active Ingredients in Rosemary: Carnosic Acid and Carnosol

Rosemary is one of the most promising, versatile, and studied natural preservatives. Carnosic acid (CA) and carnosol (CARN), as the primary active ingredients of rosemary extracts, have little difference in structure, but their antioxidant activities vary significantly, depending on the system studied. The underlying molecular mechanisms remain unclear. By means of optical spectroscopies, stopped-flow, laser photolysis, and density functional theory (DFT) calculations, we have compared CA and CARN between their reaction dynamics of radical scavenging, metal ion chelation, and oxidation inhibition in lipid emulsion and beef, as well as between their interactions with β-carotene (β-Car). For reference, 3-isopropyl catechol (IC), which is structurally similar to the active groups of CA and CARN, was studied in parallel. It is found for CA that the intramolecular hydrogen bond can boost the acidity of its phenol hydroxyl and that the synergistic effect with β-Car can substantially enhance its antioxidation activity in the model systems of lipid and meat via the CA-to-β-Car electron transfer reaction. The substitution of A and B rings on the catechol group in both CA and CARN limits browning caused by their formation of oxidative products as antioxidants.

Synthesis and characterization of new unsymmetrical 'side-off' tetra and hexa coordinate homobinuclear Cu(II) and heterobinuclear Cu(II)-Zn(II) complexes: Magnetic, electrochemical and kinetic studies

A new class of phenol based unsymmetrical side-off tetra and hexa coordinate homobinuclear Cu(II) and heterobinuclear Cu(II)-Zn(II) complexes have been synthesized and characterized by elemental and spectral analysis. The electronic spectra of all the complexes show "Red shift" in LMCT band, for the ligand H(2)L(2) compared to that of the ligand H(2)L(1) due to the relatively higher electron donating nature of their substitutents. The homobinuclear Cu(II) complexes (1 and 2) illustrate an antiferromagnetic interaction (μ(eff): 1.58 and 1.60BM) at 298K with a broad EPR signal. Variable temperature magnetic moment study of the binuclear copper (II) complexes shows that the extent of antiferromagnetic coupling is greater in the case of H(2)L(2) complexes than H(2)L(1) complexes (-2J values: 192cm(-1) and 184cm(-1) respectively). The heterobinuclear Cu(II)-Zn(II) complexes (3 and 4) have a magnetic moment value close to the spin only value with four hyperfine EPR signals. Electrochemical studies of the complexes reveal that all the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an "anodic shift" in the first reduction potential of the complexes, of the ligand H(2)L(1) when compared to that of the ligand H(2)L(2) due to the presence of relatively higher electron donating N-substituents in the later case than in the former case. The catecholase activity of the complexes reveals that the homobinuclear Cu(II) complexes show higher catalytic activity than the corresponding heterobinuclear Cu(II)-Zn(II) complexes. In the hydrolysis of 4-nitrophenylphosphate, the heterobinuclear Cu(II)-Zn(II) complexes show better catalytic activity than the corresponding homobinuclear Cu(II) complexes.

N-benzoylated 1,4,8,11-tetraazacyclotetradecane and their copper(II) and nickel(II) complexes: spectral, magnetic, electrochemical, crystal structure, catalytic and antimicrobial studies

A series of N-benzoylated cyclam ligands incorporating three different benzoyl groups 1,4,8,11-tetra-(benzoyl)-1,4,8,11-tetraazacyclotetradecane (L(1)), 1,4,8,11-tetra-(2-nitrobenzoyl)-1,4,8,11-tetraazacyclotetradecane (L(2)) and 1,4,8,11-tetra-(4-nitrobenzoyl)-1,4,8,11-tetraazacyclotetradecane (L(3)) and their nickel(II) and copper(II) complexes are described. Crystal structure of L(1) is also reported. The ligands and complexes were characterized by elemental analysis, electronic, IR, (1)H NMR and (13)C NMR spectral studies. N-benzoylation causes red shift in the lambda(max) values of the complexes. The cyclic voltammogram of the complexes of ligand L(1) show one-electron, quasi-reversible reduction wave in the region -1.00 to -1.04 V, whereas that of L(2) and L(3) show two quasi-reversible reduction peaks. Nickel complexes show one-electron quasi-reversible oxidation wave at a positive potential in the range +1.05 to +1.15 V. The ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic of square-planar geometry with nuclear hyperfine spin 3/2. All copper(II) complexes show a normal room temperature magnetic moment values mu(eff) 1.70-1.73 BM which is close to the spin-only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts and hydrolysis of 4-nitrophenylphosphate using the copper(II) and nickel(II) complexes as catalysts were carried out. All the ligands and their complexes were also screened for antimicrobial activity against Gram-positive, Gram-negative bacteria and human pathogenic fungi.

