Showing papers on "Redox published in 1991"

Solubility and dissolution of iron oxides

Abstract: In most soils, FeIII oxides (group name) are the common source of Fe for plant nutrition. Since this Fe has to be supplied via solution, the solubility and the dissolution rate of the Fe oxides are essential for the Fe supply. Hydrolysis constants and solubility products (Ksp) describing the effect of pH on FeIII ion concentration in solution are available for the well-known Fe oxides occurring in soils such as goethite, hematite, ferrihydrite. Ksp values are usually extremely low ((Fe3+)·(OH)3=10−37−10−44). However, for each mineral type, Ksp may increase by several orders of magnitude with decreasing crystal size and it decreases with increasing Al substitution assuming ideal solid solution between the pure end-members. Based on such calculations a poorly crystalline goethite with a crystal size of 5 nm may well reach the solubility of ferrihydrite. The variations in Ksp are of relevance for soils because crystal size and Al substitution of soil Fe oxides vary considerably and can now be determined relatively easily. The concentration of Fe2+ in soil solutions is often much higher than that of Fe(III) ions. Therefore, redox potential strongly influences the activity of FeII. At a given pH and Eh, the activity of FeII is higher the higher Ksp of the FeIII oxide and thus also varies with the type of Fe oxide present. Besides the solubility, it is the dissolution rate which governs the supply of soluble Fe to the plant roots. Dissolution of Fe oxides takes place either by protonation, complexation or, most important, by reduction. Numerous dissolution rate studies with various FeIII oxides were conducted in strong mineral acids (protonation) and they have shown that besides the Fe oxide species, crystal size and/or crystal order and substitution are important determinative factors. For example, in soils, small amounts of a more highly soluble meta- or instable Fe oxide such as ferrihydrite with a large specific surface (several hundred m2g−1) may be essential for the Fe supply to the plant root. Its higher dissolution rate can also be used to quantify its amount in soils. Ferrihydrite can be an important component in soils with high amounts of organic matter and/or active redox dynamics, whereas highly aerated and strongly weathered soils are usually very low in ferrihydrite. On the other hand, dissolution rates of goethites decrease as their Al substitution increases. Much less information exists on the rate of reductive and chelative dissolution of Fe oxides which generally simulate soil conditions better than dissolution by protonation. Here again, type of oxide, crystal size and substitution are important factors. Organic anions such as oxalate, which are adsorbed at the surface, may weaken the Fe3+-O bonds and thereby increase reductive dissolution. As often observed in weathering, the dissolution features of the crystals appear to follow zones of weakness in the crystal.

Catalysis of the oxidative folding of ribonuclease A by protein disulfide isomerase: dependence of the rate on the composition of the redox buffer

Abstract: The velocity of the oxidative renaturation of reduced ribonuclease A catalyzed by protein disulfide isomerase (PDI) is strongly dependent on the composition of a glutathione/glutathione disulfide redox buffer. As with the uncatalyzed, glutathione-mediated oxidative folding of ribonuclease, the steady-state velocity of the PDI-catalyzed reaction displays a distinct optimum with respect to both the glutathione (GSH) and glutathione disulfide (GSSG) concentrations. Optimum activity is observed at [GSH] = 1.0 mM and [GSSG] = 0.2 mM. The apparent kcat at saturating RNase concentration is 0.46 +/- 0.05 mumol of RNase renatured min-1 (mumol of PDI)-1 compared to the apparent first-order rate constant for the uncatalyzed reaction of 0.02 +/- 0.01 min-1. Changes in GSH and GSSG concentration have a similar effect on the rate of both the PDI-catalyzed and uncatalyzed reactions except under the more oxidizing conditions employed, where the catalytic effectiveness of PDI is diminished. The ratio of the velocity of the catalyzed reaction to that of the uncatalyzed reaction increases as the quantity [GSH]2/[GSSG] increases and approaches a constant, limiting value at [GSH]2/[GSSG] greater than 1 mM, suggesting that a reduced, dithiol form of PDI is required for optimum activity. As long as the glutathione redox buffer is sufficiently reducing to maintain PDI in an active form [( GSH]2/[GSSG] greater than 1 mM), the rate acceleration provided by PDI is reasonably constant, although the actual rate may vary by more than an order of magnitude. PDI exhibits half of the maximum rate acceleration at a [GSH]2/[GSSG] of 0.06 +/- 0.01 mM.

