Characterization of the reduction steps of Fe(III) porphyrins

Polarographic studies have shown that Fe(III) porphyrins undergo successively three one-electron reduction steps in dimethylformamide. The first involves the Fe(III)/Fe(II) redox couple. The second step proceeds to a second reduction of the metal ion and is attributed to the Fe(II)/Fe(I)_couple. This new reduction state of iron porphyrins has been characterized by ESR spectra and by absorption spectra in various solvents. This compound is not axially liganded by strong nucleophilic bases but is sensitive to solvation, the lone electron being localised in the d_z2 orbital. The third reduction step is assumed to involve a reduction of the porphyrin π-electron system.All these results have been confirmed by chemical reductions in tetrahydrofuran.     

An electrochemical study on the oxidation of tris(N,N-dimethyldithiocarbamato)ruthenium(III)

The electrochemical oxidation of Ru(Me₂dtc)₃ (where Me₂dtc=N,N-dimethyldithiocarbamate) in acetone or methylene chloride solution is a one- electron process followed by a dimerisation. This is concluded from cyclic voltammetric experiments at different scan rates, concentrations and temperatures with the use of Savéant's diagnostic criteria. NMR spectra taken at different temperatures confirm this conclusion. This behaviour is in contrast with that in coordinating solvents like acetonitrile where the one-electron oxidation is followed by addition of a solvent molecule to the coordination sphere, yielding Ru(Me₂dtc)₃(CH₃CN).     

Dehydrogenase and electrochemical activity of <Emphasis Type="Italic">Escherichia coli</Emphasis> extracts

The dehydrogenase activity of Escherichia coli BB cell extracts was studied at different growth stages in the presence of different substrates and triphenyl tetrazolium chloride as an electron acceptor. It was shown that the highest degree of reduction of triphenyl tetrazolium chloride was observed during exponential growth of the bacteria when potassium isocitrate was used as a substrate. It was found that extracts of the bacteria during the exponential phase of growth on an inert glassy carbon electrode in a three-electrode liquid electrochemical cell manifested electrochemical activity in the presence of potassium citrate and methylene blue or potassium hexacyanoferrate(III) as redox mediators.     

Iron corrolates: unambiguous chloroiron(III) (corrolate)(2-.) pi-cation radicals

The structures, electron configurations, magnetic susceptibilities, spectroscopic properties, molecular orbital energies and spin density distributions, redox properties and reactivities of iron corrolates having chloride, phenyl, pyridine, NO and other ligands are reviewed. It is shown that with one very strong donor ligand such as phenyl anion the electron configuration of the metal is d(4)S=1 Fe(IV) coordinated to a (corrolate)(3-) anion, while with one weaker donor ligand such as chloride or other halide, the electron configuration is d(5)S=3/2 Fe(III) coordinated to a (corrolate)(2-.) pi-cation radical, with antiferromagnetic coupling between the metal and corrolate radical electron. Many of these complexes have been studied by electrochemical techniques and have rich redox reactivity, in most cases involving two 1-electron oxidations and two 1-electron reductions, and it is not possible to tell, from the shapes of cyclic voltammetric waves, whether the electron is added or removed from the metal or the macrocycle; often infrared, UV-Vis, or EPR spectroscopy can provide this information. (1)H and (13)C NMR spectroscopic methods are most useful in delineating the spin state and pattern of spin density distribution of the complexes listed above, as would also be expected to be the case for the recently-reported formal Fe(V)O corrolate, if this complex were stable enough for characterization by NMR spectroscopy. Iron, manganese and chromium corrolates can be oxidized by iodosylbenzene and other common oxidants used previously with metalloporphyrinates to effect efficient oxidation of substrates. Whether the "resting state" form of these complexes, most generally in the case of iron [FeCl(Corr)], actually has the electron configuration Fe(IV)(Corr)(3-) or Fe(III)(Corr)(2-.) is not relevant to the high-valent reactivity of the complex.     

