Showing papers on "Redox published in 1986"

Redox balances in the metabolism of sugars by yeasts

Abstract: The central role of the redox couples NAD+/NADH and NADP+/NADPH in the metabolism of sugars by yeasts is discussed in relation to energy metabolism and product formation. Besides their physical compartmentation in cytosol and mitochondria, the two coenzyme systems are separated by chemical compartmentation as a consequence of the absence of transhydrogenase activity. This has considerable consequences for the redox balances of both coenzyme systems and hence for sugar metabolism in yeasts. As examples, the competition between respiration and fermentation of glucose, the Crabtree effect, the Custers effect, adaptation to anaerobiosis, the activities of the hexose monophosphate pathway, and the fermentation of xylose in yeast are discussed.

Chemical and microbiological studies of sulfide‐mediated manganese reduction 1

Abstract: Laboratory studies of manganese reduction by naturally occurring reduced inorganic compounds were undertaken, both to study further possible in situ mechanisms of manganese reduction and to examine how manganese redox reactions might be coupled to other biogeochemical processes. Chemical manganese reduction by sulfide (in the presence of excess manganese oxide) was found to be rapid and complete, with all sulfide being oxidized within 5–10 min. The reduction of δMnO2 by sulfide involves a two‐electron transfer, with S° the predominant oxidized sulfur product. Using a marine sulfate‐reducing bacterium (Desulfovibrio sp.), the kinetics of sulfide‐dependent, bacterially mediated manganese reduction were studied; the rate‐limiting step was bacterial sulfide production. These findings suggest that in stratified marine environments (such as the Black Sea, Saanich Inlet, or certain coastal sediments) manganese reduction should occur just below the oxic‐anoxic (O2/H2S) interface or redox boundary as a re...

Indirect Electroorganic Syntheses—A Modern Chapter of Organic Electrochemistry [New Synthetic Methods (59)]

Abstract: The electrochemical formation and regeneration of redox agents for organic syntheses (indirect electrolysis) widens the potential of electrochemistry, as higher or totally different selectivities can often be obtained while at the same time the energy input can be lowered significantly. Higher current densities can also be obtained by preventing otherwise often encountered electrode inhibition. New types of redox catalysts can be formed in-situ and can be regenerated after reaction with the substrates. This principle is of increasing importance also for the application of already known redox agents with regard to environmental protection, since large amounts of a product can be generated in a closed circuit using only relatively small amounts of the redox reagent. Consequently the operation of such a process can be greatly simplified, and the release of ecologically objectionable spent reagents into the environment can be prevented. The broad spectrum of redox catalysts currently in use includes, inter alia, metal salts in very low or high oxidation states, halogens in various oxidation states, and, in particular, a wide variety of transition-metal complexes. A great deal of progress has recently been made in the application of organic electron transfer agents, since compounds have been found that are sufficiently stable in both the reduced as well as the oxidized state.

