Showing papers on "Redox published in 1999"

Natural engineering principles of electron tunnelling in biological oxidation-reduction.

Abstract: We have surveyed proteins with known atomic structure whose function involves electron transfer; in these, electrons can travel up to 14 A between redox centres through the protein medium. Transfer over longer distances always involves a chain of cofactors. This redox centre proximity alone is sufficient to allow tunnelling of electrons at rates far faster than the substrate redox reactions it supports. Consequently, there has been no necessity for proteins to evolve optimized routes between redox centres. Instead, simple geometry enables rapid tunnelling to high-energy intermediate states. This greatly simplifies any analysis of redox protein mechanisms and challenges the need to postulate mechanisms of superexchange through redox centres or the maintenance of charge neutrality when investigating electron-transfer reactions. Such tunnelling also allows sequential electron transfer in catalytic sites to surmount radical transition states without involving the movement of hydride ions, as is generally assumed. The 14 A or less spacing of redox centres provides highly robust engineering for electron transfer, and may reflect selection against designs that have proved more vulnerable to mutations during the course of evolution.

The Decamethylferrocenium/Decamethylferrocene Redox Couple: A Superior Redox Standard to the Ferrocenium/Ferrocene Redox Couple for Studying Solvent Effects on the Thermodynamics of Electron Transfer

Abstract: The solvent dependence of the formal redox potentials of the ferrocenium/ferrocene (Fc+/0) and 1,2,3,4,5-pentamethylferrocenium/1,2,3,4,5-pentamethylferrocene (Me5Fc+/0) couples versus the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) couple indicates that the latter is a superior redox standard for studying solvent effects on the thermodynamics of electron transfer. The couples were studied in 29 solvents and the differences in formal redox potentials between the MenFc+/0 (n = 5, 10) and Fc+/0 couples are surprisingly solvent dependent. In the case of the Fc+/0 couple versus the Me10Fc+/0 couple, the potential difference ranges from +583 mV in 2,2,2-trifluoroethanol to +293 mV in water. The positive shifts for the Me5Fc+/0 couple versus the Me10Fc+/0 couple were about half of these values. The Me10Fc+/0 redox couple can also be used in easily oxidized solvents, such as N-methylaniline and N,N-dimethylaniline, or in conjunction with a Hg working electrode. Statistical multiparameter analysis of th...

Protein-sulfenic acids: diverse roles for an unlikely player in enzyme catalysis and redox regulation.

Abstract: While it has been known for more than 20 years that unusually stable cysteine-sulfenic acid (Cys-SOH) derivatives can be introduced in selected proteins by mild oxidation, only recently have chemical and crystallographic evidence for functional Cys-SOH been presented with native proteins such as NADH peroxidase and NADH oxidase, nitrile hydratase, and the hORF6 and AhpC peroxiredoxins. In addition, Cys-SOH forms of protein tyrosine phosphatases and glutathione reductase have been suggested to play key roles in the reversible inhibition of these enzymes during tyrosine phosphorylation-dependent signal transduction events and nitrosative stress, respectively. Substantial chemical data have also been presented which implicate Cys-SOH in redox regulation of transcription factors such as Fos and Jun (activator protein-1) and bovine papillomavirus-1 E2 protein. Functionally, the Cys-SOHs in NADH peroxidase, NADH oxidase, and the peroxiredoxins serve as either catalytically essential redox centers or transient intermediates during peroxide reduction. In nitrile hydratase, the active-site Cys-SOH functions in both iron coordination and NO binding but does not play any catalytic redox role. In Fos and Jun and the E2 protein, on the other hand, a key Cys-SH serves as a sensor for intracellular redox status; reversible oxidation to Cys-SOH as proposed inhibits the corresponding DNA binding activity. These functional Cys-SOHs have roles in diverse cellular processes, including signal transduction, oxygen metabolism and the oxidative stress response, and transcriptional regulation, as well as in the industrial production of acrylamide, and their detailed analyses are beginning to provide the chemical foundation necessary for understanding protein-SOH stabilization and function.