Synthesis, characterization and kinetic studies of the cis-[RuCl2(cyclen)] complex

We report here the synthesis, characterization and kinetic studies of cis-[RuCl₂(cyclen)]⁺ in aqueous solution, where cyclen is the macrocyclic ligand 1,4,7,10-tetraazacyclododecane. The complex releases one Cl⁻ producing cis-[RuCl(OH)(cyclen)]⁺ in aqueous solution at pH 4.60. The product of this reaction was characterized by Ultraviolet-Visible (UV-Vis) spectrum in comparison to the synthesized cis-[RuCl(OH)(cyclen)](BF₄)·2H₂O. The electrochemical data showed that E_pc of the Ru(III/II) peak increases as the macrocycle ring size decreases and also when the trans conformation is changed to cis. The chloride affinity of Ru(III) depends on the macrocycle ring size since cis-[RuCl₂(cyclam)]⁺ (cyclam=1,4,8,11-tetraazacyclotetradecane) does not release chloride for at least 12 h. The overall effect between cyclam and cyclen reflects the fact that the electron involved in the reduction enters a nonbonding π-d orbital and its energy is affected by the macrocyclic ligand.     

Electrochemical behaviour and reactivity of [Os(bpy)2(CO)(OTf)] in halogenated solvents

The dimethylaminopyridine (DMAP) promoted reaction between [Os(bpy)₂(CO)(OTf)]OTf (where ) and methylene chloride is reported. C-Cl bond breaking of a solvent molecule leads to the formation of the [Os(bpy)₂(CO)(Cl)]OTf complex. The reactivity and redox properties of [Os(bpy)₂(CO)(OTf)]OTf were investigated by means of room- and low-temperature electrochemical experiments. In CH₂Cl₂, at low temperature, the complex undergoes two 1e electrochemical and chemical reversible reductions (E_rE_r mechanism), but at room temperature a more complex electrochemical mechanism is observed, leading to the electro-synthesis of [Os(bpy)₂(CO)(Cl)]OTf via electrochemical reversible and chemical irreversible reduction processes (E_rC_i mechanism). The DMAP nucleophilicity was used to produce the new [Os(bpy)₂(CO)(Br)]OTf and [Os(bpy)₂(CO)(I)]OTf complexes which have been fully characterized.     

Electrochemical Gating of Tricarboxylic Acid Cycle in Electricity-Producing Bacterial Cells of Shewanella

Energy-conversion systems mediated by bacterial metabolism have recently attracted much attention, and therefore, demands for tuning of bacterial metabolism are increasing. It is widely recognized that intracellular redox atmosphere which is generally tuned by dissolved oxygen concentration or by appropriate selection of an electron acceptor for respiration is one of the important factors determining the bacterial metabolism. In general, electrochemical approaches are valuable for regulation of redox-active objects. However, the intracellular redox conditions are extremely difficult to control electrochemically because of the presence of insulative phospholipid bilayer membranes. In the present work, the limitation can be overcome by use of the bacterial genus Shewanella , which consists of species that are able to respire via cytochromes abundantly expressed in their outer-membrane with solid-state electron acceptors, including anodes. The electrochemical characterization and the gene expression analysis revealed that the activity of tricarboxylic acid (TCA) cycle in Shewanella cells can be reversibly gated simply by changing the anode potential. Importantly, our present results for Shewanella cells cultured in an electrochemical system under poised potential conditions showed the opposite relationship between the current and electron acceptor energy level, and indicate that this unique behavior originates from deactivation of the TCA cycle in the (over-)oxidative region. Our result obtained in this study is the first demonstration of the electrochemical gating of TCA cycle of living cells. And we believe that our findings will contribute to a deeper understanding of redox-dependent regulation systems in living cells, in which the intracellular redox atmosphere is a critical factor determining the regulation of various metabolic and genetic processes.     

