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The activities of critical metabolic and regulatory proteins can be altered by exposure to natural or synthetic redox-cycling compounds. Many bacteria, therefore, possess mechanisms to transport or transform these small molecules. The opportunistic pathogen Pseudomonas aeruginosa PA14 synthesizes phenazines, redox-active antibiotics that are toxic to other organisms but have beneficial effects for their producer. Phenazines activate the redox-sensing transcription factor SoxR and thereby induce the transcription of a small regulon, including the operon mexGHI-opmD, which encodes an efflux pump that transports phenazines, and PA14_35160 (pumA), which encodes a putative monooxygenase. Here, we provide evidence that PumA contributes to phenazine resistance and normal biofilm development, particularly during exposure to or production of strongly oxidizing N-methylated phenazines. We show that phenazine resistance depends on the presence of residues that are conserved in the active sites of other putative and characterized monooxygenases found in the antibiotic producer Streptomyces coelicolor. We also show that during biofilm growth, PumA is required for the conversion of phenazine methosulfate to unique phenazine metabolites. Finally, we compare ∆mexGHI-opmD and ∆pumA strains in assays for colony biofilm morphogenesis and SoxR activation, and find that these deletions have opposing phenotypic effects. Our results suggest that, while MexGHI-OpmD-mediated efflux has the effect of making the cellular phenazine pool more reducing, PumA acts on cellular phenazines to make the pool more oxidizing. We present a model in which these two SoxR targets function simultaneously to control the biological activity of the P. aeruginosa phenazine pool.

Electrochemical study of chalcogen-containing species under aqueous conditions is often hindered by unwanted oxidation and surface adsorption. Disulfides, diselenides, thiols, and selenols react readily with metallic electrodes to form self-assembled monolayers (SAMs) and conductive films. We have shown that the intrinsic reactivity between selenium and gold yields a stable and reproducible electrode surface modification that makes the reduction of selenocystine to selenocysteine accessible at neutral pH. In this work, we expand our electrochemical characterization of the selenocystine / selenocysteine redox couple to examine the pH dependence of the electron-transfer process, evaluate preparative conditions for formation of the selenium-gold adsorbate, and compare surface reactivity and solution behavior on gold and platinum. We demonstrate that the electrochemically generated surface modification can be removed via anodic stripping voltammetry and restored via subsequent cycling with little to no loss in current. X-ray photoelectron spectroscopy confirms the presence of selenium on the surface of substrates that have been reductively cycled in the presence of selenocystine. These findings mark an important advance in the electroanalysis of chalcogen-containing biological species under physiological conditions.

Carminic acid and its metal derivatives have been used widely as pigments for fabrics and art, and more recently as a colorant for food. An undergraduate teaching laboratory is described in which students are instructed to design and execute experimental studies to obtain detailed information about the electronic structure, metal complex formation, redox properties, and photochemical stability of extracted carminic acid. The lab maintains room for student innovation and is well-suited to upper-level undergraduates in an advanced spectroscopy lab. Students are invited to apply knowledge previously gained through highly directed experiments to the analysis of an unfamiliar, complex, and relevant problem. The laboratory lends itself to flexibility in implementation but is designed to deepen students’ understanding of UV–vis spectroscopy, fluorescence spectroscopy, IR spectroscopy, electrochemistry, pH and metal spectrophotometric titrations, and experimental determination of the kinetic behavior of UV-induce...

The voltammetric response of seleno-l-cystine has been studied at gold substrates under physiological conditions. The reactivity of diselenides is utilized to generate a modified electrode surface, which in turn allows for electroanalysis of solution-based selenium species. Stable and reproducible voltammetry is observed for selenocystine reduction at pH 7 on selenium-modified gold substrates (Emid = −486 mV vs. Ag/AgCl) and is consistent with a diffusionally controlled process. Modification of the gold surface is readily achieved via electrochemical cycling in the presence of a diselenide source at conventional scan rates. These studies afford voltammetric characterization of the selenocystine/selenocysteine redox couple under physiologically relevant conditions and highlight the potential utility of selenium-modified substrates for electroanalysis of chalcogen-containing species. Keywords: Selenium, Selenocystine, Diselenide, Voltammetry

