Catalytic Redox Cycle Research Articles

Kinetic Modeling of the Oxidation of p-Hydroxybenzoic Acid by Fenton's Reagent: Implications of the Role of Quinones in the Redox Cycling of Iron

As Fenton (and Fenton-like) chemistry is increasingly implicated in a variety of areas and applications, an understanding of the mechanism and rates governing the system becomes relevant for a growing number of disciplines and purposes. In this work a kinetic model capable of describing species concentrations measured experimentally during the Fenton-mediated oxidation of p-hydroxybenzoic acid (pHBA) is presented and discussed. Experiments were conducted in the dark at low pH using reagent and substrate concentrations ranging from 100 microM to 2 mM. Analysis of the experimental and modeling results reveals that redox reactions between Fe and quinone or quinone-like compounds are essential for the model to qualitatively predict species concentration profiles observed in the laboratory. The quinone and quinone-like compounds generated as byproducts during the oxidation of pHBA act as reducing agents toward Fe(III), thereby assisting the redox cycling of Fe and increasing degradation of the target substrate. The experimental and kinetic modeling results presented highlight the role quinones play in the catalytic redox cycling of iron and the overall effect on the oxidative treatment performance of the system.

The pH-dependence of oxygen reduction on quinone-modified glassy carbon electrodes

The electrochemical reduction of oxygen has been studied on quinone-modified glassy carbon (GC) electrodes as a function of solution pH using the rotating disk electrode (RDE) technique. The surface of GC was grafted with anthraquinone (AQ) and phenanthrenequinone (PQ) by electrochemical reduction of their diazonium derivatives and the oxygen reduction measurements were carried out at different pHs (pH 7–14). The redox-potentials of surface-bound quinones were determined using cyclic voltammetry (CV). The kinetic parameters of oxygen reduction on GC/AQ and GC/PQ electrodes were determined considering a surface redox catalytic cycle model for quinone-modified electrodes.

High throughput screening for small molecule inhibitors of heparin-induced tau fibril formation

A library of ∼51,000 compounds was interrogated by high throughput screening (HTS) using a heparin-induced tau fibrillization assay. HTS was conducted with bacterially expressed recombinant tau fragment K18 and the reaction was monitored by thioflavine T fluorescence. Hits meeting criteria set for selection in HTS were further evaluated in a panel of assays designed (a) to confirm the initial results and (b) to identify possible false positives arising from non-specific mechanisms or assay-dependent artifacts. Two 2,3-di(furan-2-yl)-quinoxalines were confirmed as inhibitors of tau fibrillization with IC 50s in the low micromolar range (l–3 μM). Among false positive hits, members of the pyrimidotriazines, benzofurans, porphyrins, and anthraquinone, inhibited tau fibrillization by generating peroxides via catalytic redox cycles due to the reducing agent dithiothreitol (DTT) in the assay. This study delineates focused strategies for HTS of tau fibrillization inhibitors that are relevant to drug discovery for Alzheimer’s disease and related tauopathies.

Rational design of the carbon nanofiber catalysts for oxidative dehydrogenation of ethylbenzene

Abstract The rational design of the carbon nanofiber catalysts for oxidative dehydrogenation of ethylbenzene (ODE) has been discussed based on a reviewed reaction mechanism. The synthesized carbon nanofibers with systematically varied graphitic platelet orientations have been used as a model system for the rational catalyst design, in respect to the carbon nanofiber properties such as the amount of basic groups, the crystallinity, and the ratio between the prismatic area and the basal plane area. Well defined carbon nanofibers have been synthesized by hydrocarbon or carbon monoxide decomposition on the sub-iron group catalysts and at selected preparation conditions. The catalytic test in a fixed bed reactor at a temperature of 400 °C shows that the stable activity and selectivity can be obtained on the well-defined carbon nanofibers catalysts. Removing the residual metal in carbon nanofibers does not evidently influence the catalytic behavior whereas the lower conversion of ethylbenzene can be observed on the high-temperature treated carbon nanofiber samples. No clear evidences are found for the catalytic behavior being related to the surface area and graphitization degree of carbon nanofibers. The basic oxygen-containing groups are identified as the active sites by TPD–MS and a special method suggested by Boehm. The ratio between the prismatic surface area and the basal area has been identified as the most important parameter for the rational catalyst design. The fish-bone catalysts CNF 20 seems to be the best catalyst in the samples studied in the present work, due to the large number of basic edge groups, the optimum ratio between the prismatic and basal plane area. It can result in a delicate balance in the redox catalytic cycle. As a consequence, the formation of CO2 can be reduced.

