Cyclic voltammetric determination of free radical species from nitroimidazopyran: a new antituberculosis agent

PA-824 (2-nitro-6-(4-trifluoromethoxy-benzyloxy)-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine) is being tested as antituberculosis drug. Little is known on the action mechanism of PA-824; however the reduction of the nitro group seems to be a key step in the metabolic activation, as is observed for the well-known bactericidal metronidazole. Consequently, this paper is focused on the cyclic voltammetric behavior of PA-824 with the aim of revealing the formation and stability of the corresponding nitro radical anion and its comparison with the metronidazole behavior.Both compounds PA-824 and metronidazole reveal, in aprotic medium (DMSO + 0.1 tetrabutylammonium hexafluorophosphate), a similar reduction pattern showing a well-resolved couple due to nitro reduction to form the corresponding nitro radical anion. The electrode reaction obeys an EC₂ mechanism with a dimerization reaction as the chemical step in aprotic medium. Using cyclic voltammetry theory for a dimerization reaction we have calculated the second-order decay constants, k_2,dim, and the half-life time, t_1/2, for the nitro radical anions formed from PA-824 and metronidazole. We have obtained k_2,dim values of 2.22 × 10² and 2.58 × 10⁴ M^?1s^?1 for metronidazole and PA-824, respectively. Our voltammetric results show that the PA-824 nitro radical anion requires more energy for formation (about 200 mV) and it is approximately 100 times less stable than the metronidazole radical anion.     

The electrochemistry and thin-layer luminescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode

We report on the first direct electrochemistry and fluorescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode. The value of n determined in spectropotentiostatic experiments at 1.87×10^?6 mol l^?1 of rhodamine 6G in 0.20 mol l^?1 KCl solution is 1.15, and the experimental value obtained for E⁰′ is ?0.787 V versus Ag ∣ AgCl ∣ KCl_sat, which agrees very well with the value (E⁰′=?0.791 V) obtained using cyclic voltammetry at a modified gold electrode. The values of the diffusion coefficients D_O and D_R for the oxidized and reduced forms of rhodamine 6G calculated from results of potential step and in situ fluorescence measurement experiments are 4.0×10^?6 cm² s^?1 and 4.2×10^?6 cm² s^?1, respectively. Cyclic voltammograms of rhodamine 6G show that the peak current I_p is proportional to the square root of the potential scan rate v^1/2, the ratio of the reduction to the oxidation peak height is about unity, and the separation of both reduction and reoxidation peak potentials ΔE_P is essentially constant at 135 mV at low scan rates. These results indicate that electrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode is a quasi-reversible one-electron electrode process.     

Note on the oxidation of methanol at Pt(1 1 1) and Pt(3 3 2) electrodes in alkaline solutions

The cyclic voltammetric behavior of different concentrations of CH₃OH has been studied in alkaline solutions on Pt(1 1 1) and Pt(3 3 2). The oxidation of CH₃OH gives well-defined current density–potential curves at around the potential where the adsorption of OH occurs on Pt.The current–potential behavior of the hydrogen adsorption–desorption shows that CH₃OH or related species adsorbs on the step sites of (3 3 2) surface, but less on the (1 1 1) surface.The analysis of the voltammograms with less concentrated methanol solutions is suggested in order to understand the mechanism of CH₃OH oxidation reaction in alkaline media.     

Influence of the leaving group and of the annelation in the electroreduction of 2-methyl-substituted quinones

