Electron Self-exchange Rate Research Articles

Redox Properties of Bacillus subtilis Ferredoxin:NADP+ Oxidoreductase: Potentiometric Characteristics and Reactions with Pro-Oxidant Xenobiotics.

Bacillus subtilis ferredoxin:NADP+ oxidoreductase (BsFNR) is a thioredoxin reductase-type FNR whose redox properties and reactivity with nonphysiological electron acceptors have been scarcely characterized. On the basis of redox reactions with 3-acetylpyridine adenine dinucleotide phosphate, the two-electron reduction midpoint potential of the flavin adenine dinucleotide (FAD) cofactor was estimated to be -0.240 V. Photoreduction using 5-deazaflavin mononucleotide (5-deazaFMN) as a photosensitizer revealed that the difference in the redox potentials between the first and second single-electron transfer steps was 0.024 V. We examined the mechanisms of the reduction of several different groups of non-physiological electron acceptors catalyzed by BsFNR. The reactivity of quinones and aromatic N-oxides toward BsFNR increased when increasing their single-electron reduction midpoint redox potentials. The reactivity of nitroaromatic compounds was lower due to their lower electron self-exchange rate, but it exhibited the same trend. A mixed single- and two-electron reduction reaction was characteristic of quinones, whereas reactions involving nitroaromatics proceeded exclusively via the one-electron reduction reaction. The oxidation of FADH• to FAD is the rate-limiting step during the oxidation of fully reduced FAD. The calculated electron transfer distances in the reaction with nitroaromatics were close to those of other FNRs including the plant-type enzymes, thus demonstrating their similar active site accessibility to low-molecular-weight oxidants despite the fundamental differences in their structures.

Electron transfer kinetics of a series of copper complexes with tripodal tetradentate guanidine quinolinyl ligands.

Copper complexes of tripodal ligands have been used as model systems for electron transfer proteins for decades, displaying a broad range of electron self-exchange rates. We herein report a group of six tripodal tetradentate triarylamine ligands which display a varying number of guanidine and 2-methylquinolinyl moieties. Their corresponding Cu(I) complexes have been (re)synthesized and studied with regard to their electron transfer properties. While their molecular structures in the solid state are four-coordinate and display an uncommon umbrella distortion, DFT studies of the Cu(II) systems reveal that they gain an additional ligand in the form of a solvent molecule and exhibit a range of possible conformers that likely co-exist in thermal equilibrium. The redox-couples' electron self-exchange rates were analyzed using Marcus theory and vary over four orders of magnitude which cyclic voltammetry studies suggest to be due to a gated addition-oxidation electron transfer mechanism. This mechanism deviates from previously studied systems, likely due to the structural anomalies of the Cu(I) systems. This demonstrates that the chosen path of tripodal model systems can be influenced by molecular design.

Position of substituents directs the electron transfer properties of entatic state complexes: new insights from guanidine-quinoline copper complexes.

In a previous study, we showed that the properties and the ability as an entatic state model of copper guanidine quinoline complexes are significantly influenced by a methyl or methyl ester substituent in the 2-position. To prove the importance of the 2-position of the substituent, two novel guanidine quinoline ligands with a methyl or methyl ester substituent in the 4-position and the corresponding copper complexes were synthesized and characterized in this study. The influence of the substituent position on the copper complexes was investigated with various experimental and theoretical methods. The molecular structures of the copper complexes were examined in the solid state by single-crystal X-ray diffraction (SCXRD) and by density functional theory (DFT) calculations indicating a strong dependency on the substituent position compared to the systems substituted in the 2-position from the previous study. Further, the significantly different influence on the donor properties in dependency on the substituent position was analyzed with natural bond orbital (NBO) calculations. By the determination of the redox potentials, the impact on the electrochemical stabilization was examined. With regard to further previously analyzed guanidine quinoline copper complexes, the electrochemical stabilization was correlated with the charge-transfer energies calculated by NBO analysis and ground state energies, revealing the substituent influence and enabling a comparatively easy and accurate possibility for the theoretical calculation of the relative redox potential. Finally, the electron transfer properties were quantified by determining the electron self-exchange rates via the Marcus theory and by theoretical calculation of the reorganization energies via Nelsen's four-point method. The results gave important insights into the dependency between the ability of the copper complexes as entatic state model and the type and position of the substituent.

