Showing papers on "Redox published in 2004"

Radical generation by the interaction of transition metals with common oxidants.

Abstract: Nine transition metals were tested for the activation of three oxidants and the generation of inorganic radical species such as sulfate, peroxymonosulfate, and hydroxyl radicals. From the 27 combinations, 14 M/Ox couples demonstrated significant reactivity toward transforming a model organic substrate such as 2,4-dichlorophenol and are further discussed here. It was found that Co(II) and Ru(III) are the best metal catalysts for the activation of peroxymonosulfate. As expected on the basis of the Fenton reagent, Fe(III) and Fe(II) were the most efficient transition metals for the activation of hydrogen peroxide. Finally, Ag(I) showed the best results toward activating persulfate. Quenching studies with specific alcohols (tert-butyl alcohol and ethanol) were also performed to identify the primary radical species formed from the reactive M/Ox interactions. The determination of these transient species allowed us to postulate the rate-determining step of the redox reactions taking place when a metal is coupled with an oxidant in aqueous solution. It was found that when Co(II), Ru(III), and Fe(II) interact with peroxymonosulfate, freely diffusible sulfate radicals are the primary species formed. The same was proven for the interaction of Ag(I) with persulfate, but in this case caged or bound to the metal sulfate radicals might be formed as well. The conjunction of Ce(III), Mn(II), and Ni(II) with peroxymonosulfate showed also to generate caged or bound to the metal sulfate radicals. A combination of sulfate and hydroxyl radicals was formed from the conjunction of V(III) with peroxymonosulfate and from Fe(II) with persulfate. Finally, the conjunction of Fe(III), Fe(II), and Ru(III) with hydrogen peroxide led primarily to the generation of hydroxyl radicals. It is also suggested here that the redox behavior of a particular metal in solution cannot be predicted based exclusively on its size and charge. Additional phenomena such as metal hydrolysis as well as complexation with other counterions present in solution might affect the thermodynamics of the overall process and are further discussed here.

Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor

Abstract: The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding o...

Biofuel Cells Select for Microbial Consortia That Self-Mediate Electron Transfer

Abstract: Microbial fuel cells hold great promise as a sustainable biotechnological solution to future energy needs. Current efforts to improve the efficiency of such fuel cells are limited by the lack of knowledge about the microbial ecology of these systems. The purposes of this study were (i) to elucidate whether a bacterial community, either suspended or attached to an electrode, can evolve in a microbial fuel cell to bring about higher power output, and (ii) to identify species responsible for the electricity generation. Enrichment by repeated transfer of a bacterial consortium harvested from the anode compartment of a biofuel cell in which glucose was used increased the output from an initial level of 0.6 W m−2 of electrode surface to a maximal level of 4.31 W m−2 (664 mV, 30.9 mA) when plain graphite electrodes were used. This result was obtained with an average loading rate of 1 g of glucose liter−1 day−1 and corresponded to 81% efficiency for electron transfer from glucose to electricity. Cyclic voltammetry indicated that the enhanced microbial consortium had either membrane-bound or excreted redox components that were not initially detected in the community. Dominant species of the enhanced culture were identified by denaturing gradient gel electrophoresis and culturing. The community consisted mainly of facultative anaerobic bacteria, such as Alcaligenes faecalis and Enterococcus gallinarum, which are capable of hydrogen production. Pseudomonas aeruginosa and other Pseudomonas species were also isolated. For several isolates, electrochemical activity was mainly due to excreted redox mediators, and one of these mediators, pyocyanin produced by P. aeruginosa, could be characterized. Overall, the enrichment procedure, irrespective of whether only attached or suspended bacteria were examined, selected for organisms capable of mediating the electron transfer either by direct bacterial transfer or by excretion of redox components.