Electron transport in an in vitro-reconstituted bacterial photophosphorylating system

Photooxidation of endogenous cytochrome(s) c, photoreduction of endogenous cytochrome(s) b and photobleaching of bacteriochlorophyll have been demonstrated in an in vitro reconstituted system, previously demonstrated to support photophosphorylation. The kinetic responses of these redox reactions to substrate and antimycin A in these particles are characteristic of electron transport processes, and strongly support the contention that all, or a part of, the oxidative phosphorylation electron transport pathway can be coupled to reaction center photopigment complex in a manner which supports photophosphorylation. In addition, a succinate-supported light dependent reduction of NAD⁺ was found.     

Synthesis, characterization and photophysical properties of mixed ligand tris(polypyridyl)chromium(III) complexes, [Cr(phen)2L]

A series of new heteroleptic, tris(polypyridyl)chromium(III) complexes, [Cr(phen)₂L]³⁺ (L = substituted phenanthrolines or bipyridines), has been prepared and characterized, and their photophyical properties in a number of solvents have been investigated. X-ray crystallography measurements confirmed that the cationic (3+) units contain only one ligand L plus two phenanthroline ligands. Electrochemical and photophysical data showed that both ground state potentials and lifetime decays are sensitive to ligand structure and the nature of the solvent with the exception of compounds containing L = 5-amino-1,10-phenanthroline (aphen) and 2,2′-bipyrimidine (bpm). Addition of electron-donating groups in the ligand structure shifts redox potentials to more negative values than those observed for the parent compound, [Cr(phen)₃]³⁺. Emission decays show a complex dependence with the solvent. The longest lifetime was observed for [Cr(phen)₂(dip)]³⁺ (dip = 4,7-diphenylphenanthroline) in air-free aqueous solutions, τ = 273 μs. Solvent effects are explained in terms of the affinity of hydrophobic complexes for non-polar solvent molecules and the solvent microstructure surrounding chromium units.     

Density functional computational studies on (E)-2-[(2-Hydroxy-5-nitrophenyl)-iminiomethyl]-4-nitrophenolate

Density functional calculations of the structure, atomic charges, molecular electrostatic potential and thermodynamic functions have been performed at B3LYP/6-31G(d,p) level of theory for the title compound (E)-2-[(2-hydroxy-5-nitrophenyl)-iminiomethyl]-4-nitrophenolate. The results show that the phenolate oxygen atom and all of the nitro group oxygen atoms have bigger negative charges, and the coordination ability of these atoms differs in different solvents. The energetic behavior of the title compound in solvent media has been examined using B3LYP method with the 6-31G(d,p) basis set by applying the Onsager method and the isodensity polarized continuum model (IPCM). The results obtained with these methods reveal that the IPCM method yielded a more stable structure than Onsager’s method. In addition, natural bond orbital and frontier molecular orbital analysis of the title compound were performed using the B3LYP/6-31G(d,p) method.     

Role of Met58 in the regulation of electron/proton transfer in trihaem cytochrome PpcA from Geobacter sulfurreducens

The bacterium Gs (Geobacter sulfurreducens) is capable of oxidizing a large variety of compounds relaying electrons out of the cytoplasm and across the membranes in a process designated as extracellular electron transfer. The trihaem cytochrome PpcA is highly abundant in Gs and is most probably the reservoir of electrons destined for the outer surface. In addition to its role in electron transfer pathways, we have previously shown that this protein could perform e⁻/H⁺ energy transduction. This mechanism is achieved by selecting the specific redox states that the protein can access during the redox cycle and might be related to the formation of proton electrochemical potential gradient across the periplasmic membrane. The regulatory role of haem III in the functional mechanism of PpcA was probed by replacing Met⁵⁸, a residue that controls the solvent accessibility of haem III, with serine, aspartic acid, asparagine or lysine. The data obtained from the mutants showed that the preferred e⁻/H⁺ transfer pathway observed for PpcA is strongly dependent on the reduction potential of haem III. It is striking to note that one residue can fine tune the redox states that can be accessed by the trihaem cytochrome enough to alter the functional pathways.     