Phonon- Versus Electron-Mediated Desorption and Oxidation of CO on Ru(0001)

Abstract: Heating of a ruthenium surface on which carbon monoxide and atomic oxygen are coadsorbed leads exclusively to desorption of carbon monoxide. In contrast, excitation with femtosecond infrared laser pulses enables also the formation of carbon dioxide. The desorption is caused by coupling of the adsorbate to the phonon bath of the ruthenium substrate, whereas the oxidation reaction is initiated by hot substrate electrons, as evidenced by the observed subpicosecond reaction dynamics and density functional calculations. The presence of this laser-induced reaction pathway allows elucidation of the microscopic mechanism and the dynamics of the carbon monoxide oxidation reaction.

Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen

Abstract: Terminally oxidized hydrocarbons are of considerable interest as potential feedstocks for the chemical and pharmaceutical industry, but the selective oxidation of only the terminal methyl groups in alkanes remains a challenging task. It is accomplished with high efficiency and selectivity by some enzymes; but inorganic catalysts, although inferior in overall performance under benign conditions, offer significant advantages from a processing standpoint1. Controlled partial oxidation is easier to achieve with ‘sacrificial’ oxidants, such as hydrogen peroxide2, alkyl hydroperoxides oriodosylbenzene3, than with molecular oxygen or air. These sacrificial oxidants, themselves the product of oxidation reactions, have been used in catalytic systems involving tailored transition-metal complexes in either a homogeneous state4,5,6, encapsulated in molecular sieves7,8,9 or anchored to the inner surfaces of porous siliceous supports10. Here we report the design and performance of two aluminophosphate molecular sieves containing isolated, four-coordinated Co(III) or Mn(III) ions that are substituted into the framework and act, in concert with the surrounding framework structure, as regioselective catalysts for the oxidation of linear alkanes by molecular oxygen. The catalysts operate at temperatures between 373 K and 403 K through a classical free-radical chain-autoxidation mechanism. They are thus able to use molecular oxygen as oxidant, which, in combination with their good overall performance, raises the prospect of using this type of selective inorganic catalyst for industrial oxidation processes.

The Steady-State Internal Redox State (NADH/NAD) Reflects the External Redox State and Is Correlated with Catabolic Adaptation in Escherichia coli

Abstract: Escherichia coli MC4100 was grown in anaerobic glucose-limited chemostat cultures, either in the presence of an electron acceptor (fumarate, nitrate, or oxygen) or fully fermentatively. The steady-state NADH/NAD ratio depended on the nature of the electron acceptor. Anaerobically, the ratio was highest, and it decreased progressively with increasing midpoint potential of the electron acceptor. Similarly, decreasing the dissolved oxygen tension resulted in an increased NADH/NAD ratio. As pyruvate catabolism is a major switch point between fermentative and respiratory behavior, the fluxes through the different pyruvate-consuming enzymes were calculated. Although pyruvate formate lyase (PFL) is inactivated by oxygen, it was inferred that the in vivo activity of the enzyme occurred at low dissolved oxygen tensions (DOT ≤ 1%). A simultaneous flux from pyruvate through both PFL and the pyruvate dehydrogenase complex (PDHc) was observed. In anaerobic cultures with fumarate or nitrate as an electron acceptor, a significant flux through the PDHc was calculated on the basis of the redox balance, the measured products, and the known biochemistry. This result calls into question the common assumption that the complex cannot be active under these conditions. In vitro activity measurements of PDHc showed that the cellular content of the enzyme varied with the internal redox state and revealed an activity for dissolved oxygen tension of below 1%. Whereas Western blots showed that the E3 subunit of PDHc (dihydrolipoamide dehydrogenase) did not vary to a large extent under the conditions tested, the E2 subunit (dihydrolipoamide acetyltransferase) amount followed the trend that was found for the in vitro PDHc activity. From this it is concluded that regulation of the PDHc is exerted at the E1/E2 operon (aceEF). We propose that the external redox state (measured as the midpoint potentials of those terminal acceptors with which the cell has sufficient capacity to react) is reflected by the internal redox state. The latter may subsequently govern both the expression and the activity of the two pyruvate-catabolizing enzymes.