Palladium Oxidase Catalysis: Selective Oxidation of Organic Chemicals by Direct Dioxygen-Coupled Turnover

Abstract: Selective aerobic oxidation of organic molecules is a fundamental and practical challenge in modern chemistry. Effective solutions to this problem must overcome the intrinsic reactivity and selectivity challenges posed by the chemistry of molecular oxygen, and they must find application in diverse classes of oxidation reactions. Palladium oxidase catalysis combines the versatility of Pd(II)-mediated oxidation of organic substrates with dioxygen-coupled oxidation of the reduced palladium catalyst to enable a broad range of selective aerobic oxidation reactions. Recent developments revealed that cocatalysts (e.g. Cu(II), polyoxometalates, and benzoquinone) are not essential for efficient oxidation of Pd(0) by molecular oxygen. Oxidatively stable ligands play an important role in these reactions by minimizing catalyst decomposition, promoting the direct reaction between palladium and dioxygen, modulating organic substrate reactivity and permitting asymmetric catalysis.

Visible light-induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions

Abstract: The photocatalytic degradation of a model azo-dye (Acid Orange, AO7) in aerated aqueous TiO2 dispersion has been studied under visible light (λ>400 nm) irradiation. The presence and role of oxidative species, such as singlet oxygen ( 1 O 2 ), superoxide (O2− ) and hydroperoxy (HO2 ) radicals was examined with the use of appropriate quenchers of these species. The reaction pathway of dye degradation was also investigated by monitoring the temporal evolution of intermediates and final products on both the photocatalyst surface and in solution, with the use of a variety of techniques, including GC–MS, FTIR and UV-Vis spectroscopies. It has been found that complete decolorization of the solution may be achieved, accompanied by a substantial decrease of the chemical oxygen demand (COD) of the solution. Evidence is presented that the main oxidative species is O2− (or HO2 ), while singlet oxygen, when formed, is also active. The adsorbed dye molecule is initially cleaved in the vicinity of the azo-bond and the resulting fragments are oxidized toward compounds of progressively lower molecular weight and, eventually, to CO2 and inorganic ions. However, when the solution is bleached, formation of active oxidative species does not take place, oxidation reactions cease and the concentrations of the dye intermediates remain practically stable upon further exposure to visible light irradiation. Formation of photoinduced hydrogen peroxide, which is also generated under the present conditions, also stops when the dye concentration in solution drops to very low levels. This behavior has been explained evoking the photosensitization mechanism of wide band-gap semiconductors, according to which the reaction is triggered by excitation of the dye molecule by visible light photons, followed by charge injection to the conduction band of the semiconductor and subsequent production of active oxygen radicals. Formation of the latter oxidizing species is possible only in the presence of visible light-absorbing compounds and cannot take place after fragmentation of the parent AO7 molecule in the vicinity of the azo-bond and decolorization.

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Abstract: The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as the archetypal TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). The different types of reactivity observed are rationalized by considering the bond dissociation energies (BDEs) of the respective N-hydroxy precursors, substrates and reaction intermediates. Reactive diacylnitroxyl radicals are generated in situ from the corresponding N-hydroxy compound. The protagonist, NHPI, catalyzes a wide variety of free radical autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal co-catalysts improved conversions and selectivities are obtained in the autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. For example, cyclohexylbenzene afforded the 1-hydroperoxide in 97.6% selectivity at 32% conversion when the autoxidation was performed in the presence of 0.5 mol % NHPI, and the product hydroperoxide as initiator, at 100 °C. This forms the basis for a potential coproduct-free route from benzene to phenol. In combination with transition metal co-catalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin NHS, afford effective catalytic systems for the effective autoxidation of hydrocarbons, e.g., toluenes to carboxylic acids, under mild conditions. In the case of the less reactive cycloalkanes, NHS proved to be a more active catalyst than NHPI which is attributed to the higher reactivity of the intermediate nitroxyl radical, resulting from the replacement of a carbonyl group in NHPI by the more strongly electron-attracting sulfonyl group. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g., alcohols, with single oxygen donors such as hypochlorite and organic peracids. The reactions involve the intermediate formation of the corresponding oxoammonium cation as the active oxidant. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols. These reactions involve metal-centered dehydrogenations and the role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu) and oxidation of the alcohol (Cu) via hydrogen abstraction or one-electron oxidation processes. Detailed mechanistic investigations, including kinetic isotope effects, revealed that oxoammonium cations are not involved as intermediates in these reactions. In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme. Similarly, N-hydroxy compounds such as NHPI also act as mediators in laccase-catalyzed oxidations of alcohols. These reactions are assumed to involve one electron oxidation of the N-hydroxy compound, leading to the formation of a proton and the nitroxyl radical, which abstracts a hydrogen atom from the substrate. However, neither of these laccase-based systems has, as yet, attained the activity and scope of the TEMPO/hypochlorite system.