(Invited) Charge Propagation in Highly Concentrated Iodine/Iodide Solutions as Potential Redox Electrolytes and Mediators for Redox Flow Batteries and Solar Cells

The technology of Redox Flow Batteries is an important option to store energy from the operating irregularly renewable energy sources. The perspectives concern stationary energy storage, including grid-scale energy storage, thanks to their high power performance, flexible design, and ease of scaling-up. The flow-based electrochemical energy storage systems utilize the appropriate electroactive species dissolved in externally flowing electrolytes which are ready to accumulate all (or part) of the charge. Among important issues is the search for highly efficient (i.e., capable of fast electron transfers) electroactive systems that would yield high power and energy densities during the systems’ operation. Highly concentrated iodine/iodide system can act as redox conducting electrolyte. Furthermore its charge mediating capabilities has been recognized in the Dye Sensitized Solar Cell technology due to the feasibilioty of fast charge propagation accompanied by efficient electron self-exchange or diffusional-type electron hopping.In the present work, we concentrate on utilization of highly concentrated iodine/iodide redox systems and their possibility to exhibit high rates of charge propagation. In practical terms, it can be combined with ZnI2, or other zinc salt, containing electrolyte. Reactions in the zinc/iodine (polyiodide) redox flow battery are as follows: Zn → Zn2+ + 2e- (E = −0.76 V vs SHE) at the negative electrode (anode), and 3I- → I3 − + 2e− (E = 0.54 V vs SHE) at the positive electrode (cathode) thus yielding a total theoretical potential output as high as ∼1.3 V. The increase of current density could be achieved not only by reducing the viscosity of the electrolyte, thus accelerating charge-carrier transport, but also – by referencing to my experience with the iodine/iodide couple as charge relay for dye-sensitized solar cells – through improvement of the dynamics of charge propagation in highly concentrated iodine/iodide solution via the catalyzed enhancement of rates of electron self-exchange (hopping) between iodine/iodide (polyiodide) redox species as well as by accelerating the interfacial kinetics at electrodes. This can be realized by choosing appropriate electrode materials and through their activation or modification. The electrochemical activities of the redox couples are usually significantly increased through application of nanostructured functionalized carbons. While dispersed in solutions they can improve electron transfers to the redox sites. The proposed chemistry has been first tested using the microelectrode methodology to determine mass-transport (effectively diffusional) coefficients for charge propagation, heterogeneous and homogeneous (electron self-exchange) rates of electron transfers. Unless catalyzed, both interfacial and bulk (self-exchange) electron transfers involving the iodine/iodide redox system are somewhat complicated; there is a need to break the I-I bond in the I3 -or I2 molecule; it has also been well-established that platinum (e.g. when deposited on the counter electrode) induces electron transfers within the iodine/iodide redox system [1]. In the presentation, we are going to explore the respective interfacial (electrocatalytic) phenomena on nanostructured metal oxides (e.g. zirconia), in addition to traces of expensive platinum or palladium nanoparticles (provided that catalytic centers are three-dimensionally distributed in the electrolyte phase), and we will utilize them to enhance iodine/iodide electron transfers to develop a new generation of redox mediators or ultra-fast components of redox electrolytes. Our results show that the catalyzed iodine/iodide system can reach extremely high electron self-exchange rates, namely on the level of109 – 1010 mol-1 dm3 s-1 . [1] I.A. Rutkowska I.A., M. Marszalek, J. Orlowska, W. Ozimek, S.M. Zakeeruddin, P.J. Kulesza, M. Grätzel, ChemSusChem 8 (2015) 2560-2568.

(Invited) Microelectrode-Based Diagnosis of Charge Propagation and Redox Transitions in Concentrated Polyoxometallate Electrolyte of Potential Utility for Redox Flow Battery

Concentrated solutions of Keggin-type silicotungstic acid, as well as the system’s single crystals (H4SiW12O40*31H2O) and their colloidal suspensions have been tested using the microelectrode methodology to determine mass-transport, electron self-exchange and apparent (effective) diffusion-type coefficients for charge propagation and homogeneous (electron self-exchange) rates of electron transfers. Silicotungstic acid facilitates proton conductivity, and undergoes fast, reversible, multi-electron electron transfers leading to the formation of highly conducting, mixed-valence (tungsten(VI,V) heteropoly blue) compounds. To develop useful electroanalytical diagnostic criteria, electroanalytical approaches utilizing microdisk electrodes have been adapted to characterize redox transitions of the system and to determine kinetic parameters. Combination of micoroelectrode-based experiments performed in two distinct diffusional regimes: radial (long-term experiment; e.g., slow scan rate voltammetry or long-pulse chronoamperometry) and linear (short-term experiment; e.g., fast scan rate voltammetry or short-pulse chronocoulometry) permits absolute determination of such parameters as effective concentration of redox centers (C0 ) and apparent transport (diffusion) coefficient (Dapp ). The knowledge of these parameters, in particular of [Dapp 1/2 C0 ] seems to be of importance to the evaluation of utility of redox electrolytes for charge storage.While current densities which reflect dynamics of electrochemical processes have an influence on the systems’ current densities, the viscosity of the electrolyte and the mass transport dynamics are also affected by the nature of the redox-active material and its concentration. Trying to develop useful electroanalytical diagnostic approaches, we have successfully utilized microelectrode-based probes, as well as the historical concepts of charge propagation in semi-solid or semi-liquid systems developed for mixed-valence polynuclear materials in order to characterize concentrated redox electrolytes. Among important parameters are concentration of redox centers (C0 ) and apparent transport (diffusion) coefficient (Dapp ). The knowledge of these parameters and, in particular of [Dapp 1/2 C0 ], are crucial when it comes to evaluation of the diffusional-type charge propagation dynamics in the concentrated electrolyte which may reflect both physical mass transport and electron self-exchange (electron-hopping) contributions. Both potential-step (chronocoulometry, chronoamperometry) and cyclic voltammetric experiments utilizing microdisk electrodes have been adapted to characterization (identification of redox transitions and determination of kinetic parameters) of model inorganic redox electrolytes, namely highly-concentrated solutions or colloidal suspensions of Keggin-type polyoxometallate, silicotungstic acid.