Psychrophilic organisms play a significant role in global nutrient cycling and bioremediation, and thus there is great interest in understanding the details of their cellular processes. Here, we report the first electrochemical measurements of cytochrome c552 isolated from the model psychrophile, Colwellia psychrerythraea. Quasi-reversible redox behavior is observed at thiol-modified and bare gold electrodes with midpoint potentials of 247 and 277 mV vs SHE, respectively. Comparison of the voltammetric response on the two substrates illustrates the need for a modified surface to direct protein orientation and facilitate electron transfer in a reproducible manner. Temperature-dependence measurements of the current profile illustrate the protein’s ability to participate in electron transfer reactions over a 50° range, which is supported by recent structural evidence of enhanced flexibility. These electrochemical findings allow the behavior of this psychrophilic cytochrome to be evaluated in the context of m...

The direct electrochemical oxidation of ammonia has been examined in both the organic solvent dimethylformamide (DMF) and the room temperature ionic liquid 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][N(Tf)2]. The corresponding voltammetric responses have been shown to be similar in each solvent with a broad oxidative wave occurring upon the introduction of ammonia to the solution and the appearance of a new reductive wave following the oxidation. The oxidative reaction process has been examined and a suitable reaction pathway has been deduced, corresponding to the formation of ammonium cations after oxidation of the ammonia. A linear response of limiting current against vol% ammonia was observed in both DMF and [EMIM][N(Tf)2], suggesting potential application for analytical methods.

Two series of homoleptic phenolate complexes with fluorinated aryloxide ligands A2[M(OAr)4] with M = Co2+ or Cu 2+, OAr- = (OC6F5)- (OArF) or {3,5-OC6H3(CF3) 2}- (OAr′), A+ = K (18-crown-6) +, Tl+, Ph4P+, Et3HN +, or Me4N+ have been synthesized. Two related complexes with nonfluorinated phenoxide ligands have been synthesized and studied in comparison to the fluorinated aryloxides demonstrating the dramatic structural changes effected by modification of OPh to OArF. The compounds {K(18-crown-6)}2[Cu(OArF)4], 1a; {K(18-crown-6)}2[Cu(OAr′)4], 1b; [Tl 2Cu(OArF)4], 2a; [Tl2Cu(OAr′) 4], 2b; (Ph4P)2[Cu(OArF) 4], 3; (nBu4N)2[Cu(OAr F)4], 4; (HEt3N)2[Cu(OAr F)4], 5; {K(18-crown-6)}2[Cu 2(μ2-OC6H5)2(OC 6H5)4], 6; {K(18-crown-6)} 2[Co(OArF)4], 7a; {K(18-crown-6)} 2[Co(OAr′)4], 7b; [Tl2Co(OAr F)4], 8a; [Tl2Co(OAr′)4], 8b; (Me4N)2[Co(OArF)4], 9; [Cp 2Co]2[Co(OAr′)4], 10; and {K(18-crown-6)}2[Co2(μ2-OC6H 5)2(OC6H5)4], 11, have been characterized with UV-vis and multinuclear NMR spectroscopy and solution magnetic moment studies. Cyclic voltammetry was used to study 1a, 1b, 7a, and 7b, X-ray crystallography was used to characterize 1b, 3, 4, 5, 6, 7a, 7b; 10, and 11. The related [MX4]2- compound (Ph 4P)2[Co(OArF)2Cl2], 12, has also been synthesized and characterized spectroscopically, as well as with conductivity and single-crystal X-ray diffraction. Use of fluorinated aryloxides permits synthesis and isolation of the mononuclear, homoleptic phenolate anions in good yield without oligomerized side products. The reaction conditions that result in homoleptic 1a and 7a with OArF upon changing the ligand to OPh result in μ2-OPh bridging phenoxides and the dimeric complexes 6 and 11. The [M(OArF)4]2- and [M(OAr′)4]2- anions in 1a, 1b, 3, 4, 5, 7a, 7b, 9, and 10 demonstrate that stable, isolable homoleptic phenolate anions do not need to be coordinatively or sterically saturated and can be achieved by increasing the electronegativity of the ligand.