Rapid reduction of iron in horse spleen ferritin by thioglycolic acid measured by dispersive X-ray absorption spectroscopy

Summary The release of iron from ferritin is important in the formation of iron proteins and for the management of diseases in both animals and plants associated with abnormal accumulations of ferritin iron. Much more iron can be released experimentally by reduction of the ferric hydrous oxide core than by chelation of Fe³⁺ which has led to the notion that reduction is also the major aspect of iron release in vivo. Variations in the kinetics of reduction of the mineral core of ferritin have been attributed to the redox potential of the reductant, redox properties of the iron core, the structure of the protein coat, the analytical method used to detect Fe²⁺ and reactions at the surface of the mineral. Direct measurements of the oxidation state of the iron during reduction has never been used to analyze the kinetics of reduction, although Mössbauer spectroscopy has been used to confirm the extent of reduction after electrochemical reduction using dispersive X-ray absorption spectroscopy (DXAS). We show that the near edge of X-ray absorption spectra (XANES) can be used to quantify the relative amounts of Fe²⁺ and Fe³⁺ in mixtures of the hydrated ions. Since the nearest neighbors of iron in the ferritin iron core do not change during reduction, XANES can be used to monitor directly the reduction of the ferritin iron core. Previous studies of iron core reduction which measured by Fe²⁺ · bipyridyl formation, or coulometric reduction with different mediators, suggested that rates depended mainly on the redox potential of the electron donor. When DXAS was used to measure the rate of reduction directly, the initial rate was faster than previously measured. Thus, previously measured differences in reduction rates appear to be influenced by the accessibility of Fe²⁺ to the complexing reagent or by the electrochemical mediator. In the later stages of ferritin iron core dissolution, reduction rates drop dramatically whether measured by DXAS or formation of Fe²⁺ complexes. Such results emphasize the heterogeneity of ferritin core structure.     

Laccase electrode for direct electrocatalytic reduction of O2 to H2O with high-operational stability and resistance to chloride inhibition

Laccase from Trametes hirsuta basidiomycete has been covalently bound to graphite electrodes electrochemically modified with phenyl derivatives as a way to attach the enzyme molecules with an adequate orientation for direct electron transfer (DET). Current densities up to 0.5mA/cm(2) of electrocatalytic reduction of O(2) to H(2)O were obtained in absence of redox mediators, suggesting preferential orientation of the T1 Cu centre of the laccase towards the electrode. The covalent attachment of the laccase molecules to the functionalized electrodes permitted remarkable operational stability. Moreover, O(2) bioelectroreduction based on DET between the laccase and the electrode was not inhibited by chloride ions, whereas mediated bioelectrocatalysis was. In contrast, fluoride ions inhibited both direct and mediated electron transfers-based bioelectrocatalytic reduction of O(2). Thus, two different modes of laccase inhibition by halides are discussed.     

Characterization of a new electrochemically active bacterium,Lysinibacillus sphaericus D-8, isolated with a WO3 nanocluster probe

Microorganisms capable of extracellular electron transfer play important roles in biogeochemical redox processes and have been of great interest in the fields of energy recovery, waste treatment, and environmental remediation. In this study, a new electrochemically active bacterium was identified with a high-throughput method using WO₃ nanoclusters as probes. The 16S rRNA gene sequence designated the strain as Lysinibacillus sphaericus D-8, a Gram-positive bacterium. Its electrochemical activity was characterized in a two-chamber microbial fuel cell and a three-electrode electrochemical cell. Strain D-8 produced 92 mW/m² of power using lactate as the electron donor. The electrochemical impedance spectroscopy results confirmed the electrochemical activity of this strain. Cyclic voltammetry analysis indicated that the presence of soluble redox active compounds might play an important role in the extracellular electron transfer by L. sphaericus D-8. This work might be the first report that demonstrates the electrochemical activity of Lysinibacillus species.     