Quantum chemical modelling of oxygen reduction on cobalt hydroxide and oxyhydroxide

Quantum chemistry has been employed to analyse the experimentally observed production of H2O2 during electrochemical reduction of O-2 on cobalt oxyhydroxide, CoOOH(s). The site for O-2 reduction was modelled using both small hydrated Co hydroxide clusters and periodic slab models of a step edge site. A catalytic site was found for the Co(II) model cluster, Co(OH)(2)(H2O)(7), which was also found in the step edge models. However, the site was found to bind O-3(2) very loosely and the Co(II) site alone displayed no electron affinity. The reduction reaction was initiated by adding an electron with O-2 present at the Co(II) site. This produces a superoxide ligand, which upon protonation is able to undergo further reduction to HO2- as Co(III) is formed at the site. Adding a second electron leads to detachment of HO2- as Co(II) is restored. A catalytic redox cycle is presented based on this understanding. The reduction behaviour explains why O-2 is readily reduced on p-type CoOOH at potentials where H2O2 is reduced at a low rate only. O-2 has the ability to introduce new charge carriers into the electrode, which means that the current is limited by charge transfer kinetics and not the electrical properties of the electrode material. H2O2 cannot produce additional charge carriers, and the electrode material limits the current.

Oxygen electroreduction on chemically modified glassy carbon electrodes in alkaline solution

Abstract The electrochemical reduction of oxygen on glassy carbon (GC) electrodes modified with aryl groups has been studied using the rotating disk electrode (RDE) technique. The GC electrodes were grafted by electrochemical reduction of diazonium salts. The diazonium derivatives of benzene, naphthalene and anthracene were used in surface modification. The blocking behaviour of aryl layers towards oxygen reduction was investigated. An unexpected order of blocking efficiency of the aryl moieties was observed for O2 reduction in 0.1 M KOH. The phenyl-modified GC electrode was less active than those electrografted with naphthalene and anthracene. Comparative measurements were carried out in 1 mM K3Fe(CN)6. A much larger extent of blocking was evident for the Fe ( CN ) 6 3 - / 4 - couple. This effect was explained by a significant difference in size between the Fe ( CN ) 6 3 - / 4 - ions and O2 molecule. A mixed film of anthraquinone (AQ) and phenyl groups was formed by diazonium reduction and its relevance in the kinetics investigations was discussed. The kinetic parameters of oxygen reduction were determined using a surface redox catalytic cycle model for quinone-modified electrodes.

Electrocatalytic oxygen reduction on glassy carbon grafted with anthraquinone by anodic oxidation of a carboxylate substituent

The electrochemical reduction of oxygen on glassy carbon (GC) electrodes modified with anthraquinone was studied using the rotating disk electrode (RDE) technique. Surface modification was performed by anodic oxidation of the carboxylate group of 9,10-anthraquinone-2-ethanoic acid in dimethylformamide (DMF). The modified electrode showed a pronounced electrocatalytic activity towards oxygen reduction in 0.1 M KOH. The kinetic parameters of oxygen reduction were determined using a surface redox catalytic cycle model and the values obtained were compared with those of surface bound anthraquinone grafted by the reduction of its diazonium salt in the absence of a –CH 2– tether. The rate constant for the reaction between the semiquinone radical and oxygen showed little dependence on the point of attachment of the quinone.

Kinetics of Oxygen Reduction on Quinone-Modified HOPG and BDD Electrodes in Alkaline Solution

The kinetics of oxygen reduction on boron-doped diamond (BDD) and highly oriented pyrolytic graphite (HOPG) electrodes modified with anthraquinone and phenanthrenequinone have been studied using the rotating disk electrode technique. The electrode surfaces were grafted by the electrochemical reduction of the corresponding diazonium salts. The kinetic parameters of oxygen reduction on the modified electrodes in 0.1 M KOH were determined using a surface redox-catalytic cycle model for quinone-modified electrodes. The results obtained further confirm the validity of the model used.

Operando Raman–GC studies of alumina-supported Sb-V-O catalysts and role of the preparation method

The interactions between Sb and V are studied by operando Raman–GC methodology during propane ammoxidation in order to understand the effect of the preparation method and reaction conditions on the structure and activity/selectivity of alumina-supported Sb-V-O catalysts. Dispersed V(V) species react with antimony species during propane ammoxidation to form VSbO 4; partially reversible transformations towards surface vanadium (V) species may account for the catalytic redox cycle. The catalytic performance is determined by the interaction between Sb and V, which is affected by the preparation method and the reaction conditions.

