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Prediction of Methyl Viologen Redox States for Biological Applications
Abstract
Methyl viologen (MV) is an electron mediator that has been shown to be beneficial for enhancing product formation in biofuel processes. For example, increased yields have been observed for ethanol and butanol production. MV has also been used in bioremediation processes such as removal of pollutants from groundwater. However, MV has also been shown to have detrimental effects including inhibition of cell growth. Unfortunately, studies have shown little information regarding the MV redox state. A thermodynamic model is presented and validated in order to predict the most likely redox state of MV depending on the redox potential, pH, and degree of MV dimerization. Model results showed that there are narrow potential ranges in which MV can change from one redox state to another. During fermentation processes in which the potential is in these narrow ranges, slight perturbations in the redox potentials can drastically change the redox state of MV. This may explain the observed variations in the positive and negative effects of MV. Thus, caution should be used when applying MV to any biological system. If needed, an electrode system may prove useful for controlling the redox potential and the associated MV redox state.
... Degradation through dimerization of viologens and quinones has also been reported to be an issue as it competes with desired reaction products and can be difficult to reverse [13,[29][30][31]. Consequently, there is a need to reliably predict the state of the charge carrier, requiring more robust theoretical models which can aid in their design [29,32,33]. ...
... While electrochemical studies reliably estimate kinetic parameters such as diffusion constant and reaction rates, the assessment of the variation in particular species long-term has not been as closely studied. To the best of the authors' knowledge, only a few recent studies have directly examined the independent speciation of products charge mediator products [29,32,39]. Expanding upon existing work can enable more focused studies on experiments and relevant parametric ranges to corroborate computational models. ...
... The thermodynamic modeling in this work is based on Nernstian equilibrium [31,32]. Further details on the transient model are detailed in the Supporting Information; here, only an overview of the key reactions involved are discussed. ...
Charge carriers have been studied for use in applications such as fuel cells, redox flow batteries, and electrochemical CO2 reactors for conversion to value-added products. Here, transient-based equilibrium models are developed for two well-known charge carriers: methyl viologen (MV) and ethyl viologen (EV). The models are simulated using Butler-Volmer kinetics until steady-state is reached. EV is favored over MV due to lower dimerization, and enabling over 2 × production of reduced EV⁺ over MV⁺. MV and EV products do not appear to significantly change, except only under sufficiently acidic conditions (pH < 4). Charge and energy input requirement are used to assess total system efficiency and potential for system scale-up via chronoamperometry. The charge and energy analysis performed with EV as the charge carrier reveals that optimal charging voltage is around − 0.8 to − 0.85 V vs. Ag/AgCl, which is above the minimum reduction voltage (around − 0.6 to − 0.7 V vs. Ag/AgCl) and suggest more favorable conditions for performing such charge carrier reductions.
Graphical abstract
... 18 Moreover, the dication (V 2+ ) is usually a pale-yellow color as its chloride salt, the radical cation (V •+ ) is dark purple, appearing almost black, and the diradical or neutral state (V 0 ) is reddish-orange as its chloride salt. These prominent redox-induced color changes make viologen-based materials ideal for electrochromic devices; 19−22 however, they have also been explored in supercapacitors, 23−25 redox-flow batteries, 26−29 and biological assays 30,31 and even as dopants in dye-sensitized solar cells 32,33 and quantum dots. 34,35 Previously, we synthesized unimolecular and water-soluble oligo-and polyviologens with alternating main-chain backbones, where each viologen subunit was spaced on either side by oligoethylene glycol or hexamethylene subunits. ...
... Either of these competing pathways would have resulted in an increase of the total rate of electron loss in the PV layer. The temporal profile of the E BL signal associated with excitation within the PTh layer at 2.38 eV is shorter with oxygen present ( Figure S27); the average decay lifetime dropped from 450 (30) ps in nitrogen to 237(5) ps with oxygen present. This change in E BL lifetime indicates there is a significant contribution of the observed recovery dynamics in the PTh layer from the back transfer of the electrons from the PV layer to the PTh layer. ...
Bipyridiniums, also known as viologens, are well-documented electron acceptors that are generally easy to synthesize on a large scale and reversibly cycle between three oxidation states (V²⁺, V•+, and V⁰). Accordingly, they have been explored in a number of applications that capitalize on their dynamic redox chemistry, such as redox-flow batteries and electrochromic devices. Viologens are also particularly useful in photoinduced electron transfer (PET) processes and therefore are of interest in photovoltaic applications that typically rely on electron-rich donors like polythiophene (PTh). However, the PET mechanism and relaxation dynamics between interfacing PTh and viologen-based thin films has not been well studied as a function of thickness of the acceptor layer. Here, a novel, bilayered thin film composite was fabricated by first spin-coating PTh onto glass slides, followed by spin-coating and curing polyviologen (PV)-based micron-sized films of variable thicknesses (0.5–11.3 μm) on top of the PTh layer. The electron-transfer mechanism and relaxation dynamics from the PTh sublayer into the upper PV film were investigated using femtosecond transient absorption (fTA) spectroscopy and electrochemistry to better understand how the charge-transfer/relaxation lifetimes could be extended using thicker PV acceptor films. The fTA experiments were performed under inert N2 conditions as well as in ambient O2. The latter shortened the lifetimes of the electrons in the PV layer, presumably due to O2 triplet-based trap sites. Contact angle measurements using H2O and MeI were also performed on top of the bilayered films to measure changes in surface free energy that would aid the assessment related to efficiency of the combined processes involving light penetration, photoexcitation, electron mobility, and relaxation from within the bilayered thin films. Insights gained from this work will support the development of future devices that employ viologen-based materials as an alternative electron-acceptor that is both easily processable and scalable.
... In summary, MV-mediated electricity-enhanced mixotrophy significantly enhanced carbon efficiency and the production of butanol and hexanol. MV has three redox states, namely, the yellow MV 0 , the blue MV + , and the colorless MV 2+ [57]. The thermodynamic model in a previous study showed that at − 900 to − 650 mV vs. Ag/AgCl and pH higher than 6, the dominant form of MV in aqueous solutions was MV + [57]. ...
... MV has three redox states, namely, the yellow MV 0 , the blue MV + , and the colorless MV 2+ [57]. The thermodynamic model in a previous study showed that at − 900 to − 650 mV vs. Ag/AgCl and pH higher than 6, the dominant form of MV in aqueous solutions was MV + [57]. In our experiment, it was observed that MV was added into the fermentation broth in the form of colorless MV 2+ . ...