The results detail a novel methodology for the electrochemical determination of ammonia based on its interaction with hydroquinone in DMF. It has been shown that ammonia reversibly removes protons from the hydroquinone molecules, thus facilitating the oxidative process with the emergence of a new wave at less positive potentials. The analytical utility of the proposed methodology has been examined with a linear range from 10 to 95 ppm and corresponding limit-of-detection of 4.2 ppm achievable. Finally, the response of hydroquinone in the presence of ammonia has been examined in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide, [EMIM][N(Tf) 2 ]. Analogous voltammetric waveshapes to that observed in DMF were obtained, thereby confirming the viability of the method in either DMF or [EMIM][N(Tf) 2 ] as solvent.

Homoleptic fluorinated alkoxide complexes have been prepared from KOC4F9, 1, via salt metathesis routes. One four-coordinate K{K(18C6)}[Co(OC4F9)4], 2, and four three-coordinate complexes: {K(18C6)}[Fe(OC4F9)3], 3, {K(18C6)}[Co(OC4F9)3], 4, {K(18C6)}[Cu(OC4F9)3], 5, and {K(18C6)}[Zn(OC4F9)3], 6, have been prepared and all except 5 have been characterized crystallographically. Compounds 3, 4, and 6 are very rare examples of monomeric, trigonal alkoxide complexes. All compounds have been characterized with UV-vis and IR spectroscopy, solution magnetic susceptibility, and elemental analysis. In solution, compound 2 exists in an equilibrium with 4 and 1, which has been probed with cyclic voltammetry, supporting energetically different Co2+/Co3+ potentials in the three-coordinate (E(p,a) = approximately 1.2 V vs Fc/Fc+) and four-coordinate (E(p,a) = approximately 0.9 V) geometries. The ligand field engendered by the perfluoro-t-butoxide ligand has been studied with DFT calculations on 4 and the hypothetical [Co(OC4H9)3]- as well as the previously reported [Co(mes)3]- and [Co{N(TMS)2}3]- showing significant -type interactions in the xy plane as well as above and below for the two alkoxide species.

The base pair stack of DNA has been demonstrated as a medium for long-range charge transport chemistry both in solution and at DNA-modified surfaces. This chemistry is exquisitely sensitive to structural perturbations in the base pair stack as occur with lesions, single base mismatches, and protein binding. We have exploited this sensitivity for the development of reliable electrochemical assays based on DNA charge transport at self-assembled DNA monolayers. Here, we discuss the characteristic features, applications, and advantages of DNA-mediated electrochemistry.

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C 4 mim][NTf 2 ]), and the conventional aprotic solvent, acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim ® was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2 Br - → Br 2 + 2 e - Br 2 + Br - ⇌ k b k f Br 3 - K eq Athigher potentials, the tribromide anion dissociates to bromine, Br 2 , and bromide, Br − , which is immediately oxidised, leading to the emergence of a second anodic wave. Irreversible electrode kinetics were inferred from Tafel analysis and removal of the first electron is believed to be the rate-determining step in the formation of bromine. The equilibrium constant, K eq , where K eq = [ Br 3 - ] [ Br 2 ] [ Br - ] was found to be 3 × 10 3 and 9 × 10 6 M −1 in the ionic liquid and in acetonitrile, respectively. An equilibrium constant, K complex for the complexation of 1-ethyl-3-methylimidazolium bromide in acetonitrile was found to be 35.1 M −1 .

The reactivity of electrogenerated bromine with cyclohexene has been studied on a platinum microelectrode by linear sweep and cyclic voltammetry in both the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, and the conventional aprotic solvent, acetonitrile. Variation in the voltammetric response was observed in the two solvents, indicating that the bromination reaction proceeded via separate mechanisms. To identify the different products, electrolysis was conducted on the preparative scale and NMR spectroscopy confirmed that while bromination of the organic substrate in the ionic liquid yields trans-1,2-dibromocyclohexane, in acetonitrile, trans-1-(N-acetylamino)-2-bromocyclohexane is instead obtained as the major product. The reaction mechanism for bromination in acetonitrile has been modeled using digital simulation.