Microelectrode-based probing of charge propagation and redox transitions in concentrated polyoxometallate electrolyte of potential utility for redox flow battery

Concentrated solutions of Keggin-type silicotungstic acid, as well as the system’s single crystals (H4SiW12O40*31H2O) and their colloidal suspensions have been tested using the microelectrode methodology to determine mass-transport, electron self-exchange and apparent (effective) diffusion-type coefficients for charge propagation and homogeneous (electron self-exchange) rates of electron transfers. Silicotungstic acid facilitates proton conductivity, and undergoes fast, reversible, multi-electron transfers leading to the formation of highly conducting, mixed-valence (tungsten(VI,V) heteropoly blue) compounds. To develop useful electroanalytical diagnostic criteria, electroanalytical approaches utilizing microdisk electrodes have been adapted to characterize redox transitions of the system and to determine kinetic parameters. Combination of microelectrode-based experiments performed in two distinct diffusional regimes: radial (long-term experiment; e.g., slow scan rate voltammetry or long-pulse chronoamperometry) and linear (short-term experiment; e.g., fast scan rate voltammetry or short-pulse chronocoulometry) permits absolute determination of such parameters as effective concentration of redox centers (C0) and apparent transport (diffusion) coefficient (Dapp). The knowledge of these parameters, in particular of [Dapp1/2 C0] seems to be of importance to the evaluation of utility of redox electrolytes for charge storage. For the colloidal suspension of silicotungstic acid (H4SiW12O40) crystals in the saturated solution, the following values have been obtained: Dapp = 1.8*10-6 cm2 s−1 and C0 = 1.1 mol dm−3, as well as the [Dapp1/2 C0] diagnostic parameter has reached the value as high as 6*10-3 mol/dm−3 cm s−1/2, provided that four electrons are involved in the H4SiW12O40 redox transitions. In this respect, the fact that crystals (dispersed solids) are characterized by high electron self-exchange rate (kex = 1.1*108 dm3 mol−1 s−1) and low activation energy (EA = 18.7 kJ mol−1) facilitating electron transfers between immobilized WVI and WV redox sites is also advantageous.

Fully Delocalized Mixed-Valent Cu1.5Cu1.5 Complex: Strong Cu-Cu Interaction and Fast Electron Self-Exchange Rate Despite Large Structural Changes.

A flexible macrocyclic ligand with two tridentate {CNC} compartments can host two Cu ions in reversibly interconvertible states, CuICuI (1) and mixed-valent Cu1.5Cu1.5 (2). They were characterized by XRD and multiple spectroscopic methods, including EPR, UV-Vis absorption and MCD, in combination with TD-DFT and CASSCF calculations. 2 features a short Cu⋯Cu distance (~2.5 Å; compared to ~4.0 Å in 1) and a very high delocalization energy of 13000 cm-1, comparable to the mixed-valent state of the biological CuA site. Electron self-exchange between 1 and 2 is rapid despite large structural reorganization, and is proposed to proceed via a sequential mechanism involving an active conformer of 1, viz. 1'; the latter has been characterized by XRD. Such electron transfer (ET) process is reminiscent of the conformationally gated ET proposed for biological systems. This redox couple is a unique pair of flexible dicopper complexes, achieving fast electron self-exchange closely related to the function of the CuA site.

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand.

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from -200 to -430 mV versus Fc+/0. The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2-4.2 M-1 s-1 compared to the high-spin to low-spin redox couple [Co(bpy)3]3+/2+ at 0.27 M-1 s-1, which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy)3]3+/2+ redox shuttle (19.8 vs 18.0 mA/cm2). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Ultramicroelectrode Based Approaches to Diagnose Utility of Redox Electrolytes in Flow Batteries