Redox-dependent structural changes in an engineered heme-copper center in myoglobin: insights into chloride binding to CuB in heme copper oxidases

The effects of chloride on the redox properties of an engineered binuclear heme-copper center in myoglobin (Cu(B)Mb) were studied by UV-vis spectroelectrochemistry and EPR spectroscopy. A low-spin heme Fe(III)-Cu(I) intermediate was observed during the redox titration of Cu(B)Mb only in the presence of both Cu(II) and chloride. Upon the first electron transfer to the Cu(B) center, one of the His ligands of Cu(B) center dissociates and coordinates to the heme iron, forming a six-coordinate low-spin ferric heme center and a reduced Cu(B) center. The second electron transfer reduces the ferric heme and causes the release of the coordinated His ligand. Thus, the fully reduced state of the heme-copper center contains a five-coordinate ferrous heme and a reduced Cu(B) center, ready for O(2) binding and reduction to water to occur. In the absence of a chloride ion, formation of the low-spin heme species was not observed. These redox reactions are completely reversible. These results indicate that binding of chloride to the Cu(B) center can induce redox-dependent structural changes, and the bound chloride and hydroxide in the heme-copper center may play different roles in the redox-linked enzymatic reactions of heme-copper oxidases, probably because of their different binding affinity to the copper center and the relatively high concentration of chloride under physiological conditions.     

Syntheses of fully sulfonated polyaniline nano-networks and its application to the direct electrochemistry of cytochrome c

Fully sulfonated polyaniline nano-particles, nano-fibrils and nano-networks have been achieved for the first time by electrochemical homopolymerization of orthanilic acid using a three-step electrochemical deposition procedure in a mixed solvent of acetonitrile (ACN) and water. The diameter of the uniform nano-particles is about 60 nm, and the nano-fibrils can be organized in two-dimensional (2D) or three-dimensional (3D) non-periodic networks with good electrical contact. Average distance between contacts is about 850 and 600nm for a 2D and 3D system, respectively. The details of the poly(orthanilic acid) (POA) nano-structure were examined with a field emission scanning electron microscope (SEM). The structure and properties of POA were characterized with FTIR, UV-vis and electrochemical methods. The 3D POA nano-networks coated platinum electrode gave a direct electrochemical behavior of horse heart cytochrome c (Cyt c) immobilized on this electrode surface, a pair of well-defined redox waves with formal potential (E( degrees ')) of -0.032 V (versus Ag/AgCl) was achieved. The interaction between Cyt c and POA makes the formal potential shift negatively compared to that of Cyt c in solution. Spectrophotometric and electrochemical methods were used to investigate the interaction of Cyt c with POA. The immobilized Cyt c in the nano-networks POA film maintained its activity, showing a surface-controlled electrode process with the electron transfer rate constant (k(s)) of 21s(-1) and a of 0.53, and could be used for the electrocatalytic reduction of hydrogen peroxide. The quantitative determination of Cyt c by differential pulse voltammetry (DPV) using the fully sulfonated 3D POA nano-networks film coated platinum electrode was also studied.     

Electrochemical Characteristics of Nitro-Heterocyclic Compounds of Biological Interest: I. The Influence of Solvent

The electrochemical properties of three nitroimidazoles, a nitropyrazole, a nitrofuran and three nitroben-zenoid compounds have been extensively investigated in a range of solvents. The reduction pathway for the nitro group is independent of the cyclic function to which it is attached, but is strongly influenced by the nature of the solvent. In aqueous media, generally, a single, irreversible 4-electron reduction occurs to give the hydroxylamine. In aprotic media (dimethylformamide, methylene chloride or dimethylsulphoxide), a reversible one-electron reduction takes place to form a stable nitro radical anion. At more negative values, a further 3-electron reduction occurs, irreversibly to give the hydroxylamine. In mixed aqueous-organic systems, intermediate behaviour is found, with the reversibility of the RNO₂/RNO₂^? couple increasing with addition of organic medium. The control of the reduction pathway, by changing the electrolytic medium is discussed in relation to the biological activities of the drugs and identification of the short-lived reduction intermediate responsible for DNA damage.     