In the present work, several 2-methylquinones, of the series benzo- (BQ), naphtho- (NQ) and anthraquinone (AQ), substituted at the C-2 side chain with groups of different nucleofugacity were prepared and submitted to electrochemical studies (cyclic voltammetry and electrolysis), in aprotic medium (DMF + 0.1 mol l^?1 TBAP), using mercury electrodes, to observe the role of the 2-methyl substituents and of the annelation in their electroreductions. The analysis of the voltammograms showed that, in each series, mainly in the naphthoquinone case, which was more extensively investigated, the presence and the nature of the leaving group influence the cathodic reduction greatly. The comparison of the electrochemical behaviour allows the classification of the quinones in three main groups. The simplest one approaches the electrochemical behaviour of the unsubstituted quinone and in the other two, the substituents work as leaving groups (LG) and are lost, after uptake of one or two electrons, generating reactive intermediates, which form electroreductive species, giving rise, then, to complex voltammograms. More specifically, the reduction mechanism involves a one electron transfer to form a radical anion, which fragments, expelling a good leaving group such as the halide (-I, -Br, -Cl), -ONO₂ and -OTs, generating radicals that can follow different and competitive pathways, including generation of quinomethides and their addition products. For leaving groups of weak nucleofugacity, like –OAc or –OCHO, the cleavage occurs only after two-electron uptake. The influence of the substituents on the first wave redox potential (E_pIc), represented by positive shifts is in the order Br > OTs ～ I > Cl > ONO₂ > OCHO ～ OAc > OH > H and is related to the nucleofugal ability or the fragility of the C-LG. The extent of annelation of the quinone moiety also influences the feature of the cyclic voltammogram, through stabilization of the electrogenerated species. The feasibility of reduction, comparing the same substituents, is BQ ～ NQ > AQ, and, the order of stability of the electrogenerated species is opposite: AQ > NQ > BQ.     

Spectral and electrochemical studies of axial ligand binding reactions of carbonylruthenium(II) meso-tetramesitylporphyrin

The spectral and electrochemical properties of carbonylruthenium(II) meso-tetramesitylporphyrin, Ru(CO)(TMP) in the presence of imidazole, N-methylimidazole and hydroxide anion as axial ligands were investigated. Spectrophotometric titration of Ru(CO)(TMP) of the nitrogeneous bases caused the absorption spectrum to shift to longer wavelengths. Larger shifts in wavelength were observed in the titration using tetrabutylammonium hydroxide. The formation constants for these ligands coordinated to the ruthenium center were calculated. The effect of axial ligand ligation caused the decrease of vibrational frequency of CO as detected from FT-IR spectroscopy. The CO stretching frequency (ν_CO) of Ru(CO)TMP in CH₂Cl₂ is 1940 cm^?1, which is lowered to 1936 and 1913 cm^?1 upon coordination of nitrogenous bases and hydroxide anion, respectively. Cyclic voltammetry of Ru(CO)(TMP) in CH₂Cl₂ showed an irreversible reduction wave at ?1.63 V and two reversible oxidations at E_1/2 = 0.78 and 1.27 V vs. Ag|AgCl, respectively. Addition of imidazole and hydroxide into Ru(CO)(TMP) solution caused shifts in the redox potential and accordingly, the binding constants of the ligands to the one- and two-electron oxidized ruthenium porphyrins were estimated and compared. Spectroelectrochemical methods were used to characterize the above electron-transfer reactions.     

Investigation of the electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN)

The electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN) was investigated by using various electrochemical techniques in 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) in acetonitrile at glassy carbon (GC) electrode. The number of electrons transferred and diffusion coefficient of the compound were estimated by using an ultramicroelectrode (UME). PN has two reduction peaks in a cyclic voltammogram. Each of them corresponds to a one-electron transfer. In this medium and at the GC electrode surface, the first peak was observed at about ?1.8 V (vs. Ag/Ag⁺) which is more stable and well defined compared to the second peak. It was determined by using cyclic voltammetry that PN is electroreduced by an EC mechanism. It was also explained by multicycle CV experiments and a dopamine test as well as by Raman spectroscopy that the homogenous chemical reaction is dimerization. The EC mechanism was also examined and the kinetics of this process were calculated by digital simulation.     