Redox flow batteries are of potential importance to both large scale energy storage and powering the electrical vehicle. Among critical challenges are low volumetric energy density of redox electrolytes, high cost and the maintenance limitations that greatly impede the wide application of conventional flow batteries. In this respect, the redox-active charge-storage material has a significant impact on the performance of a flow battery. The concentration of redox centers and their reaction kinetics have an influence on the available current densities and, thus, the power of the device.Many inorganic and organic electroactive systems have been proposed as alternatives to vanadium species in redox flow batteries. In the study, we explore the concept of highly concentrated solutions of such redox systems as iodine/iodide and poloxometallates of molybdenum and tungsten, and the possibility of optimizing their performance to exhibit high rates of charge propagation. While the iodine/iodide system can be prepared at high concentrations, the polyoxometallates of molybdenum and tungsten can serve as model examples of multi-electron systems for all-liquid redox flow batteries and related fundamental investigations. Reactions in the zinc/iodine (polyiodide) redox flow battery are as follows: Zn → Zn2+ + 2e- (E = −0.76 V vs SHE) at the negative electrode (anode), and 3I- → I3 − + 2e− (E = 0.54 V vs SHE) at the positive electrode (cathode) thus yielding a total theoretical potential output as high as ∼1.3 V. The increase of current density could be achieved not only by reducing the viscosity of the electrolyte, thus accelerating charge-carrier transport, but also – by referencing to our experience with the iodine/iodide couple as charge relay for dye-sensitized solar cells – through improvement of the dynamics of charge propagation in highly concentrated iodine/iodide solution via the catalyzed enhancement of rates of electron self-exchange (hopping) between iodine/iodide (polyiodide) redox species as well as by accelerating the interfacial kinetics at electrodes. This can be realized by choosing appropriate electrode materials and through their activation or modification. The electrochemical activities of the redox couples are usually significantly increased through application of nanostructured functionalized carbons. While dispersed in solutions they can improve electron transfers to the redox sites. The proposed chemistry has been first tested using the microelectrode methodology to determine mass-transport (effectively diffusional) coefficients for charge propagation, heterogeneous and homogeneous (electron self-exchange) rates of electron transfers. Unless catalyzed, both interfacial and bulk (self-exchange) electron transfers involving the iodine/iodide redox system are somewhat complicated; there is a need to break the I-I bond in the I3 -or I2 molecule; it has also been well-established that platinum (e.g. when deposited on the counter electrode) induces electron transfers within the iodine/iodide redox system. As far as heteropolyacids are concerned, they exhibit very strong Brønsted acidity, act as proton conductors, and undergo fast, reversible, multi-electron electron transfers leading to the formation of highly conducting, mixed-valence (e.g. tungsten(VI,V) or molybdenum(VI,V) heteropoly blue) compounds. The polyoxometallate-based redox electrolytes have different chemical identities, and they could be considered as anolytes or catholytes, depending on their redox potentials but, typically, their use would require formation of an asymmetric system with different-type redox species.While dynamics of electrochemical processes has an influence on the systems’ current densities, the viscosity of the electrolyte and the mass transport dynamics are also affected by the choice of the redox-active material and its concentration. Trying to develop useful electroanalytical diagnostic criteria, we are going to extend the historical concepts of charge propagation in semi-solid or semi-liquid systems developed for mixed-valence redox polymers and polynuclear materials to the development of redox electrolytes. Fundamental electroanalytical approaches utilizing ultramicrodisk electrodes and interdigitated arrays will be adapted to characterization of redox electrolytes.

Ultramicroelectrode Based Approaches to Diagnose Utility of Redox Electrolytes in Flow Batteries

The proposed redox electrolyte system, concentrated solutions of Keggin type silicotungstic acid (H4SiW12O40), have been tested using the microelectrode methodology to determine mass-transport (effectively diffusional) coefficients for charge propagation and homogeneous (electron self-exchange) rates of electron transfers. Silicotungstic acid (H4SiW12O40) acts as proton conductors, and undergoes fast, reversible, multi-electron electron transfers leading to the formation of highly conducting, mixed-valence (tungsten(VI,V) heteropoly blue) compounds. To develop useful electroanalytical diagnostic criteria, electroanalytical approaches utilizing ultramicrodisk electrodes have been adapted to characterization of redox electrolytes. Combination of two parallel ultramicroelectrode-based experiments performed under radial (long-term experiment; e.g., slow scan rate) and linear (short-term experiment; e.g., fast scan rate) diffusion conditions permits determination of both effective concentration of redox centers and transport (diffusion) coefficient parameters, which are crucial to evaluation of the utility of a redox electrolyte for charge storage.

(Invited) Charge Propagation in Highly Concentrated Iodine/Iodide Solutions As Potential Electrolytes for Redox Flow Batteries

