Showing papers on "Redox published in 1984"

Redox and spectral properties of monooxo polypyridyl complexes of ruthenium and osmium in aqueous media

Abstract: Preparation des complexes [(trpy) (bpy) M-OH 2 ] 2+ (M=Ru, Os). Etude de leurs proprietes redox en solution aqueuse

Mechanisms of Action of Vanadium

Abstract: Vanadium is widely distributed, the twenty-first most abundant element in the Earth's crust, with an average content of 135 ppm. In sea water, vanadium ranks thirty-fourth in abundance, with an average concentration of only 2 ppb. Because it evolved as an essential element for certain forms of life and also because of its wide industrial use, the biological actions of vanadium are of interest to scientists. Excellent accounts of the history and previous know ledge of vanadium are available. (1-7). The chemistry of vanadium is complex because the metal can exist in oxidation states from -1 to +5 and forms polymers frequently (8). Recently Rubinson (9) reviewed the material concerning the form of biochemically active vanadium. The following generalizations appear justified. At low­ normal concentrations in mammals and birds, any free vanadium will be in hydrated, monomeric form. In the body fluids at pH 4-8, the predominant species will be VOi ( + 5 oxidation state), vanadate (metavanadate). VOi may enter certain cells by an anion transport system and be reduced by glutathione to VOH (+4 oxidation state), vanadyl. By way of speculation, the oxidation­ reduction reactions may be as follows: H+ + VOi + 2GSH � Y02+ + G2S2 + OH+ e + H20. Extensive binding to extraand intracellular ligands may be expected. Since phosphate and Mg2+ are ubiquitous in biological processes, YOi as the analogue of phosphate and Y02+, which resembles the size of Mg2+ (respective ionic radii: 0.60 and 0.65 ft.), potentially have many bio­ chemical and cellular sites of action. For example, vanadium compounds in­ hibit ATP phosphohydrolases, ribonuclease, adenylate kinase, phosphofructo­ kinase, squalene synthetase, glyceraldehyde-3-phosphate dehydrogenase (10),

Relationship between lateral diffusion, collision frequency, and electron transfer of mitochondrial inner membrane oxidation-reduction components

Abstract: Fluorescence recovery after photobleaching was used to determine the diffusion coefficients of the oxidation-reduction (redox) components ubiquinone, complex III (cytochromes b-c1), cytochrome c, and complex IV (cytochrome oxidase) of the mitochondrial inner membrane. All redox components diffuse in two dimensions as common-pool electron carriers. Cytochrome c diffuses in two and three dimensions concomitantly, and its diffusion rate, unlike that of all other redox components, is modulated along with its activity by ionic strength. The diffusion coefficients established in this study reveal that the theoretical diffusion-controlled collision frequencies of all redox components are greater than their experimental maximum (uncoupled) turnover numbers. Since electron transport is slower than the theoretical limit set by the lateral diffusion of the redox components, ordered chains, assemblies, or aggregates of redox components are not necessary to account for electron transport. Rather, mitochondrial electron transport is diffusion coupled, consistent with a "random-collision model" for electron transport.

Photoreductive dissolution of colloidal iron oxide: Effect of citrate

Abstract: The light-induced dissolution of the iron oxide, lepidocrocite (γ-FeOOH), has been investigated and found to be greatly enhanced in the presence of citrate. A conceptual model of the dissolution process is presented and validated through studies of citrate adsorption to lepidocrocite, net iron oxide dissolution under a variety of conditions, and solution phase redox reactions. The initial dissolution rate is directly related to the concentration of the surface bound ferric citrate and the first order rate constant for the photodissolution process is very similar to that found for the photodissociation of soluble ferric citrate. Dissolution most likely occurs through direct excitation of charge transfer bands of the surface bound ferric citrate. At low pH (pH 4.0), a constant rate of dissolution is observed while at higher pH (pH 6.5 and 8.2), the dissolution rate decreases on continued photolysis. This decrease is due to (1) an oxygen-dependent deactivation process occurring at the surface and (2) loss of photo-produced iron from solution by “ligand-like” adsorption of soluble iron citrate complexes by the colloidal iron oxide. Superimposed on the dissolution process at these higher pH is a rapid oxidation-reduction cycle involving solution phase iron species with the reduction step induced by photodissociation of ferric citrate complexes and the oxidation step controlled by the formation of ferrous citrate.

NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c3.

