Showing papers on "Redox published in 1983"

Structure and mechanism of copper, zinc superoxide dismutase

Abstract: Copper, zinc superoxide dismutase (SOD) catalyses the very rapid two-step dismutation of the toxic superoxide radical (O−2) to molecular oxygen and hydrogen peroxide through the alternate reduction and oxidation of the active-site copper1. We report here that after refitting and further refinement of the previous 2 A structure of SOD2, analysis of the new model and its calculated molecular surface shows an extensive surface topography of sequence-conserved residues stabilized by underlying tight packing and H-bonding. There is a single, highly complementary position for O−2 to bind to both the Cu(II) and activity-important Arg 141 with correct geometry; two water molecules form a ghost of the superoxide in this position. The geometry and molecular surface of the active site, together with biochemical data, suggest a specific model for the enzyme mechanism.

Palladium in some selective oxidation reactions

Abstract: Palladium (Pd) complexes catalyzed many different types of organic reactions because the metal is able to coordinate many different organic substrates and is effective in almost all fundamental organometallic reactions such as oxidative addition, reductive elimination, nucleophilic addition, insertion reactions, and beta-elimination. The effectiveness of Pd's catalytic power to promote the formation of 1,2-and 1,4-alkenes and dienes from 1,3-dienes ad other alkenes is discussed. Nucleophilic attack on (Pi-olefins)-, (Pi-alkyl)- and (o-alkyl) palladium complexes and oxidation cleavage are of importance in these reactions.

Chemically modified electrodes and their uses

Abstract: There is disclosed an electrically conducting solid electrode on the surface of which a nitrogen-containing electron mediator, such as a viologen, and a strongly acidic cation exchange resin containing aromatic groups, such as polystyrenesulphonic acid, are immobilized. The electrodes can be used as hydrogen electrodes (when a redox system is coupled to the electron mediator), as oxygen electrodes (when a redox system such as a chloroplast is immobilized on the electrode), as voltametric detectors of biologically active substances, and as photochromic display elements.

Environmental behavior of arsenic in soils: Theoretical

Abstract: Thermodynamics data of As species were used to develop the solubility isotherms of As minerals and solution species. The effect of redox on the stability of As minerals and the distribution of As solution species was also simulated. In oxygenated and alkaline systems, Ca3(AsO4)2 was the most stable As mineral followed by Mn3(AsO4)2. It was postulated that in such environments Ca3(AsO4)2 and Mn3(AsO4)2 might precipitate. Arsenic(V) oxide was too soluble to become an important solid phase in the aqueous environment. Under reduced and acidic concitions (pe + pH 8), arsenate species were important whereas in reduced systems (pe + pH < 8), As(III) species were in abundance. Arsine gas can only form if the system was very reduced and acidic. Below pH 2.2, H3AsO4° was in abundance. As pH increased, H2AsO4− species was important, followed by AsO43− above pH 12. The results of thermodynamic stability and solubility models have been discussed in reference to soil environment.

Redox-linked hydrogen bond strength changes in cytochrome a: implications for a cytochrome oxidase proton pump.

Abstract: The heme a formyl group of cytochrome a in cytochrome oxidase appears to be involved in a hydrogen-bond interaction with a proton donor associated with the polypeptide backbone [Callahan, P.M., & Babcock, G.T. (1983) Biochemistry 22, 452-461]. Resonance Raman and optical absorption spectroscopies have been applied to the beef heart and Thermus thermophilus proteins and to heme a and copper porphyrin a models in order to assess the spectroscopic manifestations and the energetics of the hydrogen-bond interaction. We find a linear relationship between optical absorption red shift and carbonyl vibrational frequency decrease for a series of hydrogen-bonded model complexes; the magnitude of both changes increases as the hydrogen-bond strength increases. Comparison of the model compound data with analogous data for the proteins indicates that the strength of the formyl hydrogen bond in situ increases by 2-2.5 kcal/mol upon reduction of ferric cytochrome a. The selective stabilization of reduced cytochrome a by the stronger hydrogen bond is expected to increase the redox potential of this center; the energy made available as the hydrogen bond strengthens during reduction may be used to drive redox-coupled events in the protein. Thus, the linkage between cytochrome a redox state and chromophore/protein interaction energy provides a mechanism by which electron-transfer events and protein structure are coupled. Two models, which incorporate this linkage into a redox-driven proton pump centered at cytochrome a in cytochrome oxidase, are presented.

