Electrocatalytic effect of Pb and Sn adatoms on the oxidation of 1-propanol on platinized platinum electrodes: Determination of the apparent activation energy

The electrocatalytic oxidation of 1-propanol was investigated on platinum electrodes modified by submonolayers of Pb and Sn in acid media. An increase of oxidation rates observed for both Pb and Sn, and the influence of theta values was investigated. The values of the apparent activation energy evaluated from the Arrhenius plots concerning the electrochemical oxidation of 1-propanol on modified platinzed platinum electrodes, reveal a significant decrease in the presence of upd Sn and Ph adatoms. A decrease from 56 to 26 U mol(-1) in the presence of Sn. and from 78 to 25 U mol(-1) for Ph adatoms are some illustrative values which reflect the promoting effect of the upd adatoms.

The co-catalytic effect of chlorpromazine on peroxidase-mediated oxidation of melatonin: enhanced production of N-1-acetyl-N (2)-formyl-5-methoxykynuramine

Accumulating evidence points to relationships between increased production of reactive oxygen or decreased antioxidant protection in schizophrenic patients. Chlorpromazine (CPZ), which remains a benchmark treatment for people with schizophrenia, has been described as a pro-oxidant compound. Because the antioxidant compound melatonin exerts protective effects against CPZ-induced liver disease in rats, in this investigation, our main objective was to study the effect of CPZ as a co-catalyst of peroxidase-mediated oxidation of melatonin. We found that melatonin was an excellent reductor agent of preformed CPZ cation radical (CPZ(center dot+)). The addition of CPZ during the horseradish peroxidase (HRP)-catalyzed oxidation of melatonin provoked a significant increase in the rate of oxidation and production of N-1-acetyl-N-2-formyl-5-methoxykynuramine (AFMK). Similar results were obtained using myeloperoxidase. The effect of CPZ on melatonin oxidation was rather higher at alkaline pH. At pH 9.0, the efficiency of oxidation of melatonin was 15 times higher and the production of AFMK was 30 times higher as compared with the assays in the absence of CPZ. We suggest that CPZ is able to exacerbate the rate of oxidation of melatonin by an electron transfer mechanism where CPZ(center dot+), generated during the peroxidase-catalyzed oxidation, is able to efficiently oxidize melatonin.

The effect of pH on horseradish peroxidase-catalyzed oxidation of melatonin: production of N-1-acetyl-N-2-formyl-5-methoxykynuramine versus radical-mediated degradation

There is a g-rowing body of evidence that melatonin and its oxidation product, N-1-acetyl-N-2-formyl-5-methoxykynuramine (AFMK), have anti-inflammatory properties. From a nutritional point of view, the discovery of melatonin in plant tissues emphasizes the importance of its relationship with plant peroxidases. Here we found that the pH of the reaction mixture has a profound influence in the reaction rate and products distribution when melatonin is oxidized by the plant enzyme horseradish peroxidase. At pH 5.5. 1 mm of melatonin was almost completely oxidized within 2 min, whereas only about 3% was consumed at pH 7.4. However, the relative yield of AFMK was higher in physiological pH. Radical-mediated oxidation products, including 2-hydroxymelatonin a dimer of, 2-hydroxymelatonin and O-demethylated dimer of melatonin account for the fast consumption of melatonin at pH 5.5. The higher production of AFMK at pH 7.4 was explained by the involvement of compound III of peroxidases as evidenced by spectral studies. on the other hand, the fast oxidative degradation at pH 5.5 was explained by the classic peroxidase cycle.

Oxidation of melatonin and its catabolites, N-1-acetyl-N (2)-formyl-5-methoxykynuramine and N-1-acetyl-5-methoxykynuramine, by activated leukocytes