Abstract: The proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio gigas were examined while varying the pH and the redox potential The analysis of the NMR reoxidation pattern was based on a model for the electron distribution between the four haems that takes into account haem-haem redox interactions The intramolecular electron exchange is fast on the NMR time scale (larger than 105 s−1) The NMR data concerning the pH dependence of the chemical shift of haem methyl resonances in different oxidation steps and resonance intensities are not compatible with a non-interacting model and can be explained assuming a redox interaction between the haems A complete analysis at pH*= 72 and 96, shows that the haem-haem interacting potentials cover a range from -50mV to +60 mV The midpoint redox potentials of some of the haems, as well as some of their interacting potentials, are pH-dependent The physiological relevance of the modulation of the haem midpoint redox potentials by both the pH and the redox potential of the solution is discussed

Solvent, Ligand, and Ionic Charge Effects on Reaction Entropies for Simple Transition-Metal Redox Couples.

Abstract: : The dependence of the reaction entropies for simple M(III)/(II) redox couples, with M = Ru, Fe, Os, Cr, upon the nature of the ligands and the solvent is examined with a view towards correlating the entropies with simple physical parameters. For couples containing ammine, ethylenediamine, polypyridine, cyclopentadiene or psuedohalide ligands reaction entropy in a given solvent is found to correlate well with a function of the charge numbers of the oxidized and reduced forms, and with 1/r, where r is the effective radius of the redox couple. This suggests that shortrange ligand-solute interactions do not provide a predominant contribution to reaction entropy for these systems, although this effect is probably important for aquo redox couples in water. The dependence of reaction entropy upon the solvent correlates reasonably well with the solvent acceptor number and other solvent polarity parameters.

Variation of transition-state structure as a function of the nucleotide in reactions catalyzed by dehydrogenases. 2. Formate dehydrogenase.

Abstract: Since hydride transfer is completely rate limiting for yeast formate dehydrogenase [Blanchard, J.S., & Cleland, W. W. (1980) Biochemistry 19, 3543], the intrinsic isotope effects on this reaction are fully expressed. Primary deuterium, 13C, and 18O isotope effects in formate and the alpha-secondary deuterium isotope effect at C-4 of the nucleotide have been measured for nucleotide substrates with redox potentials varying from -0.320 (NAD) to -0.258 V (acetylpyridine-NAD). As the redox potential gets more positive, the primary deuterium isotope effect increases from 2.2 to 3.1, the primary 13C isotope effect decreases from 1.042 to 1.036, the alpha-secondary deuterium isotope effect drops from 1.23 to 1.06, and Vmax decreases. The 18O isotope effects increase from 1.005 to 1.008 per single 18O substitution in formate (these values are dominated by the normal isotope effect on the dehydration of formate during binding; pyridinealdehyde-NAD gives an inverse value, possibly because it is not fully dehydrated during binding). These isotope effects suggest a progression toward earlier transition states as the redox potential of the nucleotide becomes more positive, with NAD having a late and acetyl-pyridine-NAD a nearly symmetrical transition state. By contrast, the I2 oxidation of formate in dimethyl sulfoxide has a very early transition state (13k = 1.0154; Dk = 2.2; 18k = 0.9938), which becomes later as the proportion of water in the solvent increases (13k = 1.0265 in 40% dimethyl sulfoxide and 1.0362 in water). alpha-secondary deuterium isotope effects with formate dehydrogenase are decreased halfway to the equilibrium isotope effect when deuterated formate is the substrate, showing that the bending motion of the secondary hydrogen is coupled to hydride transfer in the transition state and that tunneling of the two hydrogens is involved. The 15N isotope effect of 1.07 for NAD labeled at N-1 of the nicotinamide ring suggests that N-1 becomes pyramidal during the reaction. 18O fractionation factors for formate ion relative to aqueous solution are 1.0016 in sodium formate crystal, 1.0042 bound to Dowex-1, and 1.0040 as an ion pair (probably hydrated) in CHCl3. The CO2 analogue azide binds about 10(4) times better than the formate analogue nitrate to enzyme-nucleotide complexes (even though the Ki values for both and the affinity for formate vary by 2 orders of magnitude among the various nucleotides), but the ratio is not sensitive to the redox potential of the nucleotide. Thus, not the nature of the transition state but rather the shape of the initial binding pocket for formate is determining the relative affinity.(ABSTRACT TRUNCATED AT 400 WORDS)

Redox activity at the surface of oat root cells.

Abstract: Electron transport activity at the cell surface of intact oat seedlings (Avena sativa L. cv Garry) was examined by measuring the oxidation and/or reduction of agents in the medium bathing the roots. Oxidation of NADH with or without added electron acceptors and reduction of ferricyanide by an endogenous electron donor were detected. The activities appear to be due to electron transfer at, or across, the plasma membrane and not due to reagent uptake or leakage of oxidants or reductants. NADH-ferricyanide oxidoreductase activity was also detected in plasma membrane-enriched preparations from Avena roots. Based on redox responses to pH, various ions, and to a variety of electron donors and acceptors, the results indicate that more than one electron transport system is present at the plasma membrane.