Anaerobic, non-photosynthetic mixotrophy (ANP mixotrophy) is a process whereby glucose consumption and CO2 fixation via the Wood-Ljungdahl pathway occur concurrently. ANP mixotrophy supplemented with exogenous reducing power and CO2 shows the potential of reducing the carbon loss from glycolysis and enhancing the production of biofuels. Previous studies used H2 and syngas as the exogenous source of reducing power, however, the low solubility of H2 and CO impeded their efficient utilization by bacteria. This study used a bio-electrochemical system (BES) to supply electrons to fix CO2 and shift the metabolite profiles towards more-reduced products in acetogenic bacteria Clostridium carboxidivorans. C. carboxidivorans showed no electroactivity, but the electricity-enhanced fermentation without electron mediator showed higher alcohol production and carbon efficiency compared to open-circuit fermentation. C. carboxidivorans could fix CO2 autotrophically with the electrical current and electron mediator MV or biochar. Electricity-enhanced mixotrophy with MV showed a higher alcohol/acid ratio and carbon efficiency in liquid products (0.402 mol/mol and 82.8%, respectively) compared to those without MV and electricity (0.067 mol/mol and 52.2%, respectively). The addition of biochar in electricity-enhanced mixotrophy increased the carbon efficiency to 80.3%, and promoted the synthesis of butyrate and hexanoate. This study investigated the electricity-enhanced ANP mixotrophic fermentation for the first time, and demonstrated its potential for enhancing carbon efficiency and alcohol production with C. carboxidivorans.
... The voltage-responsive MSNs S3 are depicted in Scheme 1. Surface of rhodamine B loaded MSNs was functionalized with a positively charged N,N′-dimethyl-4,4′-bipyridinium derivative (also known as paraquat or methyl viologen) and capped with the negatively charged polysaccharide heparin via electrostatic interactions. Methyl viologen was selected because its well-known redox properties and easy reduction at relatively low potentials (vide infra) [54,55]. Moreover, heparin (a sulfonated polysaccharide) is a biocompatible cap that for instance has been used to improve neuronal growth, cell proliferation and migration [56,57]. ...
The characteristics and electromechanical properties of conductive polymers together to their biocompatibility have boosted their application as a suitable tool in regenerative medicine and tissue engineering. However, conducting polymers as drug release materials are far from being ideal. A possibility to overcome this drawback is to combine conducting polymers with on-command delivery particles with inherent high-loading capacity. In this scenario, we report here the preparation of conduction polymers containing gated mesoporous silica nanoparticles (MSN) loaded with a cargo that is delivered on command by electro-chemical stimuli increasing the potential use of conducting polymers as controlled delivery systems. MSNs are loaded with Rhodamine B (Rh B), anchored to the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly[(4-styrenesulfonic acid)-co-(maleic acid)], functionalized with a bipyridinium derivative and pores are capped with heparin (P3) by electrostatic interactions. P3 releases the entrapped cargo after the application of −640 mV voltage versus the saturated calomel electrode (SCE). Pore opening in the nanoparticles and dye delivery is ascribed to both (i) the reduction of the grafted bipyridinium derivative and (ii) the polarization of the conducting polymer electrode to negative potentials that induce detachment of positively charged heparin from the surface of the nanoparticles. Biocompatibility and cargo release studies were carried out in HeLa cells cultures.
Coupling a photoredox module and a bio-inspired non-heme model to activate O2 for oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O2 at the catalytic unit. We found that the efficient oxidative quenching mechanism between [Ru(bpy)3]2+ chromophore and a reversible electron mediator, methyl viologen (MV2+), to form the reducing species methyl viologen radical (MV•+) can convey an electron to O2 to form the superoxide radical and resetting an Fe(III) species in a catalytic cycle to the Fe(II) state in an aqueous solution. The formation of the Fe(III)-hydroperoxo (FeIII-OOH) intermediate therefrom to evolve to highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate. Such a strategy allows to bypass the challenging task of charge accumulation at the molecular catalytic unit for the two-electron activation of O2. The FeIII-OOH catalytic precursor was trapped and characterized by EPR spectroscopy pertaining a metal assisted catalysis. Importantly, we found that the substrate itself can act as the electron donor to reset the photooxidzed chromophore in the initial state closing the photocatalytic loop hence excluding the use of a sacrificial electron donor. Laser Flash Photolysis (LFP) studies and spectroscopic monitoring during photocatalysis lend credence to the proposed catalytic cycle.
Cellulose, which accounts for more than half of the carbon content in plants, has become a popular feedstock for biofuel production. In this work, direct electricity generation from dissolved cellulosic biomass in an alkaline fuel cell is explored without energy-intensive process. The effect of different cellulose dissolution solvents on the electricity production is investigated. Results show dissolution treatment can remarkably affect the fuel cell performance. When NaOH/urea/thiourea is chosen as a solvent, the specific capacity of cellulose is about sixfold higher than that using only NaOH as solvent. The limiting current density and the maximum power density reach 0.85 mA cm−2 and 0.07 mW cm−2, respectively. This power density surpasses those of any existing biotic or abiotic designs. Electrochemical characterizations demonstrate that the remarkable activity improvement towards the cellulose oxidation reaction in NaOH/urea/thiourea aqueous solution is due to the lower charger transfer resistance and higher energy transfer efficiency.
The rate constants for the dimerization of the reduced products of methyl viologen dication (MV²⁺) of the processes (a) methyl viologen cation radical (MV+••), (MV²⁺ → MV+••) and 2MV+• →(MV+••)2 and (b) methyl viologen neutral (MV), (MV+••→ MV) and 2MV → (MV)2 were obtained through linear scan-/cyclic-voltammetry and the use of Olmstead-Hamilton-Nicholson theory. The rate constants for the two dimerization processes are reported here. Rate constant for the dimerization process (b) above, of the novel species methyl viologen neutral is being reported first time.
Majority of the reported electrografting of aryldiazonium salts results in the formation of covalently attached films with a limited surface coverage below 5 nmol.cm-2. Herein, we reported the preparation of higher thickness redox active viologen grafted electrodes from electroreduction of viologen phenyl diazonium salts, by either cyclic voltammetric (CV) sweeps or electrolysis using a fixed potential. Both of the methodologies, were successfully applied for various conductive surfaces, including glassy carbon, gold disc, ITO glass, mesoporous TiO2 electrodes and 3D compacted carbon fibers. A robust maximal viologen coverage, Γviologen= 9.5 nmol was achieved on a glassy carbon electrode by CV electro-reduction. Electro-reduction holding at a fixed potential at Eappl. = -0.3V, can fabricate viologen grafted electrode with Γviologen in the range of 0~37 nmol.cm-2 in a controllable way, by simply adjusting the electrodeposition time tappl.. Time dependent Γviologen were found as: 10 nmol.cm-2@2 min, 20 nmol.cm-2@4.2 min, 30 nmol.cm-2@7 min. Furthermore, TiO2 electrode coupled with Γviologen of 140 nmol.cm-2 exhibited electrochromic performance with color changes from pale yellow to blue and red-brown.