The technology of Redox Flow Batteries is an important option to store energy from the operating irregularly renewable energy sources. The perspectives concern stationary energy storage, including grid-scale energy storage, thanks to their high power performance, flexible design, and ease of scaling-up. The flow-based electrochemical energy storage systems utilize the appropriate electroactive species dissolved in externally flowing electrolytes which are ready to accumulate all (or part) of the charge. Among important issues is the search for highly efficient (i.e., capable of fast electron transfers) electroactive systems that would yield high power and energy densities during the systems’ operation.In the present work, we concentrate on utilization of highly concentrated iodine/iodide redox systems and their possibility to exhibit high rates of charge propagation. In practical terms, it can be combined with ZnI2, or other zinc salt, containing electrolyte. Reactions in the zinc/iodine (polyiodide) redox flow battery are as follows: Zn → Zn2+ + 2e- (E = −0.76 V vs SHE) at the negative electrode (anode), and 3I- → I3 − + 2e− (E = 0.54 V vs SHE) at the positive electrode (cathode) thus yielding a total theoretical potential output as high as ∼1.3 V. The increase of current density could be achieved not only by reducing the viscosity of the electrolyte, thus accelerating charge-carrier transport, but also – by referencing to my experience with the iodine/iodide couple as charge relay for dye-sensitized solar cells – through improvement of the dynamics of charge propagation in highly concentrated iodine/iodide solution via the catalyzed enhancement of rates of electron self-exchange (hopping) between iodine/iodide (polyiodide) redox species as well as by accelerating the interfacial kinetics at electrodes. This can be realized by choosing appropriate electrode materials and through their activation or modification. The electrochemical activities of the redox couples are usually significantly increased through application of nanostructured functionalized carbons. While dispersed in solutions they can improve electron transfers to the redox sites. The proposed chemistry has been first tested using the microelectrode methodology to determine mass-transport (effectively diffusional) coefficients for charge propagation, heterogeneous and homogeneous (electron self-exchange) rates of electron transfers. Unless catalyzed, both interfacial and bulk (self-exchange) electron transfers involving the iodine/iodide redox system are somewhat complicated; there is a need to break the I-I bond in the I3 -or I2 molecule; it has also been well-established that platinum (e.g. when deposited on the counter electrode) induces electron transfers within the iodine/iodide redox system [1]. In the presentation, we are going to explore the respective interfacial (electrocatalytic) phenomena on nanostructured metal oxides (e.g. zirconia), in addition to traces of expensive platinum or palladium nanoparticles (provided that catalytic centers are three-dimensionally distributed in the electrolyte phase), and we will utilize them to enhance iodine/iodide electron transfers to develop a new generation of redox mediators or ultra-fast components of redox electrolytes. Our results show that the catalyzed iodine/iodide system can reach extremely high electron self-exchange rates, namely on the level of 109 – 1010 mol-1 dm3 s-1 . [1] I.A. Rutkowska I.A., M. Marszalek, J. Orlowska, W. Ozimek, S.M. Zakeeruddin, P.J. Kulesza, M. Grätzel, ChemSusChem 8 (2015) 2560-2568.

Ionic Effect on Electrochemical Behavior of Water-Soluble Radical Polyelectrolytes

Water-soluble redox-active polymers (RAPs) have emerged as attractive electroactive materials for aqueous redox flow batteries because these systems rely on readily available size-exclusion membranes rather than expensive ion-selective membranes. While incorporating ionic units is the most effective strategy for forming water-soluble redox polyelectrolytes, little is known about how charges dictate their electrochemical behavior. Here, we design a series of water-soluble TEMPO (2,2,6,6-tetramethylpiperidyl-1-oxy) radical polyelectrolytes with identical radical distribution but various ionic groups through a sequential postmodification method. Physical and electrochemical characterizations show disparate diffusion coefficients (D) and charge transfer kinetics (k0) of these radical polyelectrolytes at various pH. Particularly, pH exerts a strong impact on k0 of the negatively charged polymer (i.e., with carboxylic acid groups). The bimolecular reaction rate kex determined from redox-induced polymer films shows electrostatic interactions between charged segments can enhance the electron self-exchange reaction rate by ten folds. The results suggest that charge effects are of great importance when designing water-soluble redox polymers for electrochemical applications.

Thioredoxin Reductase-Type Ferredoxin: NADP+ Oxidoreductase of Rhodopseudomonas palustris: Potentiometric Characteristics and Reactions with Nonphysiological Oxidants.

Rhodopseudomonas palustris ferredoxin:NADP+ oxidoreductase (RpFNR) belongs to a novel group of thioredoxin reductase-type FNRs with partly characterized redox properties. Based on the reactions of RpFNR with the 3-acetylpyridine adenine dinucleotide phosphate redox couple, we estimated the two-electron reduction midpoint potential of the FAD cofactor to be −0.285 V. 5-Deaza-FMN-sensitized photoreduction revealed −0.017 V separation of the redox potentials between the first and second electron transfer events. We examined the mechanism of oxidation of RpFNR by several different groups of nonphysiological electron acceptors. The kcat/Km values of quinones and aromatic N-oxides toward RpFNR increase with their single-electron reduction midpoint potential. The lower reactivity, mirroring their lower electron self-exchange rate, is also seen to have a similar trend for nitroaromatic compounds. A mixed single- and two-electron reduction was characteristic of quinones, with single-electron reduction accounting for 54% of the electron flux, whereas nitroaromatics were reduced exclusively via single-electron reduction. It is highly possible that the FADH· to FAD oxidation reaction is the rate-limiting step during the reoxidation of reduced FAD. The calculated electron transfer distances in the reaction with quinones and nitroaromatics were close to those of Anabaena and Plasmodium falciparum FNRs, thus demonstrating their similar “intrinsic” reactivity.