Homogeneous and heterogeneous photocatalysis

Abstract: Photoinduced charge separation: Requirements needed for ideal relays and photosensitizers- Photoinduced charge separation: Towards the design of supermolecular systems based on transition metal complexes- Fundamental studies into primary events in photocatalysis employing CdS and TiO semiconductors: Photoluminescence, laser flash photolysis and pulse radiolysis- Dynamics of interfacial electron transfer reactions in colloidal semiconductor systems and water cleavage by visible light- Electronic Spectroscopy of semiconductor clusters- Catalysed decay of oxidising radicals in water- Photocatalysis and light-induced electron transfer reactions of tertiary amines- Photocatalytic production of ascorbic acid A secondary photosynthesis in plants- Spectroscopic and electrochemical studies of photochemical electron transfer in linked donor-acceptor molecules- Bifunctional porphyrins: Redox photochemistry of: meso(tritolyl(pyridyl Ru(III) (NH 3) 4 L')) porphyrin: (L'=NH 3, pyridine, 5-Cl-pyridine)- Linear chain platinum complexes as photocatalysts- Electron transfer reactions on extremely small semiconductor colloids studied by pulse radiolysis- Nafion stabilized aqueous solutions of colloidal semiconductors- In situ generated catalyst-coated colloidal semiconductor CdS particles in surfactant vesicles and polymerized surfactant vesicles- Development of catalysts for water photoreduction: Improvement, poisoning and catalytic mechanism- Radiolytic methods of preparation of colloidal and heterogeneous redox catalysts and their application in light-induced H2 generation from water- The effect of promoters on the photochemical water cleavage in suspensions of Pt-loaded TiO2 with increased light to chemical conversion efficiency- Metallic catalysts on semiconductors: Transparency and electrical contact properties- On the nature of the inhibition of electron transfer at illuminated p-type semiconductor electrodes- The importance of solution kinetics in photoelectrochemical phenomena- Mechanistic aspects of semiconductor photoelectrochemistry: The behavior of Si and GAAs in nonaqueous solvents- Charge injection into semiconductor particles - Importance in photocatalysis- Hydrogen evolution and selective organic photosynthesis catalyzed by zinc sulfide- Photocatalytic organic synthesis by use of semiconductors or dyes- A Hiskia and A Ioannidis Photocatalytic oxidation of organic compounds with heteropoly electrolytes Aspects on photochemical utilization of solar energy- Photoformation of hydrogen in liquid water in the presence of Pt/TiO2 catalyst and organic impurities- Surface properties of catalysts Iron and its oxides: Surface chemistry, photochemistry and catalysis- Photocatalysis over clay supports- Photoreduction and-oxidation of molecular nitrogen on titanium dioxide and titanium containing minerals- Photoassisted reduction of carbon and nitrogen compounds: with semiconductors- Adsorption and desorption processes in photocatalysis- Photo-induced processes at titanium dioxide surfaces- Photolysis of adsorbed phases: Ethanoic acid-insulators and semiconductors systems Influence of acid-base properties- Cadmium sulfide photocatalyzed hydrogen production from aqueous solutions of sulfite- Environmental photochemistry of chlorinated aromatics in aqueous media A review of data- Photodegradation of wastes and pollutants in aquatic environment- Heterogeneous photocatalysis for water purification: Prospects and problems- A Gas Research Institute perspective on inorganic synthesis of gaseous fuels- Putting photocatalysis to work- Panel discussion on the Selective activation and conversion of molecules- Panel discussion on Sensitization and immobilization of catalysts on various supports- Panel discussion on Electrocatalysis and photocatalysis- Final Remarks- Author Index

Rapid microbial mineralization of toluene and 1,3-dimethylbenzene in the absence of molecular oxygen.

Abstract: Up to 0.4 mM 1,3-dimethylbenzene (m-xylene) was rapidly mineralized in a laboratory aquifer column operated in the absence of molecular oxygen with nitrate as an electron acceptor. Under continuous flow conditions, the degradation rate constant (pseudo-first order) was >0.45 h/sup -1/. Based on a carbon mass balance with (ring-/sup 14/C)m-xylene and a calculation of the electron balance, m-xylene was shown to be quantitatively (80%) oxidized to CO/sub 2/ with a concomitant reduction of nitrate. The mineralization of m-xylene in the column also took place after reducing the redox potential, E', of the inflowing medium with sulfide to <-0.11 V. Microorganisms adapted to growth on m-xylene were also able to degrade toluene under denitrifying conditions. These results suggest that aromatic hydrocarbons present in anoxic environments such as lake sediments, sludge digestors, and ground water infiltration zones from landfills and polluted rivers are not necessarily persistent but may be mineralized in the absence of molecular oxygen.