Catechin antioxidant action at various pH studied by cyclic voltammetry and PM3 semi-empirical calculations

The mechanism of catechin electro-oxidation at various pH was studied using cyclic voltammetry (CV) on the glassy carbon (GC) electrode and PM3 semi-empirical calculations. The influence of activation of the surface of the GC electrode on CV results has been discussed. Mixed adsorption–diffusion control has been observed by applying mechanistic criteria of CV to the results obtained at the activated electrode. The calculated catechin diffusion coefficient D = 2.78 × 10^?6 cm² s^?1. A linear increase of the current peak has been observed with the increase of substrate concentration up to 40 μmol dm^?3 (surface coverage Γ ～ 10^?11 mol cm^?2). In the whole investigated pH range, the dE/dpH value is very close to the anticipated Nernstian dependence of ?59 mV/pH indicating that the slope is not affected by the different sequences of e^? and H⁺ transfer. Molecular modeling results show a decrease of ≈5 kcal mol^?1 in ΔHoF (between radical and parent molecule) and a decrease of ≈6 eV in IP (of the parent molecule) when the parent molecule is changed from neutral to monoanionic form of catechin showing that both processes – hydrogen and electron abstraction are facilitated by deprotonation. Electrochemical oxidation of catechin is known to proceed as a two step one-electron oxidation of the B-ring of o-phenolic groups. Upon an increase in the pH, the mechanistic pathway of catechin electro-oxidation in both oxidation steps changes from an eH to the He process. In the reaction with a free radical, this may induce the change from hydrogen to electron donation.     

Electrocatalytic oxidation of nitrite on a carbon paste electrode modified with Co(II) porphyrin adsorbed on SiO2/SnO2/Phosphate prepared by the sol–gel method

This paper describes the immobilization of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphyrin ion on SiO₂/SnO₂/Phosphate obtained by the sol–gel processing method. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co(II) ion. The porphyrin metallation process was followed using UV–Vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. A carbon paste electrode modified with material containing metallated porphyrin was used to study the electro-catalytic oxidation of nitrite ions by means of cyclic voltammetry, chronoamperometry and RDE voltammetry. The modified electrode was very stable and exhibited the electro-catalytic oxidation of nitrite ions at 0.72 V vs. SCE by a two electron mechanism producing nitrate ions at pH 5.4. The kinetic parameters of the electrode reaction process were calculated; (1 ? α)n_a was 0.479, D was (5.3 ± 0.11) × 10^?5 cm s^?1, and k⁰ could be determined as (5.4 ± 0.14) × 10^?3 cm s^?1.     

Reactions of the nitro radical anion of metronidazole in aqueous and mixed solvent: a cyclic voltammetric study

Cyclic voltammetry was used to investigate the electrochemical reduction of metronidazole (2-methyl-5-nitro-1H-imidazole-1-ethanol) at glassy carbon and gold electrodes at different pHs in aqueous solution as well as in mixed solvent viz., aqueous dimethyl formamide. The electrogenerated nitro radical anion undergoes a disproportionation reaction, the rate constant of which is dependent on the pH, solvent composition and electrode material. The interactions of the nitro radical anion with thymine and cytosine were also investigated using a cyclic voltammetric technique. Both the bases were found to react with metronidazole nitro radical anion. The rate constants for such reactions in aqueous solutions were 3.5 × 10³ and 3.0 × 10³ dm³ mol^?1 s^?1 for thymine and cytosine, respectively.     

Electrochemical properties of [MnIII(terpy)(N3)3] (terpy=2,2′:6′,2″-terpyridine) in CH3CN: Electrogeneration of dimanganese(II) di-μ-azido and dimanganese(IV) di-μ-oxo complexes