Electron Transfer between Glucose Oxidase and Electrodes via Redox Mediators Bound with Flexible Chains to the Enzyme Surface

Abstract: : Electrical communication between redox centers of glucose oxidase and vitreous carbon electrodes is established through binding to oligosaccharides, at the periphery of the enzyme, ferrocene functions pendant on flexible chains. Communication is effective when the chains are long (>10 bonds), but when the chains are short (<5 bonds). When attached to long flexible chains the peripherally bound relays penetrate the enzyme to a sufficient depth to reduce the electron transfer distances between a redox center of the enzyme and the relay and between the relay and electrode, thereby increasing the rate of electron transfer.

Amperometric glucose microelectrodes prepared through immobilization of glucose oxidase in redox hydrogels.

Abstract: Glucose microelectrodes have been formed with glucose oxidase immobilized in poly[(vinylpyridine)Os(bipyridine)2Cl] derivative-based redox hydrogels on beveled carbon-fiber microdisk (7 microns diameter) electrodes. In the resulting microelectrode, the steady-state glucose electrooxidation current density is 0.3 mA cm-2 and the sensitivity is 20 mA cm-2 M-1. The current density and sensitivity are 10 times higher than in macroelectrodes made with the same hydrogel. Furthermore, the current is less affected by a change in the partial pressure of oxygen. The higher current density and lower oxygen sensitivity point to the efficient collection of electrons through their diffusion in the redox hydrogel to the electrode surface. These results contrast with those observed for enzyme electrodes based on diffusing mediators, where loss of the enzyme-reduced mediator by radial diffusion to the solution decreases the current densities of microelectrodes relative to similar macroelectrodes.

Redox Transfer across the Inner Chloroplast Envelope Membrane

Abstract: In leaves of spinach plants (Spinacia oleracea L.) grown in ambient CO2 the subcellular contents of adenylates, pyridine nucleotides, 3-phosphoglycerate, dihydroxyacetone phosphate, malate, glutamate, 2-oxoglutarate, and aspartate were assayed in the light and in the dark by nonaqueous fractionation technique. From the concentrations of NADP and NADPH determined in the chloroplast fraction of illuminated leaves the stromal NADPH to NADP ratio is calculated to be 0.5. For the cytosol a NADH to NAD ratio of 10−3 is calculated from the assay of the concentrations of NAD, malate, glutamate, aspartate, and 2-oxoglutarate on the assumption that the reactions catalyzed by the cytosolic glutamate oxaloacetate transaminase and malate dehydrogenase are not far away from equilibrium. For the transfer of redox equivalents from the chloroplastic NADPH to the cytosolic NAD two metabolite shuttles are operating across the inner envelope membrane: the triosephosphate-3-phosphoglycerate shuttle and the malate-oxaloacetate shuttle. Although both shuttles would have the capacity to level the redox state of the stromal and cytosolic compartment, this apparently does not occur. To gain an insight into the regulatory processes we calculated the free energy of the enzymic reactions and of the translocation steps involved. From the results it is concluded that the triosephosphate-3-phosphoglycerate shuttle is mainly controlled by the chloroplastic reaction of 3-phosphoglycerate reduction and of the cytosolic reaction of triosephosphate oxidation. The malate-oxaloacetate shuttle is found to be regulated by the chloroplastic NADP-malate dehydrogenase and also by the translocating step across the envelope membrane.

Enzymatic versus nonenzymatic mechanisms for Fe(III) reduction in aquatic sediments

Abstract: The potential for nonenzymatic reduction of Fe(III) either by organic compounds or by the development of a low redox potential during microbial metabolism was compared with direct, enzymatic Fe(III) reduction by Fe(III)-reducing microorganisms. At circumneutral pH, very few organic compounds nonenzymatically reduced Fe(III). In contrast, in the presence of the appropriate Fe(III)-reducing microorganisms, most of the organic compounds examined could be completely oxidized to carbon dioxide with the reduction of Fe(III). Even for those organic compounds that could nonenzymatically reduce Fe(III), microbial Fe(III) reduction was much more extensive. The development of a low redox potential during microbial fermentation did not result in nonenzymatic Fe(III) reduction. Model organic compounds were readily oxidized in Fe(III)-reducing aquifer sediments, but not in sterilized sediments. These results suggest that microorganisms enzymatically catalyze most of the Fe(III) reduction in the Fe(III) reduction zone of aquatic sediments and aquifers.