Oxidative Stress-Induced Calcium Signaling in Arabidopsis

Abstract: Many environmental stresses result in increased generation of active oxygen species in plant cells. This leads to the induction of protective mechanisms, including changes in gene expression, which lead to antioxidant activity, the recovery of redox balance, and recovery from damage/toxicity. Relatively little is known about the signaling events that link perception of increased active oxygen species levels to gene expression in plants. We have investigated the role of calcium signaling in H2O2-induced expression of the GLUTATHIONE-S-TRANSFERASE1 (GST1) gene. Challenge with H2O2 triggered a biphasic Ca2+ elevation in Arabidopsis seedlings. The early Ca2+ peak localized to the cotyledons, whereas the late Ca2+ rise was restricted to the root. The two phases of the Ca2+ response were independent of each other, as shown by severing shoot from root tissues before H2O2 challenge. Modulation of the height of Ca2+ rises had a corresponding effect upon H2O2-induced GST1 expression. Application of the calcium channel blocker lanthanum reduced the height of the first Ca2+ peak and concomitantly inhibited GST1 expression. Conversely, enhancing the height of the H2O2-triggered Ca2+ signature by treatment with L-buthionine-[S,R]-sulfoximine (an inhibitor of glutathione synthesis) lead to enhancement of GST1 induction. This finding also indicates that changes in the cellular redox balance constitute an early event in H2O2 signal transduction as reduction of the cellular redox buffer and thus the cell's ability to maintain a high GSH/GSSG ratio potentiated the plant's antioxidant response.

Monitoring disulfide bond formation in the eukaryotic cytosol

Abstract: Glutathione is the most abundant low molecular weight thiol in the eukaryotic cytosol. The compartment-specific ratio and absolute concentrations of reduced and oxidized glutathione (GSH and GSSG, respectively) are, however, not easily determined. Here, we present a glutathione-specific green fluorescent protein–based redox probe termed redox sensitive YFP (rxYFP). Using yeast with genetically manipulated GSSG levels, we find that rxYFP equilibrates with the cytosolic glutathione redox buffer. Furthermore, in vivo and in vitro data show the equilibration to be catalyzed by glutaredoxins and that conditions of high intracellular GSSG confer to these a new role as dithiol oxidases. For the first time a genetically encoded probe is used to determine the redox potential specifically of cytosolic glutathione. We find it to be −289 mV, indicating that the glutathione redox status is highly reducing and corresponds to a cytosolic GSSG level in the low micromolar range. Even under these conditions a significant fraction of rxYFP is oxidized.

Nitrate, NO and haemoglobin in plant adaptation to hypoxia: an alternative to classic fermentation pathways

Abstract: The role of nitrate reduction to produce nitric oxide (NO) and its subsequent oxidation by oxyhaemoglobin as a mechanism to maintain plant cell energetics during hypoxia is examined. Nitrate reduction in hypoxic conditions can be considered as an alternative respiratory pathway, with nitrate as an intermediate electron acceptor, contributing to the oxidation of NADH. NO, produced in the reaction, does not accumulate due to the induction of hypoxia-induced (class 1) haemoglobins. These haemoglobins remain in the oxyhaemoglobin form, even at oxygen tensions two orders of magnitude lower than necessary to saturate cytochrome c oxidase. They act, probably in conjunction with a flavoprotein, as NO dioxygenases converting NO back to nitrate, consuming NAD(P)H in the process. The overall system oxidizes 2.5 moles of NADH per one mole of nitrate recycled during the reaction, leading to the maintenance of redox and energy status during hypoxia and resulting in the reduced production of ethanol and lactic acid.