The electrochemical behavior of [Mn^III(terpy)(N₃)₃] (1) (terpy=2,2′:6′,2″-terpyridine), a structural model of the azide complex of the manganese superoxide dismutase (MnSOD), has been investigated in acetonitrile (CH₃CN) solution. In CH₃CN containing either 0.1 M tetra-n-butylammonium perchlorate (Bu₄NClO₄) or tetraethylammonium trifluoroacetate (Et₄NCF₃CO₂) as supporting electrolytes, the cyclic voltammogram of 1 exhibits one quasi-reversible reduction wave at E_1/2=?0.170 V versus Ag ∣ 10 mM Ag⁺ and one quasi-reversible oxidation wave at E_1/2=+0.675 V. These are both one-electron waves, corresponding to the Mn(III)/Mn(II) and Mn(III)/Mn(IV) redox couples respectively. To evaluate the stability of the oxidized and reduced species of 1, exhaustive electrolyses have been carried out. Controlled-potential reductions at ?0.35 V of solutions of 1 in CH₃CN containing 0.1 M Bu₄NClO₄ or 0.1 M Et₄NCF₃CO₂ lead to the quantitative conversion of 1 into the bridging N₃^? dimanganese(II) complex, [(N₃)(terpy)Mn^II(μ-N₃)₂Mn^II(terpy)(N₃)] (2). This transformation is chemically reversible by an oxidation process. Controlled-potential oxidation at 0.8 V of a solution of 1 in CH₃CN+0.1 M Bu₄NClO₄ produces a new mononuclear Mn(IV) complex characterized by electron paramagnetic resonance spectroscopy, which is stable only at or below ?15°C. If this oxidation is conducted in CH₃CN+0.1 M Et₄NCF₃CO₂, the stable dimanganese(IV) di-μ-oxo complex [(CF₃CO₂)(terpy)Mn^IV(μ-O)₂Mn^IV(terpy)(CF₃CO₂)]²⁺ (3) is formed quantitatively owing to the presence of an excess of the coordinating CF₃CO₂^? anions and residual H₂O in the CH₃CN solution.     

Molybdenocene–DNA interaction studies using electrochemical analysis

The binding interactions of molybdenocene dichloride (Cp₂MoCl₂) and [Cp₂Mo(L)_n]Cl₂ (n = 1, L = 6-mercaptopurine, 6-mercaptopurineribose, 2-amine-6-mercaptopurine and 2-amine-6-mercaptopurineribose and n = 2, L = d-penicillamine) complexes with calf-thymus DNA have been investigated by cyclic voltammetry. Cp₂MoCl₂ belongs to a group of metallocene antitumor agents and [Cp₂Mo(L)_n]Cl₂ complexes are structural modifications of molybdenocene dichloride that have also been shown to possess antitumor properties. From the mechanistic point, there is interest in discovering whether molybdenocene dichloride binds DNA or not. To investigate this issue in more detail, we carried out molybdenocene–DNA titrations monitored by cyclic voltammetry. The changes in oxidation potentials (E_pa) allowed us to determine the degree of Mo–DNA interaction. (Cp₂MoCl₂) and [Cp₂Mo(L)_n]Cl₂ (n = 1, L = 2-amine-6-mercaptopurine and 2-amine-6-mercaptopurineribose and n = 2, L = d-penicillamine) complexes showed weak DNA bindings (3.2–10.1%) while the complexes containing the ligands 6-mercaptopurine and 6-mercaptopurineribose showed negligible interactions. ICP-AES was used to corroborate the CV results.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.     

Pseudopolarography of trace metals. Part II. The comparison of the reversible,quasireversible and irreversible electrode reactions

The theoretical and experimental pseudopolarographic curves of reversible, quasireversible and irreversible electrochemical reactions were compared and evaluated. The measurements were performed on a stationary mercury drop electrode (SMDE, PAR 303A), using differential pulse anodic stripping voltammetry (DPASV). A good agreement between the theoretical and the experimental shift of the half-wave potential with an increasing accumulation time was obtained for the reversible pseudopolarograms of 10^?7 mol dm^?3 Cd(II) (in 0.1 mol dm^?3 NaClO₄, pH ～2). As compared with the curve of the logarithmic analysis of the polarogram, the corresponding curve of the pseudopolarogram is steeper in the region of the half-wave potential. It has been shown that even though the pseudopolarograms are quasireversible or irreversible, there is a range at the foot of the curves with a reversible slope (usually below 10% of the total/limiting current). It has been verified that the range of this reversible slope can be extended by increasing the accumulation time, lowering the mercury drop size and diminishing the thickness of the diffusion layer. The estimated value for its approximative evaluation is about 1% of the total/limiting current. This is essential for the determination of the corresponding electrochemical parameters, such as: the formal potential (E°^′), transfer coefficient (α) and rate constant (k_s). From the experimentally obtained reversible slope of the (pseudo)polarographic curves of Zn(II) (in 1 mol dm^?3 NaClO₄, pH 4.7 ± 0.1), the parameters for the quasireversible electrochemical reactions were estimated as follows: E°^′=?0.964 ± 0.002 V, α=0.24 ± 0.02 and k_s～2–3×10^?3 cm s^?1. It is shown that an accurate transfer coefficient can be calculated from the curves of the logarithmic analysis of the quasireversible pseudopolarograms, which is not the case for the polarographic curves. The irreversible system, tested on the electrochemical reaction of the CdNTA complex (in 0.1 mol dm^?3 NaClO₄, pH 7.9 ± 0.1), shows relatively good agreement between the experimental and the theoretical dependences. The (pseudo)polarographic measurements enabled approximate estimation of the electrochemical parameters (E°^′=?0.835 ± 0.010 V, α=0.55 ± 0.02 and k_s=1.0 ± 0.4 × 10^?4 cm s^?1) which are in fairly good agreement with the literature data.     