Thermal and photoinduced electron transfer reactions of phthalocyanine complexes of Zn(II) and Cu(II) in acetonitrile.

Phthalocyanine that has four peripheral 2-methoxyphenyl substituents at the α-position and its Zn(II) and Cu(II) complexes were synthesized. Chemical oxidation by the Cu(II) ion and electrochemical oxidation of these metal complexes were investigated spectrophotometrically in acetonitrile. The UV-visible absorption spectra of these metal complexes and their one-electron oxidized π-cation radicals showed no concentration dependence, indicating that these species exist as monomers in solution. Kinetics of the thermal electron transfer reaction from each phthalocyanine complex to Cu2+ and the photoinduced electron transfer reaction of the Zn(II) phthalocyanine complex with V(V) and V(IV) Schiff base complexes were studied using conventional spectrophotometric and transient absorption techniques, and the electron transfer rate constants were analysed using the Marcus cross relationship. The obtained rate constants of the electron self-exchange reaction between the parent phthalocyanine complexes and their π-cation radicals were in the order of 109 to 1011 M-1 s-1 at T = 298.2 K. These large electron self-exchange rate constants are consistent with the phthalocyanine-centred redox reactions where small reorganization energies are required with little structural change during the electron transfer process.

Develepment of Catalystic Systems for Electroreduction of Highly Inert Reactants Such As Arsenate, Nitrite and Oxohalogenates

Noble metal nanoparticles (Pt, Rh, Ir Pd), when deposited on an inert electrode substrates, exhibit electrocatalytic properties during both oxidation of As(III) and reduction of As(V) in acid medium (e.g. 0.5 mol dm-3 H2SO4). For example, upon application of the appropriate potential, both As(III) and As(V) are preconcentrated on surfaces of Pt nanoparticles. Consequently, their presence can monitored under voltammetric conditions. Population of protons does affect the systems’ electrochemical characteristics. The nature of arsenic voltammetric peaks is strongly dependent on the preconcentration potential. While the reduction and deposition of As(III) is induced by adsorbed hydrogen on Pt formed at potentials lower than 0.25 V (vs. RHE), the adsorption and preconcentration of As(V) can be readily achieved at Pt oxides generated at Pt nanoparticles at potentials higher than 0.85 V (vs. RHE). From mechanistic point of view, voltammetric oxidation of bulk As(III) is catalyzed by Pt oxo species but electrocatalytic reduction of As(V) requires its prior adsorption. Electroreduction of the resulting As(V) adsorbates is catalyzed by both metallic platinum and by platinum on which hydrogen atoms are adsorbed; the resulting voltammetric peak current densities are dependent on concentration of As(V) in solution. A unique feature of the adsorbed As(III) and As(V) oxo species is the ability to agglomerate and form stable polynuclear electroactive films on Pt nanoparticles. Their performance resembles behavior of redox conducting polymers, and their redox transitions are fast and reversible. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy.In conclusion, the present observations demonstrate the utility of Pt nanoparticles as the catalytic system for inducing of (otherwise highly inert) electrode processes of As(III) and As(V) in acid medium. While the electrocatalytic activity of platinum (particularly PtO) toward electrooxidation of As(III) was previously postulated, we clearly show here that arsenates(V) can be adsorbed, preconcentrated, and electrocatalytically reduced (without prior chemical treatment) and, thus, detected on surfaces of Pt nanostructures. On mechanistic grounds, As(V) must be first adsorbed on bare (metallic) Pt surface before the actual electron transfer can take place. In addition to the structural importance, ivolvement of protons is crucial to effective electrocatalysis. It is also noteworthy that the current signals can be correlated to concentration of As(V) in bulk solution. The present study also provides evidence of obtaining a stable redox-polymer-like As(III,V) oxo film capable of fast charge propagation (electron-self exchange rate on the level 8*106 dm3 mol-1 s-1) within the adsorbed layer existing on surfaces of platinum nanoparticles. The latter observation is of both fundamental and analytical significance. Observations made during electrocatalytic and photoelectrochemical reductions of bromates, nitrites and carbon dioxide with use of various metal nanoparticles and nanostructured metal oxides can serve as guides for such research.

Reactions of Plasmodium falciparum Ferredoxin:NADP+ Oxidoreductase with Redox Cycling Xenobiotics: A Mechanistic Study.