Arsenic and selenium chemistry as affected by sediment redox potential and pH

Abstract: The influence of sediment redox potential and pH on As and Se speciation and solubility was studied. Hyco Reservoir (North Carolina) sediments were equilibrated under controlled redox (500, 200, 0, and −200 mV) and pH (5, natural, and 7.5) conditions. Redox potential and pH affected both speciation and solubility of As and Se. Under oxidized conditions As solubility was low and 87% of the As in solution was present as As(V). Upon reduction, As(III) became the major As species in solution, and As solubility increased. Total As in solution increased approximately 25 times upon reduction to −200 mV. No organic arsenicals were detected. In contrast to As, Se solubility reached a maximum under highly oxidized (500 mV) conditions and decreased significantly upon reduction. Selenium(VI) was the predominant dissolved Se species present at 500 mV. At 200 and 0 mV, Se(IV) became the most stable oxidation state of Se. Under strongly reduced conditions (−200 mV) oxidized Se species were no longer detectable and Se solubility was controlled by the formation of elemental Se and/or metal selenides. Biomethylation of Se was important under oxidized and moderately reduced conditions (500, 200, and 0 mV). More alkaline conditions (pH 7.5) resulted in both greater As and Se concentrations in solution. Dissolved As and Se increased up to 10 and 6 times, respectively, as compared to the more acidic equilibrations.

A Cu(I)-semiquinone state in substrate-reduced amine oxidases.

Abstract: The role of copper in copper-containing amine oxidases has long been a source of debate and uncertainty. Numerous electron paramagnetic resonance (EPR) experiments, including rapid freeze-quench studies, have failed to detect changes in the copper oxidation state in the presence of substrate amines. One suggestion that copper reduction might occur, has never been confirmed. Copper amine oxidases contain another cofactor, recently identified as 6-hydroxydopa quinone (topa quinone), which is reduced by substrates. Copper has been implicated in the reoxidation of the substrate-reduced enzyme, but the failure to detect any copper redox change has led to proposals that Cu(II) acts as a Lewis acid, that it has an indirect role in catalysis, or that it serves a structural role. We present evidence for the generation of a Cu(I)-semiquinone state by substrate reduction of several amine oxidases under anaerobic conditions, and suggest that the Cu(I)-semiquinone may be the catalytic intermediate that reacts directly with oxygen.

Amperometric glucose biosensors based on redox polymer-mediated electron transfer

Abstract: Electrical communication between the flavin adenine dinucleotide redox centers of glucose oxidase and a conventional carbon paste electrode has been achieved by using electron-transfer relay systems based on polysiloxanes. Six materials for amperometric biosensors are described in which ferrocene and dimethylferrocene electron relays are covalently attached to insoluble siloxane polymers. Sensors containing these polymeric relay systems and glucose oxidase respond rapidly to glucose, with steady-state current responses achieved in less than 10 s. The dependence of the sensor response on the nature of the siloxane polymer and the type of polymer-bound relay is discussed

Growth inhibition of

Abstract: ~Malaria parasites have been shown to be more susceptible to oxidative stress than their host erythrocytes. In the present work, a chloroquine resistant malaria parasite, PIasmodium,falciparum (FCR-3) was found to be susceptible in vitro to a pyridoxal based iron chelator - (I-[N-ethoxycarbonylmethylpyridoxlidenium]-2-[2’-pyridyl] hydrazine bromide - (code named L2-9). 2h exposure to 20 pM L2-9 was sufficient to irreversibly inhibit parasite growth. Desferrioxamine blocked the drug effect, indicating the requirement for iron. Oxygen however, was not essential. Spectrophotometric analysis showed that under anoxic conditions, L2-9-Fe(II) chelate undergoes an intramolecular redox reaction which presumably involves a one electron transfer and is expected to result in the formation of free radical. Spin trapping coupled to electron spin resonance (ESR) studies of L2-9-iron chelate showed that L2-9-Fe(II) produced free radicals both in the presence and absence of cells, while L2-9-Fe(III) produced free radicals only in the presence of actively metabolising cells.