Electroreduction of halogenated fumaric esters: a combined electrochemical and computational investigation

Diethyl fumarate and related halogenated derivatives have been studied by cyclic voltammetry and electrolysis, on mercury and glassy carbon electrodes, in DMF + 0.1 mol L^?1 TBAP and in acetonitrile + water (3:4) with 0.1 mol L^?1 NaCl or 0.1 mol L^?1 TEAP. For compounds with two reducible functionalities, the electron-deficient olefin and the C–X group, the grade of substitution on the halogenated carbon causes differences on the site involved in the first electron transfer thus determining the chemical reactivity. Enediester compounds with –CH₂– groups insulating the gem-trihalide groups (–CX₃, X=Br and Cl) suffer reduction, and the C–X cleavage is the reaction of choice. The reaction pathway also depends on the nature of the electrode and significant positive potentials shifts for compounds with a gem-tribromide group are evident in the cyclic voltammogram on a mercury electrode, in relation to results on a glassy carbon electrode. The formation of easily reduced organomercurials is the main reason for this intense positive shift in the first electron transfer. On a vitreous carbon electrode, factors directly related to the strength of the C–X bond play a determinant role. Concerning electrolysis, the gem-tribromo derivative furnishes H₂CCBr₂ and ethyl hydrogenfumarate, through hydrolysis produced by initial cleavage of the C–Br. For the other polyfunctional compounds (–CHX₂, –CH₂X), the olefin is reduced first to yield dimers and/or hydrogenated products depending upon the reaction conditions. Quantum chemical calculations performed on the anion radical of these compounds yield spin densities located on the –CBr₃/–CCl₃ group and on the enediester group for the –CCl₂/–CBr₂, –CH₂Br/–CH₂Cl derived compounds. Upon geometry optimisation, the C–Br bond in compounds containing –CBr₃ groups are longer, suggesting that this bond can easily be cleaved, yielding organomercurial compounds, in the case of the use of a mercury electrode or leading to bond cleavage, for inert electrodes. For the other substituents, the optimisation basically does not affect the geometry of the radical anion compared to the neutral substrate. These results have been confirmed by the calculated dissociation energy of the C–Br bond, which for the –CBr₃ group is approximately 100 kJ mol^?1 as against 240 kJ mol^?1 for the same bond in the –CH₂Br group. The coherence between the experimental and calculated data reinforces the usefulness of these computational tools in rationalising the reactivity of electrogenerated species as well as in predicting the electrochemical reaction outcome.     