The reduction of oxygen in the presence of carbon dioxide has been investigated by cyclic voltammetry at a gold microdisk electrode in the two room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222] [N(Tf)2]). With increasing levels of CO2, cyclic voltammetry shows an increase in the reductive wave and diminishing of the oxidative wave, indicating that the generated superoxide readily reacts with carbon dioxide, The kinetics of this reaction are investigated in both ionic liquids. The reaction was found to proceed via a DISP1 type mechanism in [EMIM][N(Tf)2], with an overall second-order rate constant of 1.4 ± 0.4 × 103 M-1 s-1. An ECE or DISP1 mechanism was determined to be the most likely pathway for the reaction in [N6222][N(Tf)2], with an overall second-order rate constant of 1.72 ± 0.45 × 103 M-1 s -1.

The electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis-((trifluoromethyl)sulfonyl)imide ([N6222][N(Tf)2]) was investigated by cyclic voltammetry at a gold microdisk electrode. Chronoamperometric measurements were made to determine the diffusion coefficient, D, and concentration, c, of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. [Aoki, K.; et al. J. Electroanal. Chem. 1981, 122, 19]. A theory and simulation designed for cyclic voltammetry at microdisk electrodes was then employed to determine the diffusion coefficient of the electrogenerated superoxide species, O2·-, as well as compute theoretical voltammograms to confirm the values of D and c for neutral oxygen obtained from the transients. As expected, the diffusion coefficient of the superoxide species was found to be smaller than that of the oxygen in both ionic liquids. The diffusion coefficients of O2 and O2·- in [N6222][N(Tf)2], however, differ by more than a factor of 30 (DO2 = 1.48 × 10-10 m2 s-1, DO2·- = 4.66 × 10-12 m2 s-1), whereas they fall within the same order of magnitude in [EMIM]-[N(Tf)2] (DO2 = 7.3 × 10-10 m2 s-1, DO2·- = 2.7 × 10-10 m2 s-1). This difference in [N6222][N(Tf)2] causes pronounced asymmetry in the concentration distributions of oxygen and superoxide, resulting in significant differences in the heights of the forward and back peaks in the cyclic voltammograms for the reduction of oxygen. This observation is most likely a result of the higher viscosity of [N6222][N(Tf)2] in comparison to [EMIM][N(Tf)2], due to the structural differences in cationic component.

Three examples of unsupported short, strong hydrogen bonds (SSHBs) between phenolic moieties in aprotic media are reported in three salts of the biphenoxide [{3,5-OC 6 (CF 3 ) 2 H 3 } 2 H] − anion, analogous to bifluoride, [FHF] − . The compounds were synthesized with three different cobalticinium cations and characterized via solution NMR and UV–Vis spectroscopies, elemental analyses, and single-crystal X-ray diffraction. These SSHB protons do not exchange with the aprotic solvent in solution or in the solid state and are important contributors to the study of SSHBs in enzyme active sites that often exclude water.

We modulate the conductance of electrochemically inactive molecules in single-molecule junctions using an electrolytic gate to controllably tune the energy level alignment of the system. Molecular junctions that conduct through their highest occupied molecular orbital show a decrease in conductance when applying a positive electrochemical potential, and those that conduct though their lowest unoccupied molecular orbital show the opposite trend. We fit the experimentally measured conductance data as a function of gate voltage with a Lorentzian function and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations of transmission probability across single-molecule junctions. This work shows that electrochemical gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transport properties.