Ferredoxin:NADP+ oxidoreductase from Plasmodium falciparum (PfFNR) catalyzes the NADPH-dependent reduction of ferredoxin (PfFd), which provides redox equivalents for the biosynthesis of isoprenoids and fatty acids in the apicoplast. Like other flavin-dependent electrontransferases, PfFNR is a potential source of free radicals of quinones and other redox cycling compounds. We report here a kinetic study of the reduction of quinones, nitroaromatic compounds and aromatic N-oxides by PfFNR. We show that all these groups of compounds are reduced in a single-electron pathway, their reactivity increasing with the increase in their single-electron reduction midpoint potential (E17). The reactivity of nitroaromatics is lower than that of quinones and aromatic N-oxides, which is in line with the differences in their electron self-exchange rate constants. Quinone reduction proceeds via a ping-pong mechanism. During the reoxidation of reduced FAD by quinones, the oxidation of FADH. to FAD is the possible rate-limiting step. The calculated electron transfer distances in the reaction of PfFNR with various electron acceptors are similar to those of Anabaena FNR, thus demonstrating their similar “intrinsic” reactivity. Ferredoxin stimulated quinone- and nitro-reductase reactions of PfFNR, evidently providing an additional reduction pathway via reduced PfFd. Based on the available data, PfFNR and possibly PfFd may play a central role in the reductive activation of quinones, nitroaromatics and aromatic N-oxides in P. falciparum, contributing to their antiplasmodial action.

(Invited) Develepment of Electrocatalystic Systems for Highly Inert Reactants: Reduction of Arsenic(V), Nitrite, Oxohalogenates and CO2 in Acid Medium

Noble metal nanoparticles (Pt, Rh, Ir Pd), when deposited on an inert electrode substrates, exhibit electrocatalytic properties during both oxidation of As(III) and reduction of As(V) in acid medium (e.g. 0.5 mol dm-3 H2SO4). For example, upon application of the appropriate potential, both As(III) and As(V) are preconcentrated on surfaces of Pt nanoparticles. Consequently, their presence can monitored under voltammetric conditions. Population of protons does affect the systems’ electrochemical characteristics. The nature of arsenic voltammetric peaks is strongly dependent on the preconcentration potential. While the reduction and deposition of As(III) is induced by adsorbed hydrogen on Pt formed at potentials lower than 0.25 V (vs. RHE), the adsorption and preconcentration of As(V) can be readily achieved at Pt oxides generated at Pt nanoparticles at potentials higher than 0.85 V (vs. RHE). From mechanistic point of view, voltammetric oxidation of bulk As(III) is catalyzed by Pt oxo species but electrocatalytic reduction of As(V) requires its prior adsorption. Electroreduction of the resulting As(V) adsorbates is catalyzed by both metallic platinum and by platinum on which hydrogen atoms are adsorbed; the resulting voltammetric peak current densities are dependent on concentration of As(V) in solution. A unique feature of the adsorbed As(III) and As(V) oxo species is the ability to agglomerate and form stable polynuclear electroactive films on Pt nanoparticles. Their performance resembles behavior of redox conducting polymers, and their redox transitions are fast and reversible. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy.In conclusion, the present observations demonstrate the utility of Pt nanoparticles as the catalytic system for inducing of (otherwise highly inert) electrode processes of As(III) and As(V) in acid medium. While the electrocatalytic activity of platinum (particularly PtO) toward electrooxidation of As(III) was previously postulated, we clearly show here that arsenates(V) can be adsorbed, preconcentrated, and electrocatalytically reduced (without prior chemical treatment) and, thus, detected on surfaces of Pt nanostructures. On mechanistic grounds, As(V) must be first adsorbed on bare (metallic) Pt surface before the actual electron transfer can take place. In addition to the structural importance, ivolvement of protons is crucial to effective electrocatalysis. It is also noteworthy that the current signals can be correlated to concentration of As(V) in bulk solution. The present study also provides evidence of obtaining a stable redox-polymer-like As(III,V) oxo film capable of fast charge propagation (electron-self exchange rate on the level 8*106 dm3 mol-1 s-1) within the adsorbed layer existing on surfaces of platinum nanoparticles. The latter observation is of both fundamental and analytical significance. Observations made during electrocatalytic and photoelectrochemical reductions of bromates, nitrites and carbon dioxide with use of various metal nanoparticles and nanostructured metal oxides can serve as guides for such research.

Importance of Interfacial Activation and Proton Transfers during Redox Processes of Highly Inert Systems: Electrocatalytic Detection of As(III) and As(V) in Acid Medium