Humics as an electron donor for anaerobic respiration.

Abstract: The possibility that microorganisms might use reduced humic substances (humics) as an electron donor for the reduction of electron acceptors with a more positive redox potential was investigated. All of the Fe(III)- and humics-reducing microorganisms evaluated were capable of oxidizing reduced humics and/or the reduced humics analogue anthrahydroquinone-2,6,-disulphonate (AHODS), with nitrate and/or fumarate as the electron acceptor. These included Geobacter metallireducens, Geobacter sulphurreducens, Geothrix fermentans, Shewanella alga, Wolinella succinogenes and 'S. barnesii'. Several of the humics-oxidizing microorganisms grew in medium with AHQDS as the sole electron donor and fumarate as the electron acceptor. Even though it does not reduce Fe(III) or humics, Paracoccus denitrificans could use AHQDS and reduced humics as electron donors for denitrification. However, another denitrifier, Pseudomonas denitrificans, could not. AHODS could also serve as an electron donor for selenate and arsenate reduction by W. succinogenes. Electron spin resonance studies demonstrated that humics oxidation was associated with the oxidation of hydroquinone moieties in the humics. Studies with G. metallireducens and W. succinogenes demonstrated that the anthraquinone-2,6-disulphonate (AQDS)/AHQDS redox couple mediated an interspecies electron transfer between the two organisms. These results suggest that, as microbially reduced humics enter less reduced zones of soils and sediments, the reduced humics may serve as electron donors for microbial reduction of several environmentally significant electron acceptors.

Redox and fluorophore functionalization of water-soluble, Tiopronin- protected gold clusters

Abstract: Place-exchange and amide-forming coupling reactions represent two facile and efficient routes to poly-functionalization of water-soluble nanoparticles. In this paper, place-exchange and amide-forming coupling reactions with water-soluble tiopronin-MPCs are described and their products characterized by 1H and 31P NMR, capillary electrophoresis, electrochemistry, and fluorescence spectroscopy. Place-exchange reactions of ligands with tiopronin-MPCs yield products with about half of the ligand exchange expected on the basis of solution stoichiometry and a nonselective exchange and were not noticeably affected by steric encumbrances. Tiopronin-MPCs to which viologens are coupled (avg 36/MPC) adsorb as monolayers on Au electrodes as shown in electrochemical quartz crystal microbalance experiments. Multilayer adsorption occurs on long experimental time scales. The strong adsorption of the viologen-functionalized clusters is ascribed to increased interaction and stability in the viologen reduction products. Capi...

Control of Catechol and Hydroquinone Electron-Transfer Kinetics on Native and Modified Glassy Carbon Electrodes

Abstract: The electrochemical oxidation of dopamine, 4-methylcatechol, dihydroxyphenylacetic acid, dihydroxyphenyl ethylene glycol, and hydroquinone was examined on several native and modified glassy carbon (GC) surfaces Treatment of polished GC with pyridine yielded small ΔEp values for cyclic voltammetry of all systems studied, implying fast electron-transfer kinetics Changes in surface oxide coverage had little effect on kinetics, nor did the charge of the catechol species or the solution pH Small ΔEp values correlated with catechol adsorption, and surface pretreatments that decreased adsorption also increased ΔEp Electron transfer from catechols was profoundly inhibited by a monolayer of nitrophenyl or (trifluoromethyl)phenyl (TFMP) groups on the GC surface, so that voltammetric waves were not observed The ΔEp increased monotonically with surface coverage of TFMP groups The results indicate that catechol adsorption to GC is required for fast electron transfer for the redox systems studied Unlike Ru(NH3)6

Ab Initio Determination of Reversible Potentials and Activation Energies for Outer-Sphere Oxygen Reduction to Water and the Reverse Oxidation Reaction

Abstract: The outer-sphere reduction of oxygen to water according to O2(g) + 4H+(aq) + 4e- → 2H2O(l) (1) and its reverse reaction are analyzed using self-consistent ab initio MP2/6-31G** calculations over th...