The bacterium "Clostridium ragsdalei" previously was identified as capable of producing ethanol from CO, CO2 and H2. In order to enhance the production of ethanol from these gases, three reducing agents (methyl viologen, benzyl viologen, and neutral red) were added to fermentation medium at a concentration of 0.1 mM. The medium was inoculated with "C. ragsdalei," and the headspace gas was exchanged daily with a gas mix of 20% CO, 15% CO2, 5% H2, and 60% N2. Reducing agents were added to the culture after 91 h of cell growth. Benzyl viologen caused cell death, and no ethanol production was detected. Methyl viologen promoted ethanol production as compared to the control containing no reducing agent, while neutral red neither promoted nor detracted from ethanol production when compared to the control. Addition of 0.2 or 0.3 mM methyl viologen delayed the production of ethanol as compared with 0.1 mM methyl viologen.
Nitrifier denitrification is the conversion of nitrite to nitrous oxide by ammonia-oxidizing organisms. This process, which
is distinct from denitrification, is active under aerobic conditions in the model nitrifier Nitrosomonas europaea. The central enzyme of the nitrifier dentrification pathway is a copper nitrite reductase (CuNIR). To understand how a CuNIR,
typically inactivated by oxygen, functions in this pathway, the enzyme isolated directly from N. europaea (NeNIR) was biochemically and structurally characterized. NeNIR reduces nitrite at a similar rate to other CuNIRs but appears
to be oxygen tolerant. Crystal structures of oxidized and reduced NeNIR reveal a substrate channel to the active site that
is much more restricted than channels in typical CuNIRs. In addition, there is a second fully hydrated channel leading to
the active site that likely acts a water exit pathway. The structure is minimally affected by changes in pH. Taken together,
these findings provide insight into the molecular basis for NeNIR oxygen tolerance.
Batch cultures of Thermotoga maritima were performed in a bioreactor equipped with instruments adapted for experiments performed at 80°C to mimic the fluctuating oxidative conditions in the hot ecosystems it inhabits. When grown anaerobically on glucose, T. maritima was shown to significantly decrease the redox potential (Eh) of the culture medium down to about -480 mV, as long as glucose was available. Addition of oxygen into T. maritima cultures during the stationary growth phase led to a drastic reduction in glucose consumption rate. However, although oxygen was toxic, our experiment unambiguously proved that T. maritima was able to consume it during a 12-hour exposure period. Furthermore, a shift in glucose metabolism towards lactate production was observed under oxidative conditions.
Microbial electrosynthesis, a process in which microorganisms use electrons derived from electrodes to reduce carbon dioxide
to multicarbon, extracellular organic compounds, is a potential strategy for capturing electrical energy in carbon-carbon
bonds of readily stored and easily distributed products, such as transportation fuels. To date, only one organism, the acetogen
Sporomusa ovata, has been shown to be capable of electrosynthesis. The purpose of this study was to determine if a wider range of microorganisms
is capable of this process. Several other acetogenic bacteria, including two other Sporomusa species, Clostridium ljungdahlii, Clostridium aceticum, and Moorella thermoacetica, consumed current with the production of organic acids. In general acetate was the primary product, but 2-oxobutyrate and
formate also were formed, with 2-oxobutyrate being the predominant identified product of electrosynthesis by C. aceticum. S. sphaeroides, C. ljungdahlii, and M. thermoacetica had high (>80%) efficiencies of electrons consumed and recovered in identified products. The acetogen Acetobacterium woodii was unable to consume current. These results expand the known range of microorganisms capable of electrosynthesis, providing
multiple options for the further optimization of this process.
Sulfite reductase (EC 1.8.1.2) has been purified 190-fold from spinach leaves. The enzyme catalyzes the reduction of sulfite to sulfide, with the use of reduced methyl viologen. NADH and NADPH are not capable of replacing reduced methyl viologen. A stoichiometry of 6 molecules of reduced methyl viologen oxidized per molecule of sulfide formed has been found.
The enzyme, following treatment with reduced methyl viologen, is sensitive to inhibition by p-chloromercuribenzoate and cyanide. Without treatment with reduced methyl viologen the enzyme is not sensitive to either compound. Inhibition is not relieved by thiol compounds. Sensitivity of the enzyme to p-chloromercuribenzoate is lost in the presence of sulfite, while cyanide sensitivity remains. These results suggest the occurrence of two catalytically active sites, a p-chloromercuribenzoate-sensitive, sulfite-protected site, and a cyanide-sensitive site.
Nucleoside di- and triphosphates, irrespective of the kinds of base and sugar, stimulate reductase activity. Pyrophosphate gives the same stimulation as nucleoside diphosphates. This effect is observed in 2-(hydroxymethyl)-2-(N-glycine)-1,3-dihydroxypropane buffer, but not in phosphate buffer.
Paraquat, a quaternary ammonium bipyridyl herbicide, produces degenerative lesions in the lung after systemic administration to man and animals. The pulmonary toxicity of paraquat resembles in several ways the toxicity of several other lung toxins, including oxygen, nitrofurantoin and bleomycin. Although a definitive mechanism of toxicity of paraquat has not been delineated, a cyclic single electron reduction/oxidation of the parent molecule is a critical mechanistic event. The redox cycling of paraquat has two potentially important consequences relevant to the development of toxicity: generation of "activated oxygen" (e.g., superoxide anion, hydrogen peroxide, hydroxyl radical) which is highly reactive to cellular macromolecules; and/or oxidation of reducing equivalents (e.g., NADPH, reduced glutathione) necessary for normal cell function. Paraquat-induced pulmonary toxicity, therefore, is a potentially useful model for evaluation of oxidant mechanisms of toxicity. Furthermore, characterization of the consequences of intracellular redox cycling of xenobiotics will no doubt provide basic information regarding the role of this phenomena in the development of chemical toxicity.