Bis(2,9-diphenyl-1,10-phenanthroline)copper(I): a copper complex with a long-lived charge-transfer excited state

Abstract: The title compound, [Cu(dpp)2]+(dpp = 2,9-diphenyl-1,10-phenanthroline), exhibits long-lived emission at room temperature in solution, and in contrast with copper(I) complexes of less bulky ligands, the emission persists in polar media; a water soluble derivative of [Cu(dpp)2]+ is luminescent at room temperature in aqueous solution and undergoes efficient redox photochemistry in the presence of electron acceptors.

Correlation between rate constant for reduction and redox potential as a basis for systematic investigation of reaction mechanisms of electron transfer proteins.

Abstract: Rate constants for the reduction of approximately 40 electron transfer proteins by photoreduced flavins have been determined by laser flash photolysis techniques. The data for a series of 12 homologous cytochromes and 10 homologous high redox potential ferredoxins (HiPIPs) are in excellent agreement with semi-empirical equations relating rate constant and thermodynamic redox potential that have proven applicable to nonbiological electron transfer systems. These correlations allow the establishment of relative reactivities within structurally homologous classes of biological oxidation-reduction proteins, including cytochromes and HiPIPs, and a variety of nonhomologous heme-, iron-sulfur-, copper-, and flavin-containing proteins. A qualitative correspondence is shown to exist between such relative reactivity and the extent of solvent exposure of the redox centers in a particular structural class. The implications of these results are considered, and it is concluded that free energy relationships provide a sound basis for systematic analysis of reaction mechanisms of electron transfer proteins.

Redox potentials of bis(1,4,7-triazacyclononane complexes of some first transition series metals(II,III). Preparation of bis(1,4,7-triazacyclononane)nickel(III) perchlorate

Abstract: Etude, par voltammetrie cyclique, du comportement des complexes en solution aqueuse, avec determination dans ce but du potentiel redox des couples [([9] ane N 3 ) 2 M] 3+/2+ avec M=Mn, Fe, Co, Ni

Bifunctional air electrode

Abstract: Novel catalytic electrode in which the oxidation catalyst is protected from unfavorable redox potentials by an insulation/rectification matrix composed of a polymeric matrix containing an insulating and rectifying effective amount of a dispersed redox conductor.

Interactions of metal hydrous oxides with chelating agents

Abstract: Chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and its thermal decomposition products [iminodiacetic acid (IDA), N(2-hydroxyethyl)iminodiacetic acid (HIDA)], and oxalic acid, enhance the dissolution of magnetite and hermatite at temperatures above 90°C. The mechanism involves three different processes: (a) acid dissolution assisted by the chelating agent, (b) reductive dissolution brought about by internal electron transfer between the ligand and the constituent Fe(III) ion complexes, followed by the released of Fe(II), and (c) additional autocatalytic dissolution by the ferrous-carboxylate complexes generated in solution by process (b). Hydrogen peroxide quenches pathway (c) by oxidizing dissolved Fe(II). The rates dissolution in the absence and in the presence of high H 2 O 2 concentrations allow for the evaluation of the relative importance of (a) and (b). Hydrazine greatly accelarated the overall dissolution rates through direct heterogeneous redox interaction with the surface Fe(III) ions. In all cases magnetic is more reactive than hematite. Rate constants are calculated for the various dissolution pathways and, in the case of heterogeneous electron transfer, the values are compared with those for analogous homogeneous reactions.