Selenium is an essential trace element for humans, found primarily as the amino acid selenocysteine. Proteins containing selenocysteine in their active site, such as thioredoxin reductases, play an integral role in physiological processes. Mutation studies in which selenocysteine is replaced by its abundant sulfur analogue cysteine demonstrate the requirement of selenium for proper functioning. Several questions exist regarding the unique chemistry achieved by selenocysteine in the cellular context. Using a combination of electrochemical and spectroscopic techniques, we have examined the selenocysteine / selenocystine redox couple under physiological conditions. Reactivity of the diselenide affords stable modification of gold substrates, which in turn allows for voltammetric study of solution-based chalcogen species that would otherwise be impeded by surface chemistry. The potential use of selenium-modified substrates in biosensing applications will be discussed.

Redmond Red, a fluoropore containing a redox-active phenoxazine core, has been explored as a new electrochemical probe for the detection of abasic sites in double-stranded DNA. The electrochemical behavior of Redmond Red-modified DNA at gold surfaces exhibits stable, quasi-reversible voltammetry with a midpoint potential centered around −50 mV versus NHE. Importantly, with Redmond Red positioned opposite an abasic site within the DNA duplex, the electrochemical response is significantly enhanced compared to Redmond Red positioned across from a base. Redmond Red, reporting only if well-stacked in the duplex, represents a sensitive probe to detect abasic sites electrochemically in a DNA-mediated reaction.

The electrochemical oxidation of cobalt(II) at gold, boron-doped diamond, basal and edge plane pyrolytic graphite, and highly oriented pyrolytic graphite electrodes in aqueous solutions containing NH3 has been studied using cyclic voltammetry, with subsequent chemical and electrochemical processes explained in detail. Furthermore, the electroreduction of [Co(NH 3)6]3+ in the presence of different electrolytes has also been studied to obtain a better understanding of the oxidation pathway of the Co(II)-ammine complexes. In aqueous solution the mechanism can be described by the following scheme: (Chemical presented) © 2006 Wiley-VCH Verlag GmbH and Co. KGaA.

The electrochemical windows of acetonitrile solutions doped with 0.1 M concentrations of several ionic liquids were examined by cyclic voltammetry at gold and platinum microelectrodes. These results were compared with those observed in the commonly used 0.1 M tetrabutylammonium perchlorate/acetonitrile system as well as with neat ionic liquids. The use of a trifluorotris(pentafluoroethyl)phosphate-based ionic liquid, specifically, as supporting electrolyte in acetonitrile solutions affords a wider anodic window, which is attributed to the high stability of the anionic component of these intrinsically conductive and thermally robust compounds.

The electrochemical reduction of aqueous hexamminecobalt(III) is studied using cyclic voltammetry, observing the current-voltage response using a range of scan rates covering seven orders of magnitude. At lower scan rates, it is clear that the voltammetry is diffusion-controlled, but as the scan rate is increased, the system undergoes a transition to a regime that is governed by adsorption. These experimental results are compared with a previously developed theoretical treatment of adsorption (F.G. Chevallier, O.V. Klymenko, L. Jiang, T.G. Jones, R.G. Compton, J. Electroanal. Chem. 574 (2004) 217). © 2005 Elsevier B.V. All rights reserved.

Some twenty-five years after they first came to prominence as alternative electrochemical solvents, room temperature ionic liquids (RTILs) are currently being employed across an increasingly wide range of chemical fields. This review examines the current state of ionic liquid-based electrochemistry, with particular focus on the work of the last decade. Being composed entirely of ions and possessing wide electrochemical windows (often in excess of 5 volts), it is not difficult to see why these compounds are seen by electrochemists as attractive potential solvents. Accordingly, an examination of the pertinent properties of ionic liquids is presented, followed by an assessment of their application to date across the various electrochemical disciplines, concluding with an outlook viewing current problems and directions.

The attainable steady-state limiting currents and time responses of membrane-covered and membrane-independent gas sensors incorporating different electrode and electrolyte materials have been compared. A new design comprising a membrane-free microelectrode modified with a thin layer of a room temperature ionic liquid is considered. While the use of ionic liquid as electrolyte eliminates the need for a membrane and added supporting electrolyte, the slower diffusion of analyte within the more viscous medium results in slower time responses. Such sensors do, however, have potential application in more extreme operating conditions, such as high temperature and pressure, where traditional solvents would volatise.