Showing papers on "Redox published in 1979"
TL;DR: In this article, rate constants for electron-transfer quenching of Ru(bpy)32+* (bpy is 2,2'-bipyridine) by a series of organic quenchers have been determined in acetonitrile (μ = 0.77 ± 0.1 M) at 22 ± 2 °C.
Abstract: Rate constants for electron-transfer quenching of Ru(bpy)32+* (bpy is 2,2'-bipyridine) by a series of organic quenchers have been determined in acetonitrile (μ = 0.1 M) at 22 ± 2 °C. The reactions studied were based on three different series of structurally related quenchers having varying redox potentials. They include oxidative quenching both by a series of nitroaromatics (ArNCh) and by a series of bipyridinium ions (P2+) and reductive quenching by a series of aromatic amines (R^NAr). After corrections for diffusional effects, the quenching rate constant (kq') data fall into two classes both of which can be treated successfully using Marcus-Hush theory. For case 1, which includes the data for oxidative quenching by P2+ and reductive quenching by RaNAr, RT In kq varies as AG21/2 where | AGasI « /2. AG23 is the free energy change for electrontransfer quenching within an association complex between the quencher and excited state and is the vibrational contribution to the activation barrier to electron transfer. The experimental data are also consistent with Marcus-Hush theory over a more extended range in AG22 where the free energy dependence includes a quadratic term. For case II, which includes quenching by several of the nitroaromatics, RT In kq varies as AG23 and evidence is obtained from the remainder of the data for a transition in behavior from case 11 to case I. The microscopic distinction between the two cases lies in competitive electron transfer to give either groundor excited-state products following the electron-transfer quenching step. For case II, back-electron transfer (A32) to give the excited state, e.g., Ru(bpy)33+,ArNC>2— Ru(bpy)32+*,ArN02, is more rapid than electron transfer to give the ground state (A30), e.g., Ru(bpy)33+,ArN02~ Ru(bpy)32+,ArN02· For case I, electron transfer to give the ground state is more rapid. The different behaviors are understandable using electron-transfer theory when account is taken of the fact that k30 is a radiationless decay rate constant, and the electron-transfer process involved occurs in the abnormal free-energy region where — AG22 > . An appropriate kinetic treatment of the quenching rate data allows estimates to be made of redox potentials for couples involving the excited state. Formal reduction potentials in CH3CN (μ = 0.1 M) at 22 ± 2 °C are £(RuB33+-/2+*) = —0.81 ± 0.07 V and £(RuB32+‘/+) = +0.77 ± 0.07 V. Comparisons between groundand excited-state potentials show that the oxidizing and reducing properties of the Ru(bpyb2+ system are enhanced in the excited state by the excited-state energy, that the excited state is unstable with respect to disproportionation into Ru(bpy)3+ and Ru(bpy)33+, and that the excited state is thermodynamically capable of both oxidizing and reducing water at pH 7. A comparison between the estimated 0-0 energy of the excited state and the energy of emission suggests that there may be only slight differences in vibrational structure between the ground and excited states.
422 citations
TL;DR: In this paper, the arsenic pE or Eh domain obtained using published thermodynamic data for As species and the assumption of redox equilibrium, provides a basis for obtaining an indication of the redox levels within the central portion of the dominant redox field for natural waters.
313 citations
01 Aug 1979
TL;DR: In this article, a time-dependent diffusion model was used to determine the removal rate constants for Mn2+ and Cr(III) in the linear temperature-salinity region surrounding the oxygen-hydrogen sulfide interface (130 to 115 m).
Abstract: In June 1977 ten redox sensitive chemical species were determined in the oxidizing and reducing waters of Saanich Inlet, British Columbia. An oxygen-hydrogen sulfide interface was found at a depth of 130 m. The concentrations of the oxidized species: oxygen, nitrate, iodate, and chromate, and the reduced constituents: hydrogen sulfide, ferrous iron, and ammonia reached background or near background levels at this depth. Three reduced constituents, manganese (II), chromium (III), and iodide existed metastably in the oxic layer. Thermodynamic calculations of pe indicate that the redox couples in the anoxic waters approach equilibrium with each other while those in the oxygenated waters are far from equilibrium. A time-dependent diffusion model is used to determine the removal rate constants for Mn2+ and Cr(III) in the linear temperature-salinity region surrounding the oxygen-hydrogen sulfide interface (130 to 115 m). In this region I− is conservative within the error of our measurements over the period of anoxia in the inlet (estimated to be 3 to 6 months). The residence times of Cr(III) and Mn2+ in the oxic layer are one to three weeks and about two days, respectively. We argue that the dominant manganese removal mechanism is by oxidation, resulting in a calculated oxidation rate constant for Mn2+ five orders of magnitude greater than published laboratory values and widely different from previous field estimates. The rapid oxidation kinetics in Saanich Inlet are probably caused by bacterial catalysis near the oxygen-hydrogen sulfide interface.
251 citations
TL;DR: In this article, an analytical study of free radicals of high negative redox potential such as cap alpha-alcohol radicals were found to transfer electrons to colloidal silver particles stabilized by sodium dodecyl sulfate in aqueous solution.
Abstract: Organic free radicals of high negative redox potential such as ..cap alpha..-alcohol radicals were found to transfer electrons to colloidal silver particles stabilized by sodium dodecyl sulfate in aqueous solution. The colloidal particles thus became a pool of stored electrons that could reduce water to form hydrogen or react with suitable acceptors in solution. The organic radicals were produced by irradiation, using suitable scavengers for the primary radicals from the radiolysis of the aqueous solvent. The solutions initially contained silver ions at 1 x 10/sup -4/ - 2 x 10/sup -3/ M. At doses below 10/sup 5/ rd, the silver ions were completely reduced to form the colloidal catalyst. In this dose range, the corresponding hydrogen yield amounted to 1 molecule per 100 eV. It increased steeply at higher doses up to 3 molecules per 100 eV. The H/sub 2/ yield decreased with increasing dose rate and with increasing pH in alkaline solutions. It was highest at a concentration of sodium dodecyl sulfate of 1 x 10/sup -3/ M, i.e., far below the critical micelle concentration of this surfactant. Changes in the absorption spectrum of the colloid are attributed to changes in the size of the silver particles upon chargingmore » up with electrons. The competition of radical-colloid reactions with radical-radical deactivation in the bulk of solution or at the surface of the colloidal particles is also discussed. 11 figures.« less
234 citations
TL;DR: In this article, the first observations of the dynamics of intervention of the Pt particles in the redox events are presented, and a centrifuged colloidal Pt catalyst stabilised by polyvinyl alcohol showed exceptionally high activity in promoting hydrogen evolution from water via 2MV+ + H2O→PtH2 + 2OH− + 2 MV2+ where MV+ stands for reduced methylviologen.
Abstract: Noble metal dispersions are suitable for mediating light-induced hydrogen1–5 and oxygen6–8 evolution from water. We report here a dramatic improvement of the hydrogen production rate when very finely dispersed platinum is used as a mediator in reaction (1). The first observations of the dynamics of intervention of the Pt particles in the redox events are presented. A centrifuged colloidal Pt catalyst stabilised by polyvinyl alcohol showed exceptionally high activity in promoting hydrogen evolution from water via 2MV+ + H2O→PtH2 + 2OH− + 2MV2+ (1) where MV+ stands for reduced methylviologen. The latter is produced photochemically in aqueous solution containing Ru(bipy)32+ as a sensitiser and EDTA as an electron donor. At 10−3 M Pt the reoxidation of MV+ requires only 15 µs and leads to quantitative formation of H2.
228 citations
TL;DR: It is proposed that the redox state of mitochondrial pyridine nucleotides can be or is in part controlled by glutathione peroxidase and glutathion reductase and is a factor in the balance of Ca(2+) between mitochondria and medium.
Abstract: When rats are fed a selenium-deficient diet, the glutathione peroxidase activity in liver mitochondria decreases within 5 weeks to 0-6% of that of control animals fed on a diet supplemented with 0.5 ppm of selenium as sodium selenite. Analysis of the temperature dependence of energy-linked Ca2+ uptake by means of Arrhenius plots reveals two breaks (at around 11°C and 24°C) in mitochondria isolated from selenium-supplemented animals, whereas in selenium-deficient rats the break at 11°C is absent. Ca2+-loaded mitochondria of selenium-supplemented rats—i.e., with active glutathione peroxidase in the matrix—lose Ca2+ rapidly, with a concomitant oxidation of endogenous NAD(P)H, when exposed to t-butyl hydroperoxide or H2O2. In contrast, in selenium deficiency, t-butyl hydroperoxide and H2O2 induce neither a release of Ca2+ nor an oxidation of NAD(P)H. The peroxide-induced oxidation of NAD(P)H is reversible in the presence of succinate when no Ca2+ has been taken up. When Ca2+ has previously been accumulated, however, the oxidation of NAD(P)H is irreversible. Enzymatic analysis of mitochondrial pyridine nucleotides reveals that the peroxide-induced oxidation of NAD(P)H in Ca2+-loaded mitochondria leads to a loss of NAD+ and NADP+. It is proposed that the redox state of mitochondrial pyridine nucleotides can be or is in part controlled by glutathione peroxidase and glutathione reductase and is a factor in the balance of Ca2+ between mitochondria and medium.
195 citations
TL;DR: In this article, the anodic oxidation of glucose on platinum in phosphate buffer solution (pH=7.5) at potentials below 350 mV/RHE was investigated by cyclic voltammetry, chronoamperometry and adsorption measurements.
176 citations
TL;DR: Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P- 680 may be a ligated chloropyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment.
Abstract: Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl) a and its cation radicals. Oxidation potentials for Chl → Chl+. as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl+. ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P-680 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment.
131 citations
TL;DR: In this paper, the authors showed that derivatized n-type Si photoelectrode surfaces with (1, 1'-ferrocenediyl)dichlorosilane can be used to sustain the oxidation of solution-dissolved ferrocene under conditions where naked Si is incapable of doing so.
Abstract: Derivatization of n-type Si photoelectrode surfaces with (1,1'-ferrocenediyl)dichlorosilane results in the persistent attachment of photoelectroactive ferrocene species. Derivatized surfaces have been characterized by cyclic voltammetry in EtOH or H/sub 2/O electrolyte solutions. Such surfaces exhibit persistent oxidation and reduction waves, but the oxidation requires illumination as expected for an n-type semiconductor. The oxidation wave is observed at potentials approx. 300 mV more negative than at Pt, reflecting the ability to oxidize ferrocene contrathermodynamically by irradiation. Derivatized n-type Si can be used to sustain the oxidation of solution-dissolved ferrocene under conditions where naked Si is incapable of doing so. Further, derivatized n-type Si has been used in an aqueous electrolyte to oxidize Fe(CN)/sub 6//sup 4 -/. Finally, the photooxidation of solution species has been demonstrated to occur via photogeneration of holes in the Si, oxidation of the surface-attached species, and then oxidation of the solution species by the surface-attached oxidant, providing the first direct proof of mediated electron transfer for any derivatized electrode. Derivatized electrodes can be used to sustain the conversion of light to electricity but the efficiencies are low. Based on results for 632.8-nm irradiation, solar energy conversion efficiencies of approx. 1% can be obtained. 7 figures, 1 table.
130 citations
TL;DR: The relevant oxidation-reduction equilibria of the heme chromaphore are presented in terms of a thermodynamic model for the control of the observed cytochrome P450 redox potential through this modulation of the spin configuration of the five d-electrons of the ferric heme iron induced by the binding of Type I substrates.
19 Mar 1979
TL;DR: The rate constants for electron transfer from a series of substituted isomeric dihydroxy- and diaminobenzenes to different substituted phenoxyl radicals were measured by observing the decay or buildup of one of the radicals invoved.
Abstract: The rate constants for electron transfer from a series of substituted isomeric dihydroxy- and diaminobenzenes to different substituted phenoxyl radicals were measured by observing the decay or buildup of one of the radicals invoved. In many cases the electron transfer reactions were reversible and the equilibrium constants could be calculated from the individual rate constants for attainment of equilibrium and from the concentrations of the species involved at equilibrium. From the equilibrium constants the one-electron redox potentials for 15 individual Q/sup -/./Q/sup 2 -/ pairs were determined, using the value for hydroquinone (23 mV at pH 13.5) as a reference. The potential for catechol (43 mV) is near that of hydroquinone; resorcinol is oxidized much less readily (300 mV), while phenol is even a weaker reductant (>500mV). Methyl, methoxy, and hydroxy substituents decrease the redox potentials while acetyl and carboxyl substituents increase these values. Ascorbate has a potential (15mV) similar to that of hydroquinone, while TMPD (82mV) and p-phenylenediamine (183mV) are less easily oxidized.
TL;DR: Baedecker et al. as mentioned in this paper showed that if the groundwater flow is rapid relative to reaction rates, redox zones will develop downgradient, where groundwater flow velocities are low the zones will overlap to the extent that they may be indeterminate.
TL;DR: It is proposed that electron flow from NADPH to Q may occur in the dark by a pathway utilising portions of the normal cyclic and non-cyclic electron carrier sequences and the possible in vivo role in redox poising of cyclic electron transport and hence in controlling the ATP/NADPH supply ratio is discussed.
TL;DR: In this paper, the authors focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the questions in mind, such as how is propylene activated? What is the nature of the reactive oxygen species?
Abstract: Publisher Summary This chapter focuses on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the questions in mind, such as “How is propylene activated? What is the nature of the reactive oxygen species?” The selective oxidation of propylene is an important model reaction for studying oxidation reactions over oxide catalysts. Much information has been gathered over the past three decades that helps to answer these questions. Considerable evidence exists that indicates the selective oxidation of propylene proceeds via the formation of a symmetrical ally species. Subsequent steps may vary as a function of the catalyst. Some catalyst systems may abstract a second hydrogen atom before the insertion of oxygen. Others may add molecular oxygen, forming a hydroperoxide intermediate, which may then subsequently decompose into acrolein and water. While a number of oxygen species can exist on the surface of oxides, the reactive oxygen for the selective oxidation of propylene is lattice oxygen. Under reaction conditions, solid-state reactions occur that are a function of the temperature and the composition of the reacting gas mixture.
TL;DR: Isoprenoid quinones are able to translocate electrons and protons in lipid membranes, and that the side chain has a strong impact on the mechanism, and the relevance of the model reaction for electron and proton transport in photosynthesis and respiration is discussed.
TL;DR: In this paper, the redox system Cu(NH3)2+/1+x, Fe(CN) 3− 4− 6 and Fe3+/2+ and oxygen reduction have been studied on MoS2 surfaces normal to the C-axis and parallel to the c-axis, using rotating disc electrodes under potentiostatic conditions.
TL;DR: In this paper, a photochemical electron transfer across the interface of a water-in-toluene microemulsion was proposed as a model for the separation of oxidised and reduced species.
Abstract: THE creation of artificial solar energy conversion and storage systems that mimic the photosynthetic pathway has evoked great interest in recent years1. One approach to the photolysis of water involves the mediation of two photosystems in the generation of reduced and oxidised species that are the active components in the decomposition of water2,3. However, homogeneous (aqueous) solutions of these components suffer from the basic limitation that the reducing and oxidising agents can react with each other and thus no net reaction can be observed. Several kinds of synthetic ‘photosynthetic membranes’ have been suggested as a means of separating the two redox units and overcoming these fundamental difficulties3. Recently, photo-sensitised electron transfer across vesicle walls has been demonstrated and suggested as a means for generating oxidising and reducing agents in separate water compartments4. We report here a photochemical electron transfer across the interface of a water-in-toluene microemulsion, and propose this system as a model for the separation of oxidised and reduced species. It is well known that surfactant molecules aggregate to reversed micelles in organic solvents5. Reversed micelles entrap water to form ‘water pools’ in a continuous oil phase. The proposed general model for the compartmentalisation of two water phases and its utilisation in the photolysis of water is shown in Fig. 1.
TL;DR: The presence of valinomycin under certain well-defined conditions can strongly influence the value of the measured pK on the H+II binding agent.
TL;DR: In this article, a quantitative expression for the catalytic effect of electron-conducting catalysts on redox reactions was derived by using an electrochemical model, where the catalyst takes up the mixture potential (Emix) imposed on it by the interacting redox couples; from the equations of their individual current against voltage curves, it is then possible to calculate the equal anodic and cathodic currents at Emix.
Abstract: A quantitative expression is derived for the catalytic effect of electron-conducting catalysts on redox reactions by using an electrochemical model. In this the catalyst takes up the mixture potential (Emix) imposed on it by the interacting redox couples; from the equations of their individual current against voltage curves it is then possible to calculate the equal anodic and cathodic currents at Emix. These currents lead directly to the catalytic rate. If both current against voltage curves around Emix are in the Tafel region, the catalytic rate is proportional to the concentrations of the two reactants raised to fractional powers. If the curve of one of the reagents is in the limiting current region, the catalytic rate will be proportional to the concentration of that reagent alone.
TL;DR: Evidence is presented that indicates that the double-deprotonated ascorbate anion, asCorbate2-, is the reducing agent, and the pH-independent second-order rate constants for reduction by this species are given.
Abstract: The reduction of single-site haem and copper redox proteins by ascorbic acid was studied as a function of pH. Evidence is presented that indicates that the double-deprotonated ascorbate anion, ascorbate2-, is the reducing agent, and the pH-independent second-order rate constants for reduction by this species are given. Investigation of the temperature dependences of these rate constants have yielded the values of the activation parameters (delta H++ and delta S++) for reduction. These values, together with ligand-replacement studies, suggest that ascorbate2- acts as an outer-sphere reductant for these proteins. Reasons to account for the apparent inability of ascorbic acid to reduce the alkaline conformer of mammalian ferricytochrome c are suggested.
TL;DR: It is proposed that an early form of electron flow with oxidative phosphorylation (“prerespiration”), to therminal electron acceptors available in a reducing biosphere, was supplemented by a photocatalyst capable of a redox reaction, and cyclic photophosphorylation arose.
TL;DR: Evidence is presented for a membrane-bound 'photoreduction' hydrogenase in H2-induced Anacystis that apparently initiates a light-driven electron flow from H2 to various low-potential acceptors including endogenous ferredoxin.