Theoretical Contribution towards Understanding Specific Behaviour of “Simple” Protein‐film Reactions in Square‐wave Voltammetry

Surface reactions of uniformly adsorbed redox molecules at working electrode surface are seen as adequate models to studying chemical reactivity of many lipophilic enzymes. When considered under pulse voltammetric techniques, these systems show several uncommon features, whose origin is still not completely clear. The phenomena of “quasireverible maximum”, “splitting” of the net peak in square‐wave voltammetry, and the very steep descent of Faradaic currents of simple surface redox reactions exhibiting fast electron transfer are just some of the features that make these systems quite interesting for further elaborations. In this work, we present a set of theoretical calculations under conditions of square‐wave voltammetry in order try to explain some of aforementioned phenomena. The major goal of our work is to get insight to some voltammetric and chrono‐amperometric features of two considered surface reactions, i. e. (1) the “simple” surface redox reaction, and (2) surface redox reaction coupled to follow‐up irreversible chemical reaction of electrochemically generated redox species (or surface ECirr). We focus on the role of created Red(ads) (here in the reduction pulses only) to the current components of calculated square‐wave voltammograms exhibiting fast electrode reaction. We show that the irreversible chemical removal of electrochemically generated Red(ads) species, created in the potential pulses where half‐reaction of reduction Ox(ads)+ne‐?→Red(ads) is “defined” to take place, causes significant increase of all square‐wave current components. The results presented in this work show how complex the chrono‐amperometric features of surface redox reactions under pulse voltammetric conditions might be. In addition, we point out that both half reactions of a given simple surface redox process can occur, at both, “only reduction” and “only oxidation” potential pulses in square‐wave voltammetry. This, in turn, contributes to the occurrence of many phenomena observed in simple protein‐film voltammetry reactions. The effects of chemical reaction rate to the features of calculated square‐wave voltammograms of surface ECirr systems with fast electrode reaction are reported for the first time in this work.     

Two-electron-transfer redox systems,part 7: two-step electrochemical oxidation of the boron subhalide cluster dianions B6X 2- 6 (X = Cl,Br, I)

Boron subhalide cluster dianions B6X 2- 6 (X = Cl, Br, I) are electrochemically oxidized in two steps. According to cyclic voltammograms, the first step is chemically reversible and yields the corresponding radical anions B6X .- 6. The electron transfer is nearly diffusion controlled. The second, slower electron-transfer step leads to a species which we assume to be the hitherto not yet described neutral compounds B6X 2- 6. The voltammograms indicate a coupled fast catalytic reaction, producing the radical anions in a reduction by an electrolyte component. Computer simulations of the cyclic voltammograms reveal mechanistic details of the redox reactions, as well as quantitative values for formal potentials, rate constants, and diffusion coefficients. The results are compared to other BnXn redox systems.     

New Aspects of Protein‐film Voltammetry of Redox Enzymes Coupled to Follow‐up Reversible Chemical Reaction in Square‐wave Voltammetry

Protein‐film square‐wave voltammetry of uniformly adsorbed molecules of redox lipophilic enzymes is applied to study their electrochemical properties, when a reversible follow‐up chemical reaction is coupled to the electrochemically generated product of enzyme's electrode reaction. Theoretical consideration of this so‐called “surface ECrev mechanism” under conditions of square‐wave voltammetry has revealed several new aspects, especially by enzymatic electrode reactions featuring fast electron transfer. We show that the rate of chemical removal/resupply of electrochemically generated Red(ads) enzymatic species, shows quite specific features to all current components of calculated square‐wave voltammograms and affects the electrode kinetics. The effects observed are specific