Solution redox chemistry of carbon nanotubes.

Abstract: UV/vis/NIR absorbance spectra were used to monitor electron transfer between small-molecule redox reagents and carbon nanotubes (CNTs). The oxidation of (6, 5)-enriched nanotubes in water with K(2)Ir(Cl)(6) reveals a valence electron density of 0.2-0.4 e(-)/100 carbon atoms and a reduction potential of approximately 800 mV versus NHE. The reduction potential of CNTs is found to increase with increasing band gap and to decrease with the introduction of an anionic dispersant. In light of this newly revealed redox chemistry of CNTs, we propose that the previously observed bleaching of the CNT absorbance spectrum at low pH is most likely a consequence of the oxidation of the nanotubes by oxygen. These results demonstrate facile oxidation and reduction of CNTs, provide a way to quantify the population of valence electrons, and point to possible applications of CNT in the catalysis of redox reactions.

Cysteine/cystine couple is a newly recognized node in the circuitry for biologic redox signaling and control

Abstract: Redox mechanisms function in control of gene expression, cell proliferation, and apoptosis, but the circuitry for redox signaling remains unclear. Cysteine and methionine are the only amino acids in proteins that undergo reversible oxidation/reduction under biologic conditions and, as such, provide a means for control of protein activity, protein-protein interaction, protein trafficking, and protein-DNA interaction. Hydrogen peroxide and other reactive oxygen species (ROS) provide a mechanism to oxidize signaling proteins. However, oxidation of sulfur-containing side chains of cysteine and methionine by ROS can result in oxidation states of sulfur (e.g., sulfinate, sulfonate, sulfone) that are not reducible under biologic conditions. Thus, mechanisms for oxidation that protect against over-oxidation of these susceptible residues and prevent irreversible loss of activity would be advantageous. The present study shows that the steady-state redox potential of the cysteine/cystine couple (Eh = -145 mV) in cells is sufficiently oxidized (>90 mV) relative to the GSH/GSSG (-250 mV) and thioredoxin (Trx1, -280 mV) redox couples for the cysteine/cystine couple to function as an oxidant in redox switching. Consequently, the cysteine/cystine couple provides a means to oxidize proteins without direct involvement of more potent oxidants. A circuitry model incorporating cysteine as a redox node, along with Trx1 and GSH, reveals how selective interactions between the different thiol/disulfide couples and reactive protein thiols could differentially regulate metabolic functions. Moreover, inclusion of cysteine/cystine as a signaling node distinct from GSH and Trx1 significantly expands the redox range over which protein thiol/disulfide couples may operate to control physiologically relevant processes.

A Four-Electron O2-Electroreduction Biocatalyst Superior to Platinum and a Biofuel Cell Operating at 0.88 V

Abstract: O2 was electroreduced to water, at a true-surface-area-based current density of 0.5 mA cm-2, at 37 degrees C and at pH 5 on a "wired" laccase bioelectrocatalyst-coated carbon fiber cathode. The polarization (potential vs the reversible potential of the O2 /H2O half-cell in the same electrolyte) of the cathode was only -0.07 V, approximately one-fifth of the -0.37 V polarization of a smooth platinum fiber cathode, operating in its optimal electrolyte, 0.5 M H2SO4. The bioelectrocatalyst was formed by "wiring" laccase to carbon through an electron conducting redox hydrogel, its redox functions tethered through long and flexible spacers to its cross-linked and hydrated polymer. Incorporation of the tethers increased the apparent electron diffusion coefficient 100-fold to (7.6 +/- 0.3) x 10-7 cm 2 s-1. A miniature single-compartment glucose-O2 biofuel cell made with the novel cathode operated optimally at 0.88 V, the highest operating voltage for a compartmentless miniature fuel cell.