Role of Alpha-Tocopherol, Ascorbic Acid, Citric Acidand EDTA as Oxidants in Model Systems

Abstract: The effects of four widely employed “antioxidants” on iron-mediated hydroxyl radical formation and lipid peroxidation were studied in aqueous model systems. Iron and copper served as catalysts for the reactions which oxidized ascorbic acid and alpha-tocopherol and reduced oxygen. Ferrous ion spontaneously reduced oxygen to O−2 (su-peroxide anion radical) which led to OH (hydroxyl radical) and H2O2 generation and lipid peroxidation. Precipitation or sequestration of iron greatly depressed these oxidative events. Complexation by EDTA and citric acid, however, formed catalytically active iron chelates. The concomitant increase in iron solubility explained the substantial enhancement of iron-driven redox reactions by EDTA and citric acid.

Speciation, photosensitivity, and reactions of transition metal ions in atmospheric droplets

Abstract: Dissolved transition metal ions (TMI) are common constituents of atmospheric droplets. They are known to catalyze sulfur oxidation in droplets and are suspected of being involved in other chemical processes as well. We have reviewed the relevant equilibrium constants and chemical reactions of the major TMI (iron, manganese, copper, and nickel), their ability to form complexes in aqueous solution, and their potential involvement in photochemical processes in atmospheric droplets. Among the results are the following: (1) The major Fe(III) species in atmospheric water droplets are [Fe(OH)(H2O)5]2+, [Fe(OH)2(H2O)4]+, and [Fe(SO3)(H2O)5]+; the partitioning among these complexes is a function of pH. In contrast, Cu(II), Mn(II), and Ni(II) exist almost entirely in the droplets as hexaquo complexes. (2) Within the tropospheric solar spectrum, some of the complexes of Fe(III) have large absorption cross-sections. In this work we report cross-section data for several of the complexes. Absorption of solar photons by such complexes is generally followed by cleavage, which in the same process reduces the iron (III) atom and produces a reactive free radical. This mechanism has the potential to be a significant and heretofore unappreciated source of free radicals in atmospheric droplets. (3) TMI participate in redox reactions with H2O2 and its associated species HO2· and O2−. These reactions furnish the potential for catalytic cycles involving TMI in atmospheric droplets under a variety of illumination and acidity conditions. (4) A number of organic processes in atmospheric droplets may involve TMI. Among these processes are the production and destruction of alkylhydroperoxides, the chemical chains linking RO2· radicals to stable alcohols and acids, and the oxidation of aliphatic aldehydes to organic acids.

Reaction Products of Ozone: A Review

Abstract: The reaction products of ozone that form during the oxidation of compounds found in aqueous media are reviewed. Reaction products of ozone are well documented only for a limited number of substrates, and mechanistic information is quite rare. Decomposition of ozone during its reactions, sometimes induced by matrix impurities or by the by-products of the reactions, will generate highly reactive hydroxyl radicals. Thus, even reactions occurring at pH less than 7 may have radical character. More complete destruction of organic substrates may be enhanced by using catalysts, such as ultraviolet radiation or hydrogen peroxide, to accelerate radical formation. However, complete mineralization is generally not practical economically, so partially oxidized by-products can be expected under typical treatment conditions. Ozone by-products tend to be oxygenated compounds that are usually, but not always, more biodegradable and less toxic than xenobiotic precursors. Of particular interest are hydroperoxide by-products, which may have escaped detection because of their lability, and unsaturated aldehydes. Inorganic by-products tend to be in high oxidation states, which in some cases (e.g., some metallic elements) may lead to enhanced removal by flocculation and sedimentation. In other cases oxidation may lead to formation of reactive species such as hypobromous acid from bromide ion or permanganate from manganous ion. In general, more research is required before a valid assessment of the risks of ozone by-products can be made.