Role of substrate on the conformational stability of the heme active site of cytochrome P450cam: effect of temperature and low concentrations of denaturants.

The effect of 1R-camphor on the conformational stability of the heme active site of cytochrome P450cam has been investigated. The absorption spectra of the heme moiety showed the presence of two hitherto unknown intermediates formed at low urea concentrations or during small temperature perturbations. The corresponding thermodynamic parameters were obtained by global fitting of the experimental data to a generalized sequential unfolding model at different wavelengths, which showed that the active conformation of the enzyme is stabilized by binding of the substrate at the active site. Circular-dichroism spectra of the enzyme in the visible- and far-UV region were studied to identify the critical range of denaturant concentration and the temperature at which the tertiary structure around the heme center was affected with almost no change in the secondary structure of the enzyme. This critical range of urea concentration was 0-2.8 M in the presence of camphor and 0-1.5 M in the absence of camphor. The tertiary structure of the enzyme was found to undergo conformational change in the temperature range 20-60 degrees C in the presence of the substrate and 20-47 degrees C in its absence. The spectral assignments of the intermediate species of the heme active site with the intact secondary structure of the enzyme were made by deconvolution of the Soret absorption spectra, and the results were analyzed to determine stabilization of the heme active-site geometry by 1R-camphor. Results showed that subtle conformational changes due to melting of the tertiary contacts in the active site lead to formation of intermediates which are coordinatively similar to the native enzyme. Analogous intermediate species might be responsible for leakage in the redox catalytic cycle of the enzyme.

Mechanism of action of pyridazine analogues on protein tyrosine phosphatase 1B (PTP1B)

The inhibitory effect on PTP1B caused by the addition of pyridazine analogues has been investigated. Biophysical techniques, that is, mass spectrometry (MS), nuclear magnetic resonance (NMR), and isothermal titration calorimetry (ITC) were used for the characterization. Pyridazine analogues cause catalytic oxidation of the reducing agent, generating hydrogen peroxide that oxidizes the active site cysteine on the enzyme, leading to enzyme inactivation. Two additional compound classes show the same effect. We found one common structural feature in these molecules that allows the reaction with triplet molecular oxygen to be less endothermic. A proposed mechanism for the catalytic redox cycle is described.

Oxygen reduction on phenanthrenequinone-modified glassy carbon electrodes in 0.1 M KOH

The electrochemical reduction of oxygen on phenanthrenequinone-modified glassy carbon electrodes (GC) has been studied using a rotating ring–disk electrode (RRDE). Phenanthrenequinone (PQ) was covalently attached to the surface of GC by the electrochemical reduction of the corresponding diazonium salt. The redox potential of surface-bound PQ in 0.1 M KOH was 300 mV more positive than that of anthraquinone (AQ). The PQ modified electrode showed a much higher electrocatalytic activity for oxygen reduction. The results have been analysed using a surface redox catalytic cycle in which the key reactive intermediate is the semiquinone radical anion. The more positive redox potential of PQ compared with AQ and the larger value of the rate constant of reaction of the radical with O 2 result in an enhanced electrocatalytic activity of the functionalised electrode, producing H 2O 2 with a 100% yield.

Electrochemical reduction of oxygen on anthraquinone-modified glassy carbon electrodes in alkaline solution

The electrochemical reduction of oxygen on glassy carbon (GC) electrodes grafted with anthraquinone (AQ) has been studied using the rotating ring–disk electrode technique. Grafting was achieved by the electrochemical reduction of the corresponding diazonium salt and the effect of AQ surface concentration on the kinetics of oxygen reduction has been investigated. The two-electron reduction of oxygen to hydrogen peroxide was observed for all the quinone-modified electrodes studied and the catalytic activity of the electrodes for O 2 reduction was dependent on the AQ surface concentration. The kinetic parameters of oxygen reduction on GC/AQ electrodes in 0.1 M KOH were determined as a function of AQ surface concentration considering a surface redox catalytic cycle model for quinone-modified electrodes. The rate constant of the chemical reaction between the semiquinone radical anion of AQ and molecular oxygen has been determined.

Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

The identity of reactive intermediates and of active sites and the details of redox cycles and oxygen removal pathways during NO decomposition on well-characterized Cu−ZSM5 were examined by combining previous spectroscopic and steady-state kinetic studies with measurements of the rate of evolution of NO, N2O, NO2, N2, and O2 during isothermal and nonisothermal kinetic transients. The oxygen coverages measured during reversible isothermal switches from He to NO/He mixtures showed that NO decomposition involves bimolecular reactions of two NO molecules adsorbed on vicinal Cu+ species with the formation of N2O as the initial product near ambient temperature. These vicinal Cu+ species form via oxygen removal from {Cu2+−O2-−Cu2+}2+ to form {Cu+−□−Cu+}2+ using NO2 as an oxygen carrier among distant oxidized dimers. Adsorbed nitrate (NO3*) is the kinetically relevant intermediate in the formation of O2 during NO decomposition. This NO3* decomposition reaction is one of the steps involved in the equilibrated formation of NO2 observed during NO decomposition. These NO-mediated oxygen removal pathways, in which NO acts both as a reductant and as an oxidant, are significantly more rapid than recombinative desorption steps. The desorption of products and of unreacted NO during reactions of preadsorbed NO with increasing catalyst temperature confirmed the bimolecular nature and the low activation energy for N2O formation from NO. The facile nature of this reaction and the unfavorable NO adsorption thermodynamics as temperature increases combine to give the observed decrease in NO decomposition rates at high temperatures. These findings are consistent with some reported mechanism-based steady-state rate expressions and with previous infrared detection of the reaction intermediates proposed here based on isothermal and nonisothermal transients. These pathways appear to be relevant also to N2O decomposition reactions, which occur after the initial formation of N2O from NO and involve reactions of N2O with {Cu+−□−Cu+}2+ and removal of oxygen via NO-mediated desorption pathways. This study brings consensus and some clarification into the mechanistic details for NO and N2O decomposition reactions and resolves some remaining discrepancies and some contradictory conclusions in previous reports.

Polyoxometalates as mediators in the laccase catalyzed delignification

The polyoxometalate (POM)-laccase catalytic system was applied for the first time to aerobic delignification of kraft pulps at moderate (40–60 °C) temperatures. Laccase was found to readily catalyze the re-oxidation of different kinds of polyoxometalates, including those, which cannot be re-oxidized by dioxygen even at high temperatures (PMo 11V 1, SiW 11V 1, etc.). This allows a sequence of catalytic redox cycles similar to that in the laccase-mediator system (LMS) where electrons are transferred from the substrate (lignin) via POM and laccase to oxygen. Results obtained showed that the POM-laccase system could decrease κ number of eucalypt kraft pulp from 13.7 to 8.5 though the reaction rate is relatively slow. Among different POM used, SiW 11V showed the best results. The effect of the process variables on the delignification was studied. The best results in delignification of eucalypt pulp were obtained at 60 °C, oxygen pressure of 5 bar, pH 6.3, SiW 11V concentration of 4.2 mM and laccase concentration of 0.65 U/ml. The reaction temperature appears to be one of the crucial factors in the achievement of a delignification rate acceptable for practical application.

The anticancer drug-DNA complex: femtosecond primary dynamics for anthracycline antibiotics function.

The anthracycline-DNA complex, which is a potent agent for cancer chemotherapy, has a unique intercalating molecular structure with preference to the GC bases of DNA, as shown by Rich's group in studies of single-crystal x-ray diffraction. Understanding cytotoxicity and its photoenhancement requires the unraveling of the dynamics under the solution-phase, physiological condition. Here we report our first study of the primary processes of drug function. In a series of experiments involving the drug (daunomycin and adriamycin) in water, the drug-DNA complexes, the complexes with the four nucleotides (dGTP, dATP, dCTP, and dTTP), and the drug-apo riboflavin-binding protein, we show the direct involvement of molecular oxygen and DNA base-drug charge-separation-the rates for the reduction of the drug and dioxygen indicate the crucial role of drug/base/O(2) in the efficient and catalytic redox cycling. These dynamical steps, and the subsequent reactions of the superoxide product(s), can account for the photoenhanced function of the drug in cells, and potentially for the cell death.

Quinoid redox cycling as a mechanism for sustained free radical generation by inhaled airborne particulate matter

The health effects of airborne fine particles are the subject of government regulation and scientific debate. The aerodynamics of airborne particulate matter, the deposition patterns in the human lung, and the available experimental and epidemiological data on health effects lead us to focus on airborne particulate matter with an aerodynamic mean diameter less than 2.5 μm (PM2.5) as the fraction of the particles with the largest impact in health. In this article we present a novel hypothesis to explain the continuous production of reactive oxygen species produced by PM2.5 when it is deposited in the lung. We find PM2.5 contains abundant persistent free radicals, typically 1016 to 1017 unpaired spins/gram, and that these radicals are stable for several months. These radicals are consistent with the stability and electron paramagnetic resonance spectral characteristics of semiquinone radicals. Catalytic redox cycling by semiquinone radicals is well documented in the literature and we had studied in detail its role on the health effects of cigarette smoke particulate matter. We believe that we have for the first time shown that the same, or similar radicals, are not confined to cigarette smoke particulate matter but are also present in PM2.5. We hypothesize that these semiquinone radicals undergo redox cycling, thereby reducing oxygen and generating reactive oxygen species while consuming tissue-reducing equivalents, such as NAD(P)H and ascorbate. These reactive oxygen species generated by particles cause oxidative stress at sites of deposition and produce deleterious effects observed in the lung.