Synthesis of two 8-[(anthraquinone-2-yl)-linked]-2'-deoxyadenosine 3'-benzyl hydrogen phosphates for studies of photoinduced hole transport in DNA

The challenge in working with anthraquinone-2'-deoxyadenosine (AQ-dA) conjugates is that they are insoluble in water and only sparingly soluble in most organic solvents. However, water-soluble AQ-dA conjugates with short linkers are required for study of their electrochemical and intramolecular electron transfer properties in this solvent prior to their use in laser kinetics investigations of photoinduced hole (cation) transport in DNA. This article first describes the synthesis of a water-soluble, ethynyl-linked AQ-dA conjugate, 8-[(anthraquinone-2-yl)ethynyl]-2'-deoxyadenosine 3'-benzyl hydrogen phosphate, based on initial formation of a 5'-O-(4,4'-dimethoxytrityl) (5'-O-DMTr) intermediate. Because intended H2 over Pd/C reduction of the ethynyl linker in 5'-O-DMTr-protected 2'-deoxyadenosines cleaves the DMTr protecting group and precipitates multiple side products, this work also describes the synthesis of an ethylenyl-linked AQ-dA conjugate, 8-[2-(anthraquinone-2-yl)ethyl]-2'-deoxyadenosine 3'-benzyl hydrogen phosphate, starting with a 5'-O-tert-butyldiphenylsilyl protecting group.     

Electrochemistry and spectroelectrochemistry of a linear triiron cluster

This work reports electrochemical and spectroelectrochemical studies of a unique linear triiron cluster carbonyl complex, Fe₃(CO)₇L₂, where L is a -diazothioketone. Oxidation and reduction reactions have been observed in non-aqueous media over the temperature range −40 to 20 °C by differential pulse voltammetry, cyclic voltammetry, thin-layer, UV-Vis spectroelectrochemistry and ESR spectrometry. The sequence of the individual electron-transfer steps comprising the overall redox process is described, and a comparison between the electrochemistry of different non-linear ironcarbonyl complexes is discussed. A single one electron reduction produces the radical anion, [Fe₃(CO)₇L₂]-, which decomposes at temperatures greater than −10 °C to species which are reduced at a more negative potential, an ECE mechanism. A single one-electron oxidation produces the radical cation, [Fe₃(CO)₇L₂]⁺, which is unstable, decomposing completely at room temperature, an EC mechanism. Spectroscopic evidence indicates that in non-bonding solvents, the Fe₃(CO)₇L₂ framework remains intact at low temperatures for both the anion and cation radical produced electrochemically with radical stability higher than might be expected for a linear structure. Observations indicate only strongly bonding solvents disrupt the structure. Low temperature stability occurs at relatively high temperatures, with the cation radical less than stable and vulnerable to strongly bonding solvents.     

Electrochemical Properties As A Function of Ph for the Benzotriazine DI-N-Oxides

The electrochemistry of five benzotriazine di-N-oxides has been examined by cyclic voltammetry and differential pulse and dc polarographies as a function of pH. Between the pH range 8.5 and 2 the trend to less negative potentials with lowering of pH can be described by an equation of the type Ep = - apH + b. Comparison has been made with the mono- and zero-N-oxides which were found to show virtually identical trends in electron affinity with pH. The general electrochemical characteristics for the di- and mono-N-oxides under acidic conditions were found to be comparable with the zero-N-oxide. This was particularly the case on repeat scanning in the cyclic voltammetric mode. The redox mechanism involved reduction by a 4-electron addition step and subsequent loss of the A-oxide group(s) yielding the intact benzotriazine heterocycle. The helerocycle was also redox active, involving a reversible 2-electron reduction. for the di-N-oxides these two stages could be identified as separate processes at alkaline pH, but ony a single step at acidic values. The mono-N-oxide in which the electrochemical behaviour was dominated by the triazine, showed only a single reduction step, although the single N-oxide group was redox active.