STATE I‐STATE II TRANSITIONS IN THE THERMOPHILIC BLUE‐GREEN ALGA (CYANOBACTERIUM) Synechococcus lividus*

Abstract: Time courses of state I-state II transitions were measured in the thermophilic blue-green alga (Cyanobacterium), Synechococcus lividus, that was grown at 55°C. The rate of the state I–II transition using light II illumination was the same as that in the dark, and the dark state was identified to be state II. Therefore, light regulation attained by state transitions is produced by the state II–I transition induced by system I light. The redox level of plastoquinone did not affect this dark state II. Arrhenius plots of the state transitions showed a break point around 43°C that corresponded to the phase transition temperature of this alga. Since both the state I–II and II–I transitions were very much temperature-independent, we could keep the alga in either state for a long time at a “low” temperature such as room temperature. Activities of both photosystems I and II in states I and II were also measured. After a state II–I transition, the system II activity increased about 16% and at the same time, svstem I activity decreased about 30%.

Redox chemistry of metalloporphyrins in aqueous solution

Abstract: A series of water-soluble metalloporphyrins has been prepared and the redox chemistry investigated by electrochemical and pulse radiolytic techniques. All of the metalloporphyrins exhibit reasonably intense absorption transitions (upsilon = ca. 2 x 10/sup 4/ dm/sup 3/ mol/sup -1/ cm/sup -1/) in the visible region and a strong absorption (upsilon = (2-7) x 10/sup 5/ dm/sup 3/ mol/sup -1/ cm/sup -1/) around 430 nm. Cyclic voltammetry showed that the compounds undergo well-defined reduction and oxidation steps, although the oxidation processes were not really reversible. For diamagnetic metalloporphyrins, the difference in E/sub 1/2/ between addition and removal of an electron for a particular compound was 2.05 +/- 0.20 V while the difference in E/sub 1/2/ between addition of one and two electrons was 0.28 +/- 0.12 V. Similarly, E/sub 1/2/ for the removal of a second electron from the porphyrin ..pi.. system was some 0.25 +/- 0.10 V higher than that for removal of the first electron. These findings are consistent with the central metal ion exerting only an inductive effect upon the porphyrin ..pi.. levels. The absorption spectra of the one-electron reduction and oxidation products were recorded by pulse radiolysis methods. Both products exhibit broad absorption transitions stretching across themore » entire visible and near-IR regions. The reduction products were identified as ..pi..-radical anions and, in many cases, these were unstable with respect to disproportionation. The oxidation products were identified as ..pi..-radical cations and these were also unstable in aqueous solution but the decay route remains obscure. 44 references, 6 figures, 3 tables.« less

Electrochemistry of ruthenates. Part 1.—Oxygen reduction on pyrochlore ruthenates

Abstract: The reduction of dioxygen to water has been investigated on ternary ruthenium(IV) oxides having the pyrochlore structure; in particular, compounds of the general form Bi2–2xPb2xRu2O7-y have been studied. Oxygen reduction on these materials appears to proceed by a rate-determining outer-sphere electron transfer, followed by displacement of OH– groups coordinated to RuIV by the much stronger nucleophile O2H˙. The cyclic voltammetry of these oxides is interpreted in the light of the X-ray and ultraviolet photoelectron spectra, and a connection between the bandwidth in these oxides and the potential of the main redox processes observed on the voltammagrams is suggested.

Photoelectrochemical reactions of FeS2 (pyrite) with H2O and reducing agents

Abstract: The photoelectrochemical behaviour of natural and synthetic FeS2 (pyrite) has been investigated. In all cases anodic photocurrents have been observed. From photocurrent spectra an energy band gap of 0.9 eV has been determined. The reaction of photogenerated holes produced on d-states of the Fe2+ ions with water leads to the formation of SO 4 2− as a corrosion product. This is contrary to the photoreaction of RuS2 with H2O which yields oxygen, although the crystal as well as the electronic structure of both compounds is equivalent. The differences are discussed in terms of the transition metal chemistry of iron and ruthenium, respectively. Like with other d-band semiconductors a shift of the onset of photocurrents with the redox potential of electron donors has been observed. This study is part of systematic investigations of catalytic requirements for the photoelectrolysis of water.