The methyl viologen (MV)-dependent, linear electron flow from PS II to PS I was severely blocked in intact or broken, uncoupled chloroplasts when oxygen was removed from the suspension medium, as revealed by measurements of chlorophyll fluorescence and the rate of photoreduction of MV. Kinetics of the reduction of pre-oxidized P700 by a saturating light pulse showed that reduced MV in the absence of oxygen re-reduces P700+ via the intersystem electron transport chain. Since the re-reduction of P700+ was inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, the MV-mediated cyclic electron flow, in contrast to the phenazine methosulphate-catalyzed one, involves the plastoquinone pool. However, 2-n-heptyl-4-hydro-xyquinoline-N-oxide, 2-n-nonyl-4-hydroxyquinoline-N-oxide and antimycin A did not inhibit the MV-mediated flow. Thus, the inhibition of the linear electron flow in chloroplasts under anaerobic conditions suggested the overreduction of the plastoquinone pool as a result of the MV-mediated cyclic flow
Hydrogenases are nature's efficient catalysts for both the generation of energy via oxidation of molecular hydrogen and the production of hydrogen via the reduction of protons. However, their O2 sensitivity and deactivation at high potential limit their applications in practical devices, such as fuel cells. Here, we show that the integration of an O2-sensitive hydrogenase into a specifically designed viologen-based redox polymer protects the enzyme from O2 damage and high-potential deactivation. Electron transfer between the polymer-bound viologen moieties controls the potential applied to the active site of the hydrogenase and thus insulates the enzyme from excessive oxidative stress. Under catalytic turnover, electrons provided from the hydrogen oxidation reaction induce viologen-catalysed O2 reduction at the polymer surface, thus providing self-activated protection from O2. The advantages of this tandem protection are demonstrated using a single-compartment biofuel cell based on an O2-sensitive hydrogenase and H2/O2 mixed feed under anode-limiting conditions.
The kinetics of reductive electron transfer to the methyl viologen dication (MV2+) to form a radical cation (MV+.) have been investigated in the presence of outer-sphere charge-transfer complexation. The solvent was water and the electrode substrate was platinum. It was found that the rate of electron transfer does not depend on the equilibrium constant of complexation, but rather on the rate of dissociation of the solution-phase complex. Accordingly, it has been suggested that the reaction of a CE reaction with at least a partial dissociation of the complex, to separate the acceptor and donor components—prior to the successful electron-transfer reaction.
The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V2+) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V0) in the presence of H+ (from HCl), CH3COOH (pKa=4.75), ClCH2CH2COOH (pKa=4.00), HCOOH (pKa=3.75) in aqueous media was examined by cyclic voltammetry according to the EECi mechanism. To avoid the electrodeposition of V0, we used a 9:1 (v/v%) H2O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V2+, the ratio of the anodic and cathodic peak current (Ipa2/Ipc2) corresponding to V0–e−⇄V+ was plotted against log τ, where τ is the time between E1/2 and the switching potential, at various scan rates of 0.02–3.5 V s−1. The chemical reaction was found to be a parallel reaction consisting of H+-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as kH+=3.5×103 M−1 s−1, kCH3COOH=5.7 M−1 s−1, kHCOOH=4.6×101 M−1 s−1, and kClCH2CH2COOH=3.2×101 M−1 s−1 using the Nicholson–Shain method and kH2O was estimated as <3×10−6 M−1 s−1. The CVs were digitally simulated under the assumption of a two-step reaction of V0 following the two-step electrode reactions of V2+ to V0. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V0 is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V0 is in pseudo-equilibrium with H+ or HA to produce VH+ which undergoes a reaction with H2O.
Two pathways for transmembrane reduction of dihexadecylphosphate (DHP) vesicle-entrapped Co(bpy)[sub 3][sup 3+] ion by photogenerated methyl viologen radical cation have been identified by quantitative evaluation of the reaction kinetics. One pathway involves electrogenic diffusion of MV[sup +] and the other electroneutral diffusion of MV[sup 0] across the bilayer. The pathways were clearly distinguished by the self-impeding character of the electrogenic reaction, which was progressively retarded as the membrane polarization increased, and by the net stoichiometry of viologen uptake accompanying transmembrane oxidation-reduction. The first-order rate constants for transmembrane diffusion of MV[sup +] and MV[sup 0] were estimated to be 2.7 x 10[sup [minus]2] s[sup [minus]1] and 1.1 x 10[sup 3] s[sup [minus]1], respectively, at 23[degree]C. The rate constant for MV[sup +] diffusion is identical to the value previously measured by [sup 14]C-isotopic labeling methods, and other kinetic parameters were consistent with thermodynamic data obtained from measurements of Donnan equilibria. 31 refs., 7 figs., 2 tabs.
The reduction of water to H2 by reduced methyl viologen, MV˙+, mediated by a number of different redox catalysts has been studied via the oxidation of MV˙+ and concomitant generation of H2. One of the best catalysts found, in terms of H2 evolution efficiency, activity, reproducibility and stability, is an H2-pretreated sample of platinised alumina (Pt–Al2O3) which gave H2 yields of ca. 75%. The kinetics of H2O reduction by MV˙+, mediated by this catalyst have been studied as a function of pH and from the results it appeared there was a diffusion-controlled region (at pH < 3.1) and an activation-controlled region (at pH > 4). The kinetics of MV˙+ oxidation were studied in these two regions as a function of [MV˙+], [MV2+], [cat] and temperature, and the results interpreted in terms of an electrochemical model of redox catalysis in which a Nernstian oxidation reaction (i.e. MV˙+–e–→ MV2+) is coupled to an irreversible reduction reaction (i.e. H++ e–→½H2)via electron transfer through the Pt particles on the alumina support.
The process of electron transfer between methyl viologen radicals and water in photolytic systems was investigated. It was found that upon irradiation the steady-state concentration of reduced methyl viologen was pH-dependent and that hydrogen was produced without a catalyst. Electrochemical reduction of methyl viologen showed that the radical acts as an acid-base indicator. It could be shown that the electron transfer process occurs intramolecularly via a colourless methyl viologen radical-proton complex (pKa = 4.0 ∓ 0.2). Hitherto the catalyst was believed to operate as an electron acceptor for methyl viologen radicals. Now it could be proved that the noble metal acts merely as a site for hydrogen deposition and recombination for the intramolecular reduced hydrogen atoms.
Efficient electron transfer (ET) between microbes and electrodes is a key factor for electricity generation in microbial fuel cell (MFC). The utilization of reversible redox electron-mediator can enhance such extracellular ET but could result in environmental contamination and low cost-effectiveness. These limitations may be overcome by immobilizing electron-mediator molecules on electrode surface. In this paper, we present a stepwise amidation procedure to covalently immobilize neutral red (NR), which has been proved to be an appropriate mediator to harvest microbial metabolic electrons due to its excellent electrochemical reversibility and compatible redox potential to the major metabolic electron carriers (e.g., of NADH/NAD+), on carbon electrodes. In this procedure, immobilization of NR is realized by acylchlorination of the carboxylated carbon surface with thionyl chloride followed by amidation reaction with NR. It is shown that such a stepwise amidation procedure can significantly increase the amounts of NR molecules immobilized on carbon surface without altering their redox properties. In addition, the use of NR-immobilized carbon electrodes as MFC anode can significantly increase the power output and the utilization of carbon sources (organic fuel).