Mechanisms of inorganic and organometallic reactions

Abstract: Electron Transfer Reactions: Electron Transfer J.F. Endicott, et al. Redox Reactions Between Two Metal Complexes D.H. Macartney. Metal-Ligand Redox Reactions R.M.L. Warren, A.G. Lappin. Substitution and Related Reactions: Reactions of Compounds of the Nonmetallic Elements G. Steadman. Ligand Exchange Reactions of Inert-Metal Complexes-Coordination Numbers 4 and 5 R.J. Cross. Substitution Reactions of Inert-Metal Complexes-Coordination Numbers 6 and Above: Chromium D.A. House. Substitution Reactions of Inert-Metal Complexes-Coordination Numbers 6 and Above: Cobalt R.W. Hay. Substitution Reactions of Inert-Metal Complexes-Coordination Numbers 6 and Above: Other Inert Centers J. Burgess. Substitution Reactions of Labile Metal Complexes P.A. Tregloan. Reactions of Organometallic Compounds: Substitution and Insertion Reactions A.J. Poe. Metal-Alkyl and Metal-Hydride Bond Formation and Fission Oxidative Addition and Reductive Elimination R.D. Pike. 4 additional articles. Index.

Redox Behavior and Electrochromic Properties of Polypyrrole Films in Aqueous Solutions

Abstract: Polypyrrole films prepared by anodic polymerization of pyrrole on Pt or SnO2-coated glass show redox behavior in aqueous electrolyte solutions containing a supporting electrolyte alone. Transfer of electrolyte anions into and from the film is involved in the redox reaction similarly with acetonitrile solutions already reported. Insertion of divalent anions into the film occurs in two steps but that of monovalent anions in one step, as judged from corresponding cyclic voltammograms. The change in film color which occurs in the redox reaction of the film was investigated in detail as a function of electrode potential and the charge consumed in the redox reaction. Polypyrrole is a promising material for electrochromic display devices.

Structural basis for the variation of pH-dependent redox potentials of Pseudomonas cytochromes c-551.

Abstract: The redox potentials of many c-type cytochromes vary with pH over the physiological pH range. We have investigated the pH dependence of redox potential for the four homologous cytochromes c-551 from Pseudomonas aeruginosa, Pseudomonas stutzeri strain 221, Pseudomonas stutzeri strain 224, and Pseudomonas mendocina . The pH dependence is due to an ionizable group that ionizes with pKox in ferricytochrome c-551 but with a higher pK, pKred , in ferrocytochrome c-551. For P. aeruginosa cytochrome c-551 it has been shown that this ionizable group is one of the heme propionic acid substituents [Moore, G. R., Pettigrew , G. W., Pitt , R. C., & Williams, R. J. P. (1980) Biochim. Biophys. Acta 590, 261-271]but the values of pKox and pKred are significantly lower in this protein than in the other three cytochromes. NMR and chemical modification studies show that for the two P. stutzeri cytochromes c-551 and P. mendocina cytochrome c-551, this propionic acid substituent is again important for the pH dependence of the redox potential. However, a histidine occurring at position 47 in their sequences hydrogen bonds to the propionic acid and thereby raises its pK. In P. aeruginosa cytochrome c-551, His-47 is substituted by Arg-47. Hydrogen-bonding schemes involving His-47 and the propionic acid are proposed.

Properties of a transplasma membrane electron transport system in HeLa cells.

Abstract: A transmembrane electron transport system has been studied in HeLa cells using an external impermeable oxidant, ferricyanide. Reduction of ferricyanide by HeLa cells shows biphasic kinetics with a rate up to 500 nmoles/min/g w.w. (wet weight) for the fast phase and half of this rate for the slow phase. The apparentK m is 0.125 mM for the fast rate and 0.24 mM for the slow rate. The rate of reduction is proportional to cell concentration. Inhibition of the rate by glycolysis inhibitors indicates the reduction is dependent on glycolysis, which contributes the cytoplasmic electron donor NADH. Ferricyanide reduction is shown to take place on the outside of cells for it is affected by external pH and agents which react with the external surface. Ferricyanide reduction is accompanied by proton release from the cells. For each mole of ferricyanide reduced, 2.3 moles of protons are released. It is, therefore, concluded that a transmembrane redox system in HeLa cells is coupled to proton gradient generation across the membrane. We propose that this redox system may be an energy source for control of membrane function in HeLa cells. The promotion of cell growth by ferricyanide (0.33–0.1 mM), which can partially replace serum as a growth factor, strongly supports this hypothesis.