SCR of NO by NH3 over TiO2-supported V2O5-MoO3 catalysts: reactivity and redox behavior

Abstract: The reactivity in the selective catalytic reduction (SCR) reaction and the redox behavior of V2O5–MoO3/TiO2 catalysts was investigated by means of the temperature programmed reduction (TPR)/reaction technique, and compared with that of binary V2O5/TiO2 and MoO3/TiO2 catalysts having the same metal oxide loading. It was found that the ternary catalysts are more active in the SCR reaction at low temperatures compared to the corresponding binary samples: hence the ‘temperature window’ of the reaction is widened and shifted towards lower temperatures. Transient reactivity data provide clear evidence in favor of the hypothesis of a redox mechanism for the SCR reaction and point out that the ternary catalysts are more easily reduced and reoxidized than the corresponding binary samples: this indicates that the simultaneous presence of V and Mo enhances the catalyst redox properties, and thus its reactivity. Such conclusions are also in line with the results of the characterization studies pointing out the existence of electronic interactions involving the V and Mo surface oxide species. The overall picture closely resembles the one obtained in the case of the analogous V2O5–WO3/TiO2 system and indicates that the effects of the addition of WO3 and MoO3 to V2O5/TiO2 are similar, both oxides acting as ‘chemical’ promoters besides playing a ‘structural’ function as well.

Facile Preparation of Active Glassy Carbon Electrodes with Activated Carbon and Organic Solvents

Abstract: Pretreatment of glassy carbon (GC) electrodes with 2-propanol, acetonitrile, or cyclohexane had a significant effect on electrode kinetics, adsorption, and capacitance. Reagent grade solvents slowed electron transfer rates for dopamine, ascorbic acid, Fe3+/2+, and Fe(CN)63-/4- and decreased adsorption of anthraquinone-2,6-disulfonate (AQDS) and methylene blue (MB). However, if activated carbon (AC) was present in the solvent during pretreatment, the result was increased electron transfer rates and adsorption for several commonly studied redox systems. The large surface area of AC acts as a “getter” for solvent impurities and for species desorbed from the GC surface, leading to a carbon electrode surface with higher capacitance, higher adsorption of AQDS and MB, and faster electron-transfer rates for Fe(CN)63-/4-, dopamine, and ascorbic acid. In addition, the treated surfaces were more reproducible, and aged electrodes were reactivated by AC in 2-propanol. The results imply that large, polar organic impuri...

Cooperative Bimetallic Redox Reactivity

Abstract: Binucleating ligands possessing contiguous 6-coordinate and 4-coordinate sites are described, and the redox properties of their bimetallic complexes have been investigated. The design of the two parts of these ligands is based on the redox and reactivity patterns of the corresponding monometallic complexes. If the two sites were to behave as they do in the corresponding monometallic complexes, these bimetallic complexes would be expected to bind a substrate, such as dioxygen, and to reduce it by two electrons, one from each metal, as occurs in the respiratory protein, hemerythrin. This was found not to be the case. In these bimetallic complexes, oxidation of one metal leads to the deactivation of the other metal to oxidation. The origins of this mutual deactivation appear to be connected with ligand reorganization, through-bond electronic coupling, and electrostatic interactions between neighboring metals. It is suggested that, in the systems described, ligand reorganization is the dominant deactivating e...

Redox Catalysis Using Ag@SiO2 Colloids

Abstract: We show that metal particles encapsulated by thin silica shells can catalyze redox reactions on their surface. The surface charge of both silica-coated and uncoated colloidal silver particles has been varied by chemical electron injection, using controlled amounts of borohydride ions. The charging and discharging processes were monitored by time-resolved UV−visible spectroscopy, making use of the influence of the surface charge on the position and intensity of the silver plasmon band. We show that the charging and discharging rates can be controlled by means of silica coating, since they are limited by the diffusion of borohydride ions through the silica pores.