Electron transfer at a dithiolate-bridged diiron assembly: electrocatalytic hydrogen evolution.

Abstract: Electrochemical reduction of Fe2(μ-pdt)(CO)6 1 (pdt = propane-1,3-dithiolate) leads initially to a short-lived species, 1-, then subsequently to two-electron reduced products, including a CO-bridged diiron compound, 1B. The assignment of the redox level of 1- is based on EPR and UV−vis spectra together with the observation that a CO-saturated solution of 1- decays to give 1 and 1B. Hydride reduction of 1 also results in formation of 1B via a relatively long-lived formyl species, 1formyl. Despite its involvement in hydride transfer reactions, 1B is formulated as [Fe2(μ-S(CH2)3SH)(μ-CO)(CO)6]- based on a range of spectroscopic measurements together with the Fe−Fe separation of 2.527 A (EXAFS). Electrocatalytic proton reduction in the presence of 1 in moderately strong acids has been examined by electrochemical and spectroelectrochemical techniques. The acid concentration dependence of the voltammetry is modeled by a mechanism with two electron/proton additions leading to 1H2, where dissociation of dihydroge...

S-glutathionylation: From Redox Regulation of Protein Functions to Human Diseases

Abstract: Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play an integral role in the modulation of several physiological functions but can also be potentially destructive if produced in excessive amounts. Protein cysteinyl thiols appear especially sensitive to ROS/RNS attack. Experimental evidence started to accumulate recently, documenting that S-glutathionylation occurs in a number of physiologically relevant situations, where it can produce discrete modulatory effects on protein function. The increasing evidence of functional changes resulting from this modification, and the growing number of proteins shown to be S-glutathionylated both in vitro and in vivo support this contention, and confirm this as an attractive area of research. S-glutathionylated proteins are now actively investigated with reference to problems of biological interest and as possible biomarkers of human diseases associated with oxidative/nitrosative stress.

Reversible redox energy coupling in electron transfer chains

Abstract: Reversibility is a common theme in respiratory and photosynthetic systems that couple electron transfer with a transmembrane proton gradient driving ATP production. This includes the intensely studied cytochrome bc1, which catalyses electron transfer between quinone and cytochrome c. To understand how efficient reversible energy coupling works, here we have progressively inactivated individual cofactors comprising cytochrome bc1. We have resolved millisecond reversibility in all electron-tunnelling steps and coupled proton exchanges, including charge-separating hydroquinone-quinone catalysis at the Q(o) site, which shows that redox equilibria are relevant on a catalytic timescale. Such rapid reversibility renders popular models based on a semiquinone in Q(o) site catalysis prone to short-circuit failure. Two mechanisms allow reversible function and safely relegate short-circuits to long-distance electron tunnelling on a timescale of seconds: conformational gating of semiquinone for both forward and reverse electron transfer, or concerted two-electron quinone redox chemistry that avoids the semiquinone intermediate altogether.

Selective oxidation of methane to methanol catalyzed, with C-H activation, by homogeneous, cationic gold.

Abstract: Some of the most efficient homogeneous catalysts for the lowtemperature, selective oxidation of methane to functionalized products employ a mechanism involving C H activation with an electrophilic substitution mechanism. Several such systems have been reported based on the cations Hg, Pd, and Pt. These catalyst systems typically operate by two general steps that involve: A) C H activation by coordination of the methane to the inner sphere of the catalyst (E) followed by cleavage of the C Hbond by overall electrophilic substitution to generate E CH3 intermediates, and B) oxidative functionalization involving redox reactions of E CH3 to generate the desired oxidized product CH3X. [4a]