Iron-Sulfur Centers and Activities of the Photosynthetic Electron Transport Chain in Iron-Deficient Cultures of the Blue-Green Alga Aphanocapsa

Abstract: Cultures of the blue-green alga, Aphanocapsa, were grown under iron-limiting conditions and changes in concentration of redox components of the photosynthetic electron transport chain, particularly iron-sulfur centers, were monitored by spectroscopic methods. A moderate iron depletion (1/10 of the normal concentration) had little effect on photosynthetic electron transport reactions and growth. Nevertheless, the amount of membrane-bound non-heme iron decreased sharply, and ferredoxin was nearly totally replaced by a flavin-containing protein, flavodoxin. Severe iron-deficiency (1/100 of the normal concentration) was accompanied by growth inhibition and decreased rates of photosynthetic electron flow. The Photosystem I reaction center was most affected by iron depletion as evidenced by a decrease in the amounts of iron-sulfur centers A, B, and X. However, formation of other redox proteins, even those that do not contain iron, was also inhibited by severe iron deficiency.

Ascorbate-quinone interactions: Electrochemical, free radical, and cytotoxic properties (redox potential/semiquinone radical/cancer drug/electron transfer/ascites)

Abstract: Standard midpoint potentials have been deter- mined for p-benxoquinone, methoxy-p-benzoquinone and 2,3-, 2,5-, and 2,6-dimethoxy-p-benxoquinones in aqueous solution. ESR studies have been made of the ascorbate and semiqninone radicals produced when these quinones interact with sodium as- corbate. Direct correlations are found between the electrocbem- ical potentials, generated semiquinone lifetimes, and cytotoxic action in Ehrlich ascites-bearing mice. This work follows from earher considerations of the oxidation- reduction and cytotoxic properties of some methoxyquinones (1) and ofcharge-transfer reactions and electronic delo~ization effects in biological systems (2-4). The compounds of interest in these particular studies have been the methoxy-substituted p-quinones, two of which (methoxy- and 2,6-dimethoxy-p-ben- zoquinone) occur naturally in wheat germ (5). Bachur et al (6) have suggested that the cytotoxic action of quinone anticancer drugs is mediated through their free-radical metabolites. An important first step in such activity is the pro- duction of relatively stable semiquinone free radicals. Radicals result from one-electron rather than two-electron redox reac- tions and the radical concentration and rate ofproduction should depend on the difference between the electrochemical poten- tials of the electron donor and acceptor molecules involved. In this work, electrochemical and ESR measurements have been made to determine the correlation between the cytotoxic activ- ity of several quinone/ascorbate mixtures and their ability to produce stable semiquinone radicals.

Photo-oxidation of metalloporphyrins in aqueous solution

Abstract: Water-soluble, diamagnetic metalloporphyrins have been prepared which contain either zinc(II), palladium(II) or tin(IV) ions as the central metals and their photophysical properties have been measured in dilute aqueous solution. All the compounds undergo efficient intersystem crossing to form long-lived excited triplet states that can participate in electron-transfer reactions. Thus excitation of the metalloporphyrin in dilute aqueous solution containing an appropriate electron acceptor, such as iron(III), may result in formation of the metalloporphyrin π-radical cation in quite high yield. These π-radical cations are powerful oxidants, in some cases E° > 1 V vs NHE, but they undergo secondary reactions that lead to formation of π-dications and isoporphyrins. Despite the high redox potentials, it has not been possible to couple the one-electron reduction of the π-radical cations to the four-electron oxidation of water to molecular oxygen, even in the presence of a redox catalyst such as RuO2. Therefore these compounds appear to possess little promise as water oxidants in homogeneous photosystems.