Voltammetric determination of stability constants of iron(III)–glycine complexes in water solution

Determination of the stability constants of dissolved iron(III)–glycine system in water solution (I = 0.6 mol L^?1 in NaClO₄ at 25 ± 1 °C) using differential pulse cathodic voltammetry (DPCV) was performed on a static mercury drop electrode (SMDE). Iron(III) concentration of 2.5 × 10^?5 mol L^?1 and the pH range from 9.05 to 6.36 ensured the formation of enough concentration of iron(III)–glycine higher coordination complexes (1:2 and 1:3) to be measured by the applied method. The concentrations of total glycine varied from 0.1 to 0.5 mol L^?1. Cyclic voltammetry (CV) measurements were used to investigate reversibility of the iron(III)–glycine complexes which showed one-electron reversible character. The stability constants of iron(III) [Fe(Gly)₂]⁺ and Fe(Gly)₃ complexes, which had not been reported in the literature so far, were found to be log β₂ = 16.83 ± 0.47 and log β₃ = 18.64 ± 0.70, respectively. The model that best fitted the data gave two iron(II)–glycine stability constants for [FeGly]⁺ log K₁ = 3.69 ± 0.19 and for Fe(Gly)₂ log β₂ = 5.08 ± 0.60. According to the constants found, chemical distribution of iron(III) in glycine water solution, as a function of pH, was calculated and proposed.     

Measurement of Donnan potentials in gels by in situ microelectrode voltammetry

This work describes the electrochemical methodology for the determination of the Donnan potential from diffusion-limited steady-state voltammograms of acrylamide gels. The technique is based upon the measurement of gel–sol systems that have reached Donnan equilibrium and contain Cd²⁺ as a probe ion. Au-amalgam microelectrodes are used to measure the Cd concentration in the gel phase relative to the solution phase, thus permitting comparison of the Cd voltammograms obtained in both phases. This approach yields two independent measures of the Donnan potential resulting from (i) the potential shift relative to the reference electrode, and (ii) the enhancement of the Cd²⁺ wave. Two suites of acrylamide gels containing 0.2% and 0.5% Na-acrylate were studied as a function of ionic strength by varying [NaNO₃] and maintaining a constant concentration of the electroactive probe ion, [Cd²⁺] = 1 × 10^?5 mol/L in the equilibrating solutions. Independent model predictions of the Donnan potential as a function of ionic strength that consider the effects of differential swelling on the charge density, the influence of a mixed electrolyte on the potential developed in the gel at the limit of low ionic strength and the effects of incomplete dissociation of the carboxylic functional groups were in agreement with the Donnan potentials independently measured by the twofold steady-state voltammetric approach.     

Evidence of self-protonation on the electrodic reduction mechanism of an anti-Helicobacter pylori metronidazole isotere

The electrochemical reduction mechanism of 1-(2-ammoniumethyl)-2-methyl-5-nitroimidazole bromide (2) in DMSO + 0.1 mol l^?1 TBAP has been investigated by cyclic voltammetry and macroscale electrolysis, on a glassy carbon electrode, in comparison with metronidazole (1). The cyclic voltammogram of 2 is represented by three reduction waves, one of them at less negative potential, when compared to the first wave of metronidazole which indicates that it undergoes easier reduction. There is evidence for a self-protonation mechanism in the electroreduction of 2, represented by the absence of the first wave in the successive cyclic voltammogram, by the disappearance of the first reduction wave upon addition of base and increase of the same wave in the presence of exogenous proton donors. The stoichiometry of the reaction, at the first reduction wave, involves 0.8 mol electron mol^?1 and yields 0.2 mol of 4e^?/4H⁺ reduced derivative (probably an unstable hydroxylamine) and 0.8 mol of the amine derivative, the conjugated base of 2. The second and third waves are typical for nitroaromatic reduction and are related to the reduction of the nitro group in this aminoderivative.     

Effect of phase transition on the electrochemical behavior of ferredoxin embedded in an artificial lipid membrane film