The kinetic role of metal ions in environmentally relevant redox reactions of chlorite ion and sulfite ion

A brief survey is given about the reactions of chlorite and sulfite ions with some of the most common transition metal ions. The interpretation of the complex kinetic patterns observed in these systems requires the use of comprehensive data treatment and simultaneous evaluation of all experimental results. In most cases, complex formation reactions are in the core of the catalytic redox cycles initiated by the metal ions. The environmental relevance of the kinetic models proposed for catalytic decomposition of chlorite ion and auto-oxidation of sulfite ion is discussed in detail.

Contrasting effects of catecholic and O-methylated tetrahydroisoquinolines on hydroxyl radical production

Tetrahydroisoquinolines (TIQs) are intraneuronal, catecholamine-derived alkaloids that have been implicated in the etiology of Parkinson’s disease and in alcohol related disorders. The in vitro production of the cytotoxic hydroxyl radical ( ⋅OH) was recorded during the autoxidation of salsolinol (SAL) and salsolinol-1-carboxylic acid (SAL-1C), but not when these two catecholic TIQs were oxidized by tyrosinase. Significantly higher levels of the radical were produced when these catecholic TIQs were incubated with ⋅OH generating complexes, or with chelated iron. In contrast, mono- O-methylated TIQs such as salsoline (SLN) and salsoline-1-carboxylic acid (SLN-1C) did not generate ⋅OH during autoxidation or when incubated with chelated iron or tyrosinase. Radical production by ⋅OH-generating complexes was reduced in the presence of O-methylated TIQs. The neurotoxicity of TIQs may result from their propensity to autoxidize and generate reactive quinoids and ensuing oxygen radicals. The functional significance of the replacement of a hydroxyl group attached to C-7 of SAL or SAL-1C with a methoxyl group remains to be determined. This single structural modification may prevent mono- O-methylated TIQs from participating in catalytic redox cycling reactions that would otherwise augment ⋅OH production. If true, then O-methylation and other cellular mechanisms that circumvent the autoxidation of catecholamine-derived TIQs may reduce the likelihood of these substances forming cytotoxic quinoids and influencing endogenous ⋅OH-generating reactions.

Coenzyme A-disulfide reductase from Staphylococcus aureus: evidence for asymmetric behavior on interaction with pyridine nucleotides.

An unusual flavoprotein disulfide reductase, which catalyzes the NADPH-dependent reduction of CoASSCoA, has recently been purified from the human pathogen Staphylococcus aureus [delCardayré, S. B., Stock, K. P., Newton, G. L., Fahey, R. C., and Davies, J. E. (1998) J. Biol. Chem. 273, 5744-5751]. Coenzyme A-disulfide reductase (CoADR) lacks the redox-active protein disulfide characteristic of the disulfide reductases; instead, NADPH reduction yields 1 protein-SH and 1 CoASH. Furthermore, the CoADR sequence reveals the presence of a single putative active-site Cys (Cys43) within an SFXXC motif also seen in the Enterococcus faecalis NADH oxidase and NADH peroxidase, which use a single redox-active cysteine-sulfenic acid in catalysis. In this report, we provide a detailed examination of the equilibrium properties of both wild-type and C43S CoADRs, focusing on the role of Cys43 in the catalytic redox cycle, the behavior of both enzyme forms on reduction with dithionite and NADPH, and the interaction of NADP+ with the corresponding reduced enzyme species. The results of these analyses, combined with electrospray mass spectrometric data for the two oxidized enzyme forms, fully support the catalytic redox role proposed for Cys43 and confirm that this is the attachment site for bound CoASH. In addition, we provide evidence indicating dramatic thermodynamic inequivalence between the two active sites per dimer, similar to that documented for the related enzymes mercuric reductase and NADH oxidase; only 1 FAD is reduced with NADPH in wild-type CoADR. The EH2.NADPH/EH4.NADP+ complex which results is reoxidized quantitatively in titrations with CoASSCoA, supporting a possible role for the asymmetric reduced dimer in catalysis.