N-1-acetyl-N-2-formyl-5-methoxykynuramine (AFMK) and N-1-acetyl-5-methoxykynuramine (AMK), two melatonin catabolites, have been described as potent antioxidants. We aimed to follow the kinetics of AFMK and AMK formation when melatonin is oxidized by phorbol myristate acetate (PMA) and lipopolysaccharide (LPS)-activated leukocytes. An HPLC-based method was used for AFMK and AMK determination in neutrophil and peripheral blood mononuclear cell cultures supernatants. Samples were separated isocratically on a C18 reverse-phase column using acetonitrile/H2O (25:75) as the mobile phase. AFMK was detected by fluorescence (excitation 340 nm and emission 460 nm) and AMK by UV-VIS absorbance (254 nm). Activation of neutrophils and mononuclear cells with PMA produces larger amounts of AFMK than activation with LPS, probably due to the lower levels of reactive oxygen species formation and myeloperoxidase (MPO) degranulation that occurs when cells are stimulated with LPS. The concentration of AMK found in the supernatant was about 5-10% (from 18-hr cultures) compared with AFMK. This result may reflect its reactivity. Indeed AMK, but not AFMK, is easily oxidized by activated neutrophils in a MPO and hydrogen peroxide-dependent reaction. In conclusion, we defined a simple procedure for the determination of AFMK and AMK in biological samples and demonstrated the capacity of leukocytes to oxidize melatonin and AMK.

Electrochemical oxidation of an acid dye by active chlorine generated using Ti/Sn(1-x)Ir O-x(2) electrodes

The generation of active chlorine on Ti/Sn(1-x)Ir (x) O-2 anodes, with different compositions of Ir (x = 0.01, 0.05, 0.10 and 0.30 ), was investigated by controlled current density electrolysis. Using a low concentration of chloride ions (0.05 mol L-1) and a low current density (5 mA cm(-2)) it was possible to produce up to 60 mg L-1 of active chlorine on a Ti/Sn0.99Ir0.01O2 anode. The feasibility of the discoloration of a textile acid azo dye, acid red 29 dye (C.I. 16570), was also investigated with in situ electrogenerated active chlorine on Ti/Sn(1-x)Ir (x) O-2 anodes. The best conditions for 100% discoloration and maximum degradation (70% TOC reduction) were found to be: NaCl pH 4, 25 mA cm(-2) and 6 h of electrolysis. It is suggested that active chlorine generation and/or powerful oxidants such as chlorine radicals and hydroxyl radicals are responsible for promoting faster dye degradation. Rate constants calculated from color decay versus time reveal a zero order reaction at dye concentrations up to 1.0 x 10(-4) mol L-1. Effects of other electrolytes, dye concentration and applied density currents also have been investigated and are discussed.

Electrochemical characterization of the paste carbon modified electrode with KSr2Ni0.75Nb4.25O15-delta solid in catalytic oxidation of the dipyrone

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electrocatalysis of oxidation of hydrogen on platinum ordered intermetallic phases: Kinetic and mechanistic studies

An experimentally based kinetic and mechanistic study of the hydrogen oxidation reaction (HOR) on platinum and platinum ordered intermetallic materials in acid medium is presented. RDE kinetic data were re-evaluated and complemented by Tafel plots obtained from chronoamperometric measurements. Among the materials evaluated, PtSb and PtSn exhibited markedly improved kinetic current densities and exchange current densities, compared to Pt in the same experimental conditions. It is proposed that the intermetallic phase enhanced the adsorptive characteristic of the surface sites and, as a consequence, improved the kinetics of the adsorption steps (Tafel or Heyrovsky) of the mechanism involved. (c) 2006 Published by Elsevier B.V.

Oxidation of melatonin by taurine chloramine

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Oxidation of melatonin by AAPH-derived peroxyl radicals: Evidence of a pro-oxidant effect of melatonin

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Voltammetric Behavior of a Modified Electrode With Azide Copper Octa(3-Aminopropyl)Octasilsesquioxane Composite in the Oxidation of Ascorbic Acid

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electrochemical Oxidation of Hydroxylamine on Gold in Aqueous Acidic Electrolytes: An in Situ SERS Investigation

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Effects of Alloyed and Oxide Phases on Methanol Oxidation of Pt-Ru/C Nanocatalysts of the Same Particle Size

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The Oxidation of Hydroxylamine on Gold Electrodes in Mildly Acidic Aqueous Electrolytes: Electrochemical and In Situ Differential Reflectance Studies

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Photoeletrocatalytic oxidation of anionic surfactant used in leather industry on nanoporous Ti/TiO2 eletrodes

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)