The effect of the phase transition of a bilayer membrane on the electrochemical behavior of ferredoxin embedded in a cast film of artificial lipid, 2C₁₈N⁺Br⁻, was investigated. The redox potential of ferredoxin embedded in the lipid film showed a linear, positive shift from −510 (± 2) to −466 (± 3) mV (vs. Ag|AgCl|saturated KCl) with increasing temperature from 25 to 53 °C. The diffusion coefficient (D) and heterogeneous electron transfer rate constant (k°^′) were evaluated at various temperatures by means of analyzing cyclic voltammograms. The overall shape of the simulated voltammograms fitted well with the experimentally observed voltammograms at various potential sweep rates, when the estimated D and k°^′ values were used for the simulation. The results of the temperature dependence of the estimated D and k°^′ values indicated that D and k°^′ were enhanced near the phase transition temperature (T_c), 46 °C, of the lipid film. The D and k°^′ values at temperatures above the T_c were approximately two-orders and one-order of magnitude larger than those estimated at temperatures under the T_c, respectively. The estimated D and k°^′ values were 2.1–5.8 × 10⁻¹⁰ cm² s⁻¹ and 1.2–1.6 × 10⁻⁴ cm s⁻¹ at 25 °C, and 3.5–3.7 × 10⁻⁸ cm² s⁻¹ and 1.1–1.2 × 10⁻³ cm s⁻¹ at 50 °C, respectively. The electrochemical behavior of ferredoxin in the lipid film changed drastically near the T_c.     

Remarkably high activity of electrodeposited IrO2 film for electrocatalytic water oxidation

A homogenous and transparent IrO₂ film was prepared on an ITO electrode by anodic electrodeposition under galvanostatic conditions from an aqueous solution containing 2 mM K₂IrCl₆ and 40 mM oxalic acid that is aged at 37 °C and pH 10 for ca. 10 days. The absorption spectral change of the solution suggested that an IrO₂ colloid is formed in the solution during ca. 10 day-aging. The scanning electron microscopic (SEM) measurement displayed homogeneous deposition of IrO₂ particles with 100–250 nm of a diameter on the surface of the film. The X-ray diffraction (XRD) measurement indicated that IrO₂ in the film is amorphous. The cyclic voltammogram (CV) of the IrO₂-coated ITO electrode dipped in a 0.1 M KNO₃ aqueous solution exhibited a steep rise of an anodic current at 1.0 V vs SCE for catalytic water oxidation, as well as an anodic wave at 0.3 V and a corresponding cathodic wave at ?0.1 V that are assigned as an Ir^IV/Ir^V redox. The anodic current at 1.3 V on the CV was 660 times higher than that for a blank bare ITO electrode. Ir electrodeposited on the ITO electrode was also shown to be electrocatalytically active for water oxidation. However, the anodic current at 1.3 V on the CV for the Ir-coated ITO electrode was 14 times lower than that for an IrO₂-coated electrode in spite of the 34 times higher coverage of Ir. The potential static electrochemical water oxidation using the IrO₂-coated ITO electrode produced a significant amount of O₂ above 1.1 V vs Ag/AgCl, in contrast to no O₂ detected even at 1.3 V using a bare ITO electrode. The maximum turnover frequency (TOF) of the IrO₂ catalyst was provided as 16,400 ± 450 h^?1 at 1.3 V vs Ag/AgCl from the slope of the linear plots of the amount of O₂ vs coverage of IrO₂ in the range of ～1.5 × 10^?9 mol. The TOF was 450 times higher than that (36.4 ± 1.4 h^?1 at 1.3 V) for electrodeposited Ir showing the very high catalytic activity of the IrO₂ film.     

Voltammetric analysis of lipophilicity of benzodiazepine derivatives at the water ∣ 1,2-dichloroethane interface

The transfer of benzodiazepine derivatives (diazepam, bromazepam, alprazolam, oxazepam, nitrazepam, clonazepam, chlordiazepoxide, flunitrazepam, midazolam and lorazepam) across the water ∣ 1,2-dichloroethane interface was studied using cyclic voltammetry. The partition coefficients of ionic species of benzodiazepines, log P_XH⁺, were calculated from the transfer potentials measured at pH<pK_a. These values were compared with the partition coefficient of neutral species, log P_X. The difference between log P_XH⁺ and log P_X was related to the degree of charge delocalization, which depends markedly on the presence of electron acceptor substituents in the molecule. These electron acceptor groups, in turn, affect the biological activity of these drugs. The results indicated that the difference Δlog P_XH⁺=log P_XH⁺?log P_X can be used in structure–activity correlations as it takes into account the effects of substituents on the main positions within the molecule.