Noble metal nanoparticles (Pt, Rh, Ir Pd), when deposited on an inert electrode substrates, exhibit electrocatalytic properties during both oxidation of As(III) and reduction of As(V) in acid medium (e.g. 0.5 mol dm-3 H2SO4). For example, upon application of the appropriate potential, both As(III) and As(V) are preconcentrated on surfaces of Pt nanoparticles. Consequently, their presence can monitored under voltammetric conditions. Population of protons does affect the systems’ electrochemical characteristics. The nature of arsenic voltammetric peaks is strongly dependent on the preconcentration potential. While the reduction and deposition of As(III) is induced by adsorbed hydrogen on Pt formed at potentials lower than 0.25 V (vs. RHE), the adsorption and preconcentration of As(V) can be readily achieved at Pt oxides generated at Pt nanoparticles at potentials higher than 0.85 V (vs. RHE). From mechanistic point of view, voltammetric oxidation of bulk As(III) is catalyzed by Pt oxo species but electrocatalytic reduction of As(V) requires its prior adsorption. Electroreduction of the resulting As(V) adsorbates is catalyzed by both metallic platinum and by platinum on which hydrogen atoms are adsorbed; the resulting voltammetric peak current densities are dependent on concentration of As(V) in solution. A unique feature of the adsorbed As(III) and As(V) oxo species is the ability to agglomerate and form stable polynuclear electroactive films on Pt nanoparticles. Their performance resembles behavior of redox conducting polymers, and their redox transitions are fast and reversible. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy. In conclusion, the present observations demonstrate the utility of Pt nanoparticles as the catalytic system for inducing of (otherwise highly inert) electrode processes of As(III) and As(V) in acid medium. While the electrocatalytic activity of platinum (particularly PtO) toward electrooxidation of As(III) was previously postulated, we clearly show here that arsenates(V) can be adsorbed, preconcentrated, and electrocatalytically reduced (without prior chemical treatment) and, thus, detected on surfaces of Pt nanostructures. On mechanistic grounds, As(V) must be first adsorbed on bare (metallic) Pt surface before the actual electron transfer can take place. In addition to the structural importance, involvement of protons is crucial to effective electrocatalysis. It is also noteworthy that the current signals can be correlated to concentration of As(V) in bulk solution. The present study also provides evidence of obtaining a stable redox-polymer-like As(III,V) oxo film capable of fast charge propagation (electron-self exchange rate on the level 8*106 dm3 mol-1 s-1) within the adsorbed layer existing on surfaces of platinum nanoparticles. The latter observation is of both fundamental and analytical significance. Although the experimental approaches are not optimized here, the detection of As(V) and As(III) down 5*10-6 and 2*10-6 mol dm-3, respectively, is feasible. The main drawback is that the baseline uncertainty limits its routine use in static solutions to concentrations above 2-5*10-6 mol dm-3. This level is not sufficient for practical purposes. For example, the US Environmental Protection Agency (EPA) defines the permissible level of arsenic in drinking water in the United States as 10 μg dm-3 (ca. 1.3*10-7 mol dm-3). Further research is being continued along this line.

Electrocatalytic effects during redox reactions of arsenic at platinum nanoparticles in acid medium: Possibility of preconcentration, electroactive film formation, and detection of As(III) and As(V)

Platinum nanoparticles, when deposited on an inert electrode substrate, exhibit electrocatalytic properties during both oxidation of As(III) and reduction of As(V) in acid medium (0.5 mol dm−3 H2SO4). Upon application of the appropriate potential, both As(III) and As(V) are preconcentrated on surfaces of Pt nanoparticles. Consequently, their presence can monitored under voltammetric conditions. The nature of arsenic stripping voltammetric peaks is strongly dependent on the preconcentration potential. While the reduction and deposition of As(III) is induced by adsorbed hydrogen on Pt formed at potentials lower than 0.25 V (vs. RHE), the adsorption and preconcentration of As(V) can be readily achieved at Pt oxides generated at Pt nanoparticles at potentials higher than 0.85 V (vs. RHE). From mechanistic point of view, voltammetric oxidation of bulk As(III) is catalyzed by Pt oxo species but electrocatalytic reduction of As(V) requires its prior adsorption. Electroreduction of the resulting As(V) adsorbates is catalyzed by both metallic platinum and by platinum on which hydrogen atoms are adsorbed; the resulting voltammetric peak current densities are dependent on concentration of As(V) in solution. A unique feature of the preconcentrated As(III) and As(V) oxo species is the ability to agglomerate and form stable redox-polymer-like polynuclear-electroactive-films on Pt nanoparticles. Their performance resembles the behavior of redox conducting polymers, and their redox transitions are fast and reversible with the electron self-exchange rate reaching 8·106 dm3 mol−1 s−1. Under such conditions, the stripping type voltammetric currents are enhanced. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy.

Copper Guanidinoquinoline Complexes as Entatic State Models of Electron‐Transfer Proteins

The electron-transfer abilities of the copper guanidinoquinoline (GUAqu) complexes [Cu(TMGqu)2 ]+/2+ and [Cu(DMEGqu)2 ]+/2+ (TMGqu=tetramethylguanidinoquinoline, DMEGqu=dimethylethylguanidinoquinoline) were examined in different solvents. The determination of the electron self-exchange rate based on the Marcus theory reveals the highest electron-transfer rate of copper complexes with pure N-donor ligands (k11 =1.2×104 s-1 m-1 in propionitrile). This is supported by an examination of the reorganisation energy of the complexes by using Eyring theory and DFT calculations. The low reorganisation energies in nitrile solvents correspond with the high electron-transfer rates of the complexes. Therefore, the [Cu(GUAqu)2 ]+/2+ complexes act as good entatic states model of copper enzymes. The structural influence of the complexes on the kinetic parameters shows that the TMGqu system possesses a higher electron-transfer rate than DMEGqu. Supporting DFT calculations give a closer insight into the kinetics and thermodynamics (Nelsen's four-point method and isodesmic reactions) of the electron transfer.