Ligand dependence in the copper-catalyzed oxidation of hydroquinones

Transition metal-mediated oxidation of hydroquinones is an important physiologic reaction, and copper(II) effectively catalyzes the reaction in phosphate-buffered saline (PBS). Studies reported herein in phosphate buffer alone demonstrate that copper(II) is an ineffective catalyst in the absence of coordinating ligands, but that 1,10-phenanthroline and histamine facilitate the copper(II)-mediated oxidation of hydroquinone and its 2,5- and 2,6-di-tert-butyl analogs to the corresponding benzoquinones. The high concentration of chloride in PBS is the key element that allows copper(II) to work in this system. Although the bis-bathocuproine disulfonate complex of Cu(II), (BC)(2)Cu(II), is a strong stoichiometric oxidant, stoichiometric amounts of copper(II) in the presence of ligands other than BC oxidize hydroquinones very slowly under anaerobic conditions. Thus, the rapid copper(II)-catalyzed reaction operating aerobically does not involve a simple ping-pong reduction of copper(II) to copper(I) by hydroquinone and reoxidation of copper(I) by O(2).

Synthesis, structure and catecholase activity of dinuclear copper and zinc complexes with an N(3)-ligand

The preparation and characterization of dinuclear [M(II)(dbcat)(idpa)](2) (M[double bond]Zn (1), Cu (3); dbcat[double bond]3,5-di-tert-butylcatecholate; idpa[double bond]3,3'-iminobis(N,N-dimethylpropylamine)) complexes are described. Crystallographic characterization of the complex [Cu(II)(dbcat)(idpa)](2) has shown that the co-ordination geometry around copper(II) ions is distorted square pyramidal (triclinic, P-1, a=10.576(1) A, b=11.927(1) A, c=12.621(1) A, alpha=77.89(1) degrees, beta=88.65(1) degrees, gamma=70.21(1) degrees, V=1462.7(2) A(3), Z=2, R=0.0387). Both 1 and 3 were suitable catalysts for the catalytic oxidation of dbcatH(2) to dtbq (dtbq=3,5-di-tert-butyl-1,2-benzoquinone) with dioxygen at ambient conditions in good yields. However, on the basis of kinetic studies the copper- and zinc-catalyzed reactions showed different mechanisms. In the first case valence tautomerism [Cu(II)(dbcat)(idpa)]<==>[Cu(I)(dbsq)(idpa)] precedes the reaction with O(2), while with the zinc complex metal-bound catecholate reacts directly with O(2) with the formation of free superoxide anion.

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.

Design and Synthesis of New Biomimetic Materials by Sol-Gel: A Cu(II)(histidine)(2) Complex Covalently Bonded on a Silica Matrix

The synthesis of a new histidine-silane derivative, Boc-His(Boc)-CONH-(CH(2))(3)Si(OEt)(3), is reported. Hydrolysis and co-condensation of this monomer with tetraethoxysilane, via the sol-gel procedure, results in a hybrid inorganic-organic material that bears histidine molecules covalently bonded on a silica matrix. 1D-ESEEM and 2D-HYSCORE studies of its Cu(II) complex show that the copper atom is coordinated by two inequivalent histidine imidazoles. The new Cu(II) material exhibits catalytic activity for DTBQ formation in the presence of dioxygen, with considerable turnover rates and yields. In addition it is highly recyclable and shows high specific surface area.

Studies on chemiluminescence in the enzymatic and autoxidative transformation of 3,4-dihydroxyphenylalanine into eumelanins

The catechol oxidase-catalysed and autoxidative transformation of 3,4-dihydroxyphenylalanine (DOPA) to eumelanin have been studied by oxygen consumption, energy transfer, absorption and fluorescence spectroscopy. Formation of transient dopachrome (lambda max = 480 nm) and dopalutin (lambda ex = 423 nm, lambda em = 491 nm) have been found in the enzymatic and autoxidative reaction. In the enzymatic reaction, neither a photon emission with quantum yield phi greater than 10(-13) nor energy transfer to triplet and singlet energy acceptors (sensitizers such as anthracene derivatives, xanthene dyes and chlorophyll-a) in water and micellar solutions have been found. The autoxidative reaction is chemiluminescent (phi = 10(-9)), the emission occurring in the 400-600 nm range. The excitation energy is not transferred to sensitizers. The effect of various enzymes and traps of active oxygen species as well as the spectral distribution of chemiluminescence indicate that there is no emission from oxygen dimoles. Carbonates and active species of oxygen are shown to participate in the chemiexcitation reaction.