Benzoylformic acid is reduced electrocatalytically to mandelic acid in excellent yield in the presence of methyl viologen of which two-electron reduction product behaves as active reductant, and the presence of β-CD or its derivative induces enantioselectivity.
The use of methyl viologen (MV) neutral species as a mediator for the reduction of CH3I and the methanogenic bacterial nickel-containing cofactor F430 is reported. Solutions of MV0 were generated using a potential step electrolysis procedure in which MV.+ is first generated at −0.6 V (Ag/AgCl) and then MV0 at −1.0 V (Ag/AgCl). The resulting spectrum of MV0 has a prominent peak at 367 nm with a shoulder at 398 nm and no peaks in the 525 to 625 nm region. Spectral and total charge transfer comparisons from spectroelectrochemical experiments as well as cyclic voltammetric studies of MV2+ alone and with CH3I and F430, individually, provide evidence for the MV0 mediated reduction of CH3I and F430. To our knowledge, this represents the first use of MV0 as a mediator for the reduction of another species.
Tin oxide, gold and platinum film optically transparent electrodes are evaluated for use in spectro-electrochemical studies of organic oxidations and reductions in non-aqueous solvents. Acetonitrile, dimethylformamide, nitromethane, propylene carbonate, dichloroethane, monoglyme and diglyme were used, usually with tetraalkylammonium perchlorate or hexafluorophosphate as supporting electrolyte. Care was taken in purifications and all experiments were run in a controlled atmosphere dry box. The background potential limits for these electrodes depend greatly on the type and purity of solvent and supporting electrolyte, and in well-purified systems are sufficiently large to allow the study of oxidation of benzene or the reduction of phenanthrene or napthalene. Data are reported for the reduction of methyl viologen, a6nthraquinone and three nitroaromatics, and for the oxidation and reduction of several aromatic hydrocarbons. The optical windows of these electrodes have made possible the in situ observation of the intermediates and products of electrode reactions; some examples, including work using rapid scanning spectrophotometry, are presented.
The polarographic behavior of 1,1′-dimethyl- and 1,1′-dibenzyl-4,4′-bipyridinium dichloride has been investigated over the pH range of 1 to 13. Reversible reduction waves are obtained for the first single-electron step for both compounds. The second step is electrochemically irreversible but evidence is presented to indicate that the second step is fleetingly reversible at the dropping mercury electrode for methyl viologen but not benzyl viologen. Minor waves due to adsorption of the reduced viologen on the surface of the mercury have been observed.
Cyclic voltammetry of Pseudomonas aeruginosa azurin on polycrystalline gold is reversible (E0 = 360 mV vs she; 50 mM ammonium acetate) but the voltammetric signals decay with time constants of about 3 × 10-3 s-1. No signal is observed for monocrystalline Au(111). Cys3Ser azurin is electrochemically inactive on either type of gold electrode but shows a reversible although decaying peak (362 mV, 50 mM ammonium acetate; decay time constant ≈ 2 × 10-3 s-1) on edge-plane pyrolytic graphite. Ex situ and in situ atomic force microscopy (AFM) of the azurins on Au(111) show initially arrays of protein structures of lateral 100-200 Å and vertical ≈ 50 Å extension. These could be individual molecular images convoluted with the tip curvature. As scanning proceeds the structures in the ex situ mode collect into large two-dimensional assemblies while the adsorbed protein in the in situ mode is largely swept into the solution, recovering the free Au(111) surface. The cyclic voltammetry and AFM data are consistent with time dependent adsorption of the azurins on gold via the disulphide bridge (wild-type) or free thiol group (Cys3Ser mutant).
1. The electron and the proton content (measured as electrode potential [Eh] and pH) of an environment characterize this environment in many ways. In this paper the electrode potential and the pH are used as empirical parameters rather than as electrochemical data capable of thermodynamic interpretation. From published and unpublished work by the authors and from the literature, more than 6,200 pairs of characteristics were gathered, covering most types of the aqueous environment as well as the potential milieu of the chief actors in these environments: algae and bacteria. 2. It appears that the Eh-pH limits of biological systems and of the naturally occurring aqueous environment almost coincide. This would indicate that there are few, if any, sterile terrestrial environments caused by limiting Eh-pH characteristics. 3. As it seems unlikely that environments will be found outside the limits outlined in this paper, physico-chemical speculations on the sedimentary environment should be limited by this outli...
Treatment of 1-methyl-4-cyanopyridinium iodide with aqueous sodium dithionite at pH 10.5 in the absence of oxygen yields methylviologen cation radical iodide (the cation radical formed by one-electron reduction of 1,1′-dimethyl-4,4′-dipyridinium diiodide). The structure of the cation radical is shown by spectroscopic, titrimetric, and electron spin resonance methods. The mechanism of the reaction is outlined, some aspects of pyridinyl radical dimer chemistry are discussed, and complexes of the methylviologen cation radical are described.
The effect of the solvent on the standard electrode potential of the silver-silver chloride electrode or on the standard potential of the cell H2 (Pt-Pt) (1 atm.)/HCl (soln.)/AgCl-Ag, can be estimated by a new equation which is based upon the free energy change of solution of hydrochloric acid with the change of the solvent: EN0* = EN0 - [9.2/(P - 3.19)] RT ln aw in mole fraction scale and at 25°. The agreement between the calculated and the observed values is good.
With a thin-layer spectroelectrochemical technique the molar extinction coefficient (epsilon) of the methyl vilogen cation radical (MV/sup +/.) has been measured to obtain, at the red peak, 13,700, 13,800, and 13,900 Mâ»Â¹ cmâ»Â¹, in HâO, CHâOH, CâHâOH, and CHâCN, respectively. These values are 15 to 25% higher than those usually reported or cited. The present data are used to assess the quantum efficiency (phi) of photocatalytic MV/sup +/.production. The quantum efficiency value well exceeds unity for some semiconductor-solvent combinations.
Monte Carlo simulations of liquid formamide and dimethylformamide, using the united-atom OPLS potential energy functions, have been undertaken. Basic thermodynamic properties and radial distribution functions are largely unaffected by the inclusion of long-range electrostatic interactions in the form of a reaction field, whereas the dipole-dipole correlation function is observed to be very sensitive. Static dielectric constants of the liquids were calculated by measuring the polarization of the systems as a function of an applied electric field. The dielectric constant calculated for dimethylformamide is in good agreement with experiment, whereas the computed value for formamide is too low. It is likely that treatment of explicit polarization is needed for hydrogen-bonded liquids and related systems including proteins to correctly describe dielectric behavior. The dependence of the calculated dielectric constants on system size and reaction field dielectric constant was also examined.