X-ray photoelectron spectroscopic studies of polypyrrole synthesized with oxidative iron(III) salts

Abstract: Polypyrrole complexes obtained from chemical polymerizatin and oxidation of pyrrole by FeCl 3 and Fe(ClO 4 ) 3 are studied. The C12p corelevel spectrum of the FeCl 3 polymerized samples suggests that the chlorine dopant can exist in three chemical states. The XPS results also reveal that the proton modified pyrrolylium nitrogens can exist in a number of intrinsic oxidation states. The behavior of the pyrrolylium nitrogens at various redox states toward protonation/deprotonation or oxidation/reduction suggests that they are chemically similar to the nitrogens of polyaniline

Organic Electron Transfer Systems, II Substituted Triarylamine Cation‐Radical Redox Systems – Synthesis, Electrochemical and Spectroscopic Properties, Hammet Behavior, and Suitability as Redox Catalysts

Abstract: 21 triarylamines (1n – 1z, 1za, 1zb) and triarylamine analogs (2a, 2b, 3a, 3b, 4a, 4b) with substituents in at least all three p positions and some of their cation-radical hexachloroantimonates have been synthesized. The electrochemical behavior has been studied by cyclic voltammetry. Most of the compounds show chemically and electrochemically reversible first oxidation waves in the formation of the cation radicals. With the exception of 4a and 4b, the second wave for the formation of the dication is chemically irreversible. The UV spectra of the triarylamine cation radicals have been obtained in the presence of a slight excess of SbCl5. A good Hammett correlation between the first anodic potential of only p-substituted triarylamines and the σ/σ values has been established. Some redox-catalytic properties of triarylamine cation radicals are described.

Electron-transfer kinetics of redox reactions at the semiconductor/electrolyte contact. A new approach

Abstract: The model used for electron-transfer kinetics between the electronic charge carriers of a semiconductor and the species of a redox couple in an electrolyte has been refined by taking into account the statistics of forming a reaction pair at the interface. Electron transfer within such a reaction pair is described by the semiclassical theory. A comparison is made between the electron transfer in the forward direction over a semiconductor-metal and a semiconductor-redox electrolyte Schottky barrier of equal height

Iron oxide solubilization by organic matter and its effect on iron availability

Abstract: The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control. Fe(III) hydrolysis species constitute the major Fe species in solution. Other inorganic Fe complexes are present, but their concentrations are much less than the hydrolysis species. Organic complexes of Fe including those of organic acids like citrate, oxalate, and malate contribute slightly to increased Fe solubility in acid soils, but not in alkaline soils.

Novel Solid Redox Polymerization Electrodes Electrochemical Properties

Abstract: The generic redox reaction of a class of linear sulfur‐containing redox polymerization electrodes can be described asthat is, the polymer electrode can be progressively depolymerized, leading ultimately to monomeric anions, as the sulfur‐sulfur bridges between the organic R groups are cleaved during discharge and then the monomer anions can be subsequently reoxidized back to the original polymer during charge. This is the first time the process of electrodepolymerization‐electropolymerization has been exploited for energy storage, establishing a broad class of chemically flexible, low equivalent weight, and inexpensive electrodes for advanced batteries. Electrochemical investigation of a diverse group of novel solid redox polymerization electrodes indicates that these materials are excellent candidates for all‐solid‐state, thin‐film, energy‐storage systems. Some of the advantages offered by the batteries based on these materials include high energy density and rate capability, extensive utilization of positive electrode capacity, ease of fabrication, low cost, and superior reliability, and safety. In addition, these materials are reversible to lithium and sodium (as well as many alkaline earth and transition metals), allowing for a much greater choice of negative electrode materials, in stark contrast to cells based on analogous intercalation compounds. Further, and in particular, a great advantage of redox polymerization electrodes is the ability to alter the physical, chemical, and electrochemical properties of these materials in a very predictable manner through manipulation of various functional groups, electron‐withdrawing heteroatoms, and the molecular architecture.