Electron-transfer reactions of tryptophan and tyrosine derivatives

Abstract: Oxidation of tryptophan, tyrosine, and derivatives by oxidizing radicals was studied by pulse radiolysis in aqueous solutions at 20 /sup 0/C. Rate constants for the oxidation of tryptophan derivatives with .N/sub 3/ and Br/sub 2//sup -/. radicals vary from 8 x 10/sup 8/ to 4.8 and 10/sup 9/ M/sup -1/ s/sup -1/ and oxidation goes to completion; no pH dependence was observed. Oxidation rate constants for tyrosine derivatives increase upon deprotonation of the phenolic residue at higher pH. Redox potentials for the indolyl and phenoxyl radicals were derived from the measured equilibrium constants by using p-methoxyphenol (E/sub 7.5/ = 0.6 and E/sub 13/ = 0.4 V), bisulfite (E/sub 3/ = 0.84 V), and guanosine (E/sub 7/ = 0.91 V) redox couples as reference systems. Redox potentials of tryptophan derivatives were found to be in dependent on the nature of the side chain and higher than the redox potentials of tryptophan derivatives. Electron transfer from tyrosine to tryptophyl radical was found to be slow in neutral media and is suggested to proceed via multiple steps, one of which is proton transfer from tyrosine to tryptophyl radical followed by electron transfer. 26 references, 2 figures, 4 tables.

Relationship of Transplasmalemma Redox Activity to Proton and Solute Transport by Roots of Zea mays

Abstract: Transplasmalemma redox activity, monitored in the presence of exogenous ferricyanide stimulates net H+ excretion and inhibits the uptake of K+ and α-aminoisobutyric acid by freshly cut or washed, apical and subapical root segments of corn (Zea mays L. cv “Seneca Chief”). H+ excretion is seen only following a lag of about 5 minutes after ferricyanide addition, even though the reduction of ferricyanide occurs before 5 minutes and continues linearly. Once detected, the enhanced rate of H+ excretion is retarded by the ATPase inhibitors N,N′-dicyclohexylcarbodiimide, diethylstilbestrol, and vanadate. A model is presented in which plasmalemma redox activity in the presence of ferricyanide involves the transport only of electrons across the plasmalemma, resulting in a depolarization of the membrane potential and activation of an H+-ATPase. Such a model implies that this class of redox activity does not provide an additional and independent pathway for H+ transport, but that the activity may be an important regulator of H+ excretion. The 90% inhibition of K+ (86Rb+) uptake within 2 minutes after ferricyanide addition can be contrasted with the 5 to 15% inhibition of uptake of α-aminoisobutyric acid. The possibility exists that a portion of the K+ and most of the α-aminoisobutyric acid uptake inhibitions are related to the ferricyanide-induced depolarization of the membrane potential, but that the redox state of some component of the K+ uptake system may also regulate K+ fluxes.

Catalysis of electrochemical reactions at redox polymer coated electrodes: mediation of the Fe(III)/Fe(II) oxido-reduction by a polyvinylpyridine polymer containing coordinatively attached bisbipyridine chlororuthenium redox centers

Abstract: The (Ru(bpy)/sub 2/Cl-poly(4-vinylpyridine)) redox polymer catalyzes the oxidation of Fe(II) into Fe(III) in 1 M HCl in a quite efficient manner both in terms of current and potential. Quite substantial portions of the film thickness are active in the catalytic system. Providing a good example of the interest of redox polymer coatings in the catalysis of electrochemical reactions as a result of the three-dimensional dispersion of the reacting centers. The polymer is also able to catalyze, although to a lesser extent, the reduction of Fe(III) to Fe(II) in the same medium in spite of the fact that the standard potential of the Ru(III)/Ru(II) couple is positive to that of the Fe(III)/Fe(II) couple. Analysis of the experimental data and the optimization of the catalytic efficiency by means of previously developed kinetic models demonstrate the validity of these models both in terms of the limiting currents and half-wave potentials featuring the catalytic process.

Method of treating fluids

Abstract: A method for treating fluid to remove hydrogen sulfide and sulfur dioxide is disclosed. The method includes passing fluid containing the hydrogen sulfide and/or sulfur dioxide through a bed of metal particulate matter. The metal particulate matter is preferably chosen from metals having favorable redox potentials relative to the redox potentials of these undesirable constituents so as to establish conditions for spontaneous oxidation and reduction reactions between the undesirable constituents and the metal particles.

Surface-modified electrode and its use in a bioelectrochemical process

Abstract: In a bioelechemical process in which the electrons are transferred directly, without use of a mediator, such as a redox dye or cofactor, between an electrode and an electroactive biological material, such as an enzyme or a protein, in either direction, rapid electron transfer has previously been achieved between an electrode and the positively charged protein horse-heart cytochrome c by adding a surface-modifier such as 4,4'-bipyridyl or a derivative thereof. It has now been found possible to promote electron transfer to either a positively or a negatively charged protein using the same surface-modifier for either job, namely a compound of formula wherein: The puridine ring shown in substituted in the 2-, 3- or 4-position by the (methylene)hydrazinecar- bothioamide group shown; R' represents hydrogen atom(s) or one or two methyl or ethyl groups in the 2-, 3- or 4-position (when the said position is not substituted by the (methylene)hydrazinecar- bothioamide group shown); R 2 represents a hydrogen atom or a methyl group; R 3 represents a hydrogen atom or a methyl group; and R 4 represents a hydrogen atom or a methyl group.

Redox potentials of some sulfur-containing radicals

Abstract: Calcul des variations d'energie libre standard et des potentiels redox, a partir des donnees thermodynamiques, dans le cas des reactions: RS • +e − =RS − ; RS˙SR − +e − =2RS − ; RSSR+e − =RS˙SR − avec R=CH 3 , C 2 H 5 , C 3 H 7 et H

Redox processes in glass-forming melts

Abstract: Oxidation-reduction (redox) equilibria of multivalent elements in glass-forming melts can be expressed by several different, yet similar, chemical equations. Each equation attempts to describe the effects that melt temperature, base composition, imposed oxygen fugacity, multivalent element concentration, and the presence of other redox components induce on the established redox equilibrium. Redox kinetics, or the rate at which equilibrium is attained in the melt, demonstrates the interrelationship between the dissolved multivalent species and the gaseous redox components. The diffusion of redox gases, such as oxygen into or out of a static glass-forming melt controls how fast the redox state of the melt can change. Both equilibrium and kinetic redox processes are thus integral to the development of desired glass/melt properties.

Oxygen reduction at electrochemically treated glassy carbon electrodes

Abstract: The authors studied dioxygen reduction at electrodes treated electrochemically and at ones on which quinone was adsorbed. Results were compared to see if the surface redox groups introduced on the electrochemically treated electrodes could serve as mediators. The electrodes behaved very differently for oxygen reduction, especially with respect to pH. For the quinone-adsorbed electrodes, oxygen reduction via quinone was not observed at a pH less than the semiquinone pK/sub a/; for the other electrodes, reduction currents were unchanged in the pH range investigated. From these and other results, they concluded that quinones of the electrochemically treated electrodes cannot serve as mediators for oxygen reduction.

Factors influencing redox potentials of electron transfer proteins

Abstract: The redox potentials of electron transfer proteins vary over a wide range, even when the type of redox center is the same. Rees [Proc. Natl. Acad. Sci. USA (1985) 82, 3082-3085] proposed that this variation of redox potential partly reflects the different net charges of the proteins, and he presented a linear correlation between these two properties for 36 proteins. A review of the factors that influence protein redox potentials makes it clear that this linear correlation is fortuitous. The key factors influencing redox potentials are the contributions to the Gibbs energy difference between the two redox states, resulting from bonding interactions at the redox center, electrostatic interactions between the redox-center charge and polar groups within the protein and solvent, and redox-state conformational changes. The relative importance of these terms is likely to vary from protein to protein.