In many potentiometric titrations of biological electron transfer components, mediators are often employed to act as electron transfer mediators between the components and the potentiometric electrode. Certain mediators have been employed recently as electrogenerated titrants of components in coulometric titrations, particularly in some spectroelectrochemical studies. In this paper, the optical and electrochemical characteristics of some bipyridylium salts are examined as mediator-titrants. These salts (dications) undergo a reversible one electron reduction to the radical cation which can be a good reductant. Spectroelectrochemistry has also been applied to the evaluation of the rate of electron transfer from the radical to oxidized cytochrome c. The results reported herein are a part of our interest in evaluating the electron transfer properties of heme proteins found in the mammalian respiratory system.Bei vielen potentiometrischen Titrationen von Komponenten des biologischen Elektronentransport-Systems werden häufig Mediatoren eingesetzt, die als Vermittler der Elektronenübertragung zwischen der Verbindung und der potentiometrischen Elektrode wirken. Eine Reihe von Mediatoren sind kürzlich als elektrochemisch erzeugte Titranden in coulometrischen Titrationen angewandt worden, insbesondere bei einigen spektroelektrochemischen Untersuchungen. In dieser Veröffentlichung werden die optischen und elektrochemischen Eigenschaften einer Reihe von Bipyridyliumsalzen in bezug auf ihre Eignung als intern erzeugte Elektronenüberträger in coulometrischen Titrationen (mediator-titrants) untersucht. Diese Salze (Dikationen) unterliegen einer reversiblen Ein-Elektron-Reduktion zum Radikalkation, das ein gutes Reduktionsmittel darstellt. Außerdem wurde die Geschwindigkeit der Elektronenübertragung vom Radikal auf oxidiertes Cytochrom c mit Hilfe spektroelektrochemischer Methoden ermittelt. Die hier berichteten Ergebnisse sind Teil unserer Forschungen über die Wirkungsweise der Hämproteine in der Atmungskette bei Elektronenübertragungsreaktionen.
The interaction of neutral red (NR) with double-stranded calf thymus DNA was investigated by electrochemical methods using a DNA-modified glassy carbon (GC-DNA) electrode. The results were compared with those obtained at a bare glassy carbon (GC) electrode. The findings showed that although the kinetic of the electron transfer process remains reversible similar to that of using GC electrode, the formal potential of NR (E0′) was more positive when using GC-DNA electrode. GC-DNA electrode enables the enrichment of the concentration of NR for the chosen times on the electrode surface. The ratio of the binding constants for the oxidized and reduced forms of the bound species was obtained through comparing the formal potentials of NR obtained using the modified and bare surfaces. A binding isotherm for NR at GC-DNA electrode was obtained from coulometric titrations indicated a Langmuiran behavior for the NR adsorption on DNA layer, and gave an affinity constant equal to 2.76 × 104 l mol−1.
BACKGROUND: A bioelectrochemical process for trichloroethene (TCE) dechlorination was developed. In this new process, a solid-state electrode polarized to −450 mV versus the standard hydrogen electrode (SHE), in combination with a redox mediator (i.e., methyl viologen, MV) is employed as an electron donor for the microbial reductive dechlorination of TCE. In this study we compared the performance of the process with the redox mediator immobilized at the surface of electrodes or dissolved in the bulk liquid, using both a culture highly enriched in Desulfitobacterium spp., capable of dechlorinating TCE to cis-dichloroethene (cis-DCE), and a culture highly enriched in Dehalococcoides spp. capable of dechlorinating cis-DCE to ethene.RESULTS: Short-term potentiostatic (−450 mV versus SHE) experiments showed that TCE or cis-DCE was dechlorinated both in the presence of soluble (500 µmol L−1) and immobilized MV. However, TCE or cis-DCE dechlorination rates with MV-modified electrodes were remarkably lower than with soluble MV. Both cultures produced significant amounts of H2 in the presence of electrically reduced, soluble MV, whereas no H2 was produced when the mediator was immobilized at the electrode surface, regardless of the potential applied to the electrode, in the range −425 to −500 mV versus SHE.CONCLUSIONS: The process, operated with modified electrodes, supports the microbial dechlorination of TCE to ethene. Immobilization not only allows retention of the mediator within the system, but also increases process efficiency by preventing bioelectrochemical H2 formation. On the other hand, strategies to increase dechlorination rates with modified electrodes need to be developed. Copyright © 2009 Society of Chemical Industry
Controlled batch experiments performed withClostridium acetobutylicum show that methyl viologen induces solvent production at near neutral pH. At a pH of 6.8, significant ethanol production was observed in presence of methyl viologen. At pH 5, production of butanol and ethanol are favored at the expense of acetone.
Batch cultures of Clostridium acetobutylicum at controlled pH values of 5 and 5.5 were carried out in a three-electrode potentiometric system with methyl viologen (1 mM) as electron carrier. Although an irreversible loss of methyl viologen at the electrode surface was observed, a significant increase in alcohol yield was obtained. In comparison to control fermentation with or without methyl viologen addition, the butanol yield improvements were respectively of 7 or 51% at pH 5, and 56 or 467% at pH 5.5.
The nature and importance of the DNA repair system in the chloroplasts of higher plants under oxidative stress or UV radiation-induced genotoxicity was investigated via gain-of-functional approaches exploiting bacterial RecAs. For this purpose, transgenic tobacco (Nicotiana tabacum) plants and cell suspensions overexpressing Escherichia coli or Pseudomonas aeruginosa RecA fused to a chloroplast-targeting transit peptide were first produced. The transgenic tobacco plants maintained higher amounts of chloroplast DNA compared with wild-type (WT) upon treatments with methyl viologen (MV), a herbicide that generates reactive oxygen species (ROS) in chloroplasts. Consistent with these results, the transgenic tobacco leaves showed less bleaching than WT following MV exposure. Similarly, the MV-treated transgenic Arabidopsis plants overexpressing the chloroplast RecA homologue RECA1 showed weak bleaching, while the recA1 mutant showed opposite results upon MV treatment. In addition, when exposed to UV-C radiation, the dark-grown E. coli RecA-overexpressing transgenic tobacco cell suspensions, but not their WT counterparts, resumed growth and greening after the recovery period under light conditions. Measurements of UV radiation-induced chloroplast DNA damage using DraI assays (Harlow et al. 1994) with the chloroplast rbcL DNA probe and quantitative PCR analyses showed that the transgenic cell suspensions better repaired their UV-C radiation-induced chloroplast DNA lesions compared with WT. Taken all together, it was concluded that RecA-overexpressing transgenic plants are endowed with an increased chloroplast DNA maintenance capacity and enhanced repair activities, and consequently have a higher survival tolerance to genotoxic stresses. These observations are made possible by the functional compatibility of the bacterial RecAs in chloroplasts.
The electrocatalytic reduction of NAD+ using diaphorase enzyme was studied. Methyl viologen was used as an electron transfer mediator between an electrode and the enzyme. A catalytic wave for the reduction of NAD+ when all the species were in the solution was measured with cyclic voltammetry at a gold-amalgam electrode which showed low background currents at negative potentials. Steady-state currents could be obtained under the conditions of slow scan rate, low methyl viologen concentration, and high NAD+ concentration as the electrode reaction was converted to an electrochemical-catalytic (EC′) reaction. The bimolecular rate constant for the reaction of the reduced methyl viologen with the oxidized diaphorase was estimated as 7.5×103 M−1 s−1 from the slope of the current versus [MV2+] plot. Another slope of the current against the square root of the enzyme concentration also gave a close value of 6.7×103 M−1 s−1. In the calculation of the rate constant, the stoichiometric factor when it is not one-to-one was considered. With the evaluated rate constant, digital simulation using the suggested reaction mechanism was compared with the experimentally obtained voltammograms. Satisfactory agreement indicates that the evaluation methods of the rate constant and the suggested mechanism are appropriate for the mediated enzyme-catalyzed electrochemical reactions.
1. Single reduced methyl viologen (MV.+) acts as an electron donor in a number of enzyme systems. The large changes in extinction coefficient upon oxidation (λmax 600 nm; MV.+, ε = 1.3 · 104 M−1 · cm−1; oxidised form of methyl viologen (MV2+), ε = 0.0) make it ideally suited to kinetic studies of electron transfer reactions using stopped-flow and standard spectrophotometric techniques.2. A convenient electrochemical preparation of large amounts of MV.+ has been developed.3. A commercial stopped-flow apparatus was modified in order to obtain a high degree of anaerobicity.4. The reaction of MV.+ with O2 produced H2O2 (k > 5 · 106 M−1 · s−1, pH 7.5, 25 °C). H2O2 subsequently reacted with excess MV.+ (k = 2.3 · 103 M−1 · s−1, pH 7.5, 25 °C) to produce water. The kinetics of this reaction were complex and have only been interpreted over a limited range of concentrations.5. The results support the theory that the herbicidal action of methyl viologen (Paraquat, Gramoxone) is due to H2O2 (or radicals derived from H2O2) induced damage of plant cell membrane.
The effects of a weak electric field on the growth and metabolic behavior of Enterobacter dissolvens were investigated using glucose as the sole carbon source. A direct current (DC) was applied using salt bridge electrodes and platinum electrodes. The best stimulating effects in terms of cell growth and the dehydrogenase activity (DHA) were obtained when a DC of 10 mA was applied for 12 h via the platinum electrodes. In this case, the electrolysis of water was the major electrode reaction, as determined by cyclic voltammetry. The presence of the hydrogen generated a strong reductive environment and led to a reduction of NAD/NADH ratio from 7 to 3. The specific activity of dehydrogenase and glucose consumption increased 2- and 1.5-fold, respectively. The application of the DC via the platinum electrodes also led to accelerated cell death during the later stationary phase. This is possibly due to the presence of anodic intermediates including H2O2, OH and O2. These results provide more details for understanding the effect of a DC on E. dissolvens, a strain with potential application in the electro-remediation of PAHs contaminated soil.
Production of chemicals and fuels directly from CO(2) is an attractive approach to solving the energy and environmental problems. 1-Butanol, a chemical feedstock and potential fuel, has been produced by fermentation of carbohydrates, both in native Clostridium species and various engineered hosts. To produce 1-butanol from CO(2), we transferred a modified CoA-dependent 1-butanol production pathway into a cyanobacterium, Synechococcus elongatus PCC 7942. We demonstrated the activity of each enzyme in the pathway by chromosomal integration and expression of the genes. In particular, Treponema denticola trans-enoyl-CoA reductase (Ter), which utilizes NADH as the reducing power, was used for the reduction of crotonyl-CoA to butyryl-CoA instead of Clostridium acetobutylicum butyryl-CoA dehydrogenase to by-pass the need of Clostridial ferredoxins. Addition of polyhistidine-tag increased the overall activity of Ter and resulted in higher 1-butanol production. Removal of oxygen is an important factor in the synthesis of 1-butanol in this organism. This result represents the first autotrophic 1-butanol production.
The tabulation gives the normal potentials of the various indicators at 30 degrees C.; referred to the normal hydrogen electrode, the accuracy is estimated to be +/-0.002 volt. Normal potentials of the viologens at 30 degrees C.: Methyl viologen -0.446 volts Ethyl viologen -0.449 volts Betaine viologen -0.444 volts Benzyl viologen -0.359 volts Supposing some solution brings about a coloration of one of these indicators to the extent of A per cent of the maximum color, the oxidation-reduction potential of this solution is E = E(o) - 0.06 log See PDF for Equation where E(o) is the normal potential according to the above tabulation. This normal potential is independent of pH.
A mechanism for the reduction and oxidation of methyl viologen by Clostridium pasteurianum hydrogenase (hydrogen:ferredoxin oxidoreductase, EC 1.12.7.1) is proposed. Double reciprocal plots for methyl viologen reduction and oxidation at pH values 7.0-9.85 are linear, and the plots for reduction and oxidation are intersecting. Such data are consistent with a mechanism in which the H2 and one methyl viologen bind (either in order or randomly) with subsequent reduction and release of the methyl viologen. A second methyl viologen then is bound, reduced and released. Comparison of the calculated Keq' with the Haldane expression in which both methyl viologens react at the same rate show a large difference. This difference indicates that the two methyl viologens react at different rates. Addition of oxidized electron carriers inhibits the hydrogen-deuterium exchange reaction (i.e., the exchange of protons between H2 and 2H2O). CO reversibly inhibits methyl viologen reduction and is competitive vs. H2. O2 acts as an irreversible inhibitor.
Pure glutathione reductase from Saccharomyces cerevisiae catalyzed under anaerobic conditions the enzymatic reduction of GSSG using electrochemically reduced methyl viologen as electron donor. The new assay was completely dependent on the amount of active enzyme present, and involved the formation of 1 mol GSH per mole of reduced methyl viologen consumed. The enzyme followed a standard Michaelis-Menten kinetics; a Km = 230 microM for reduced methyl viologen and a turnover number of 969 mumol GSSG reduced per minute per micromole enzyme were determined. The enzymatic activity seemed to depend on the redox potential, showing half-maximal activity at -0.407 V. The enzyme was quite specific: the activity using reduced benzyl viologen as electron donor was just 1.5% of that obtained with reduced methyl viologen at the same concentration and potential. Glutathione reductase was totally inactivated after a brief anaerobic exposure with reduced methyl viologen in the absence of GSSG; a partial reactivation was observed following addition of glutathione disulfide. No inhibition of the methyl viologen-dependent activity was observed in the presence of 2',5'-ADP or 2'-P-5'-ADP-ribose, two NADP(H) analogs, at concentrations which drastically inhibited the NADPH-dependent activity, thus suggesting that the reduced viologen does not interact with the pyridine nucleotide-binding site.
Paraquat was reduced by mouse lung microsome when incubated anaerobically with NADPH. The reaction was inhibited by the addition of antibody to rat liver NADPH-cytochrome c reductase. In the presence of NADPH and NADPH-cytochrome c reductase, paraquat increased the invitro peroxidation of rat liver microsomal lipid. The peroxidation was inhibited by superoxide dismutase and the singlet oxygen trapping agent 1,3-diphenylisobenzofuran. It is suggested that paraquat toxicity may be mediated through the transfer of a single electron from reduced paraquat to oxygen and thus form superoxide ion. Singlet oxygen may form from superoxide and subsequently react with lipids to form fatty acid hydroperoxides.
The photoreduction of viologen dyes having E0′ values between −0.32 and −0.74 V was studied with isolated chloroplasts. Compared to ferricyanide, all compounds mediated photosynthetic oxygen evolution and photophosphorylation with equal rate and efficiency. The concentration required for maximum stimulation increased with decreasing normal potential.Under anaerobic conditions, an accumulation of the reduced viologens could be observed directly. Despite the interference of a back reaction with photosynthetically evolved O2, compounds with E0′ > −0.55 could be reduced completely, lower-potential agents partially. It is concluded that the normal potential of the strong reductant, generated in the long-wave photoact of photosynthesis is as low or lower than −0.7 V.
Methods are described for determining diquat and paraquat, singly and in admixture, in formulations. For determining diquat, ultraviolet absorptiometry at 310 mµ in a sodium acetate buffer solution at pH 4·05 is adopted. Paraquat is determined in a diluted solution by measuring the optical density, at 600 mµ, of the blue free radical produced by reduction with alkaline sodium dithionite. For analysing mixtures containing both diquat and paraquat, these methods are combined and use is made of a base-line correction procedure to compensate for interference of diquat in the determination of paraquat. The error for these methods is established to be within ±2 per cent.
Oxygen and carbon dioxide concentrations, internal atmospheric pressure, catalyst temperature, and time of appearance of water condensate were monitored for various time intervals at ambient (20 to 25 degrees C) temperature in a GasPak 100 Anaerobic System (BBL Microbiology Systems, Cockeysville, Md). Simultaneously, the redox potential (Eh) of various plated culture media in the system was also measured. The oxygen concentration was reduced to less than 0.4% in 100 min. The Eh of the media, corrected for hydrogen ion, reached -100 mV within 60 to 100 min, and the carbon dioxide concentration increased to between 4 and 7% in 60 min, depending on the number of plates of media present. Condensate appeared generally between 10 and 15 min, and the temperature of the lid reached a maximum between 20 and 40 min. Condensate time and lid temperature increase are important early indicators of a correctly functioning GasPak System. A characteristic pressure-vacuum-pressure profile is produced as a result of controlled release of hydrogen and carbon dioxide gases and the reaction of hydrogen and oxygen to produce water. Anaerobic conditions were achieved well before the methylene blue anaerobic indicator became decolorized, which required more than 6 h at 20 to 25 degrees C. At this time the Eh of media in the jar was well below -200 mV. Since the indicator is reduced within 5 h at 35 degrees C, the Eh of media in the jar would also be expected to decrease more rapidly at the higher temperature.
The oxidation-reduction midpoint potentials (Em) of the following compounds have been measured in the range of pH from 3 to 12 by polarography: methyl viologen; benzyl viologen; 2-hydroxy-1,4-naphthoquinone; 2-hydroxy-1,4-anthraquinone; N,N,N',N',-tetramethyl-p-phenylenediamine; 2,3,5,6-tetramethyl-p-phenylenediamine; phenazine; N-methylphenazonium methosulfate; N-methylphenazonium sulfonate methosulfate; N-ethylphenazonium ethosulfate; pyocyanine; neutral red; safranin; phenol red; chlorophenol red; cresol red; bromocresol purple; 2,5-dibromo-3-methyl-6-isopropylbenzoquinone and 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole. Many of these previously assumed to have a simple behavior in this range have proven to be rather more complicated, and several anomalous observations have been reconciled.
The ability to transfer electrons, via an extracellular path, to solid surfaces is typically exploited by microorganisms which use insoluble electron acceptors, such as iron-or manganese-oxides or inert electrodes in microbial fuel cells. The reverse process, i.e., the use of solid surfaces or electrodes as electron donors in microbial respirations, although largely unexplored, could potentially have important environmental applications, particularly for the removal of oxidized pollutants from contaminated groundwater or waste streams. Here we show, for the first time, that an electrochemical cell with a solid-state electrode polarized at -500 mV (vs standard hydrogen electrode), in combination with a low-potential redox mediator (methyl viologen), can efficiently transfer electrochemical reducing equivalents to microorganisms which respire using chlorinated solvents. By this approach, the reductive transformation of trichloroethene, a toxic yet common groundwater contaminant, to harmless end-products such as ethene and ethane could be performed. Furthermore, using a methyl-viologen-modified electrode we could even demonstrate that dechlorinating bacteria were able to accept reducing equivalents directly from the modified electrode surface. The innovative concept, based on the stimulation of dechlorination reactions through the use of solid-state electrodes (we propose for this process the acronym BEARD: Bio-Electrochemically Assisted Reductive Dechlorination), holds promise for in situ bioremediation of chlorinated-solvent-contaminated groundwater, and has several potential advantages over traditional approaches based on the subsurface injection of organic compounds. The results of this study raise the possibility that immobilization of selected redox mediators may be a general strategy for stimulating and controlling a range of microbial reactions using insoluble electrodes as electron donors.
Non-growing Clostridium saccharoperbutylacetonicum N1-4 hardly produced butanol from only butyrate. As adding glucose to the medium, butyrate utilization and butanol production were stimulated. Addition of 0.1 mM methyl viologen as electron carrier resulted in the highest yield of butanol of 0.671 mol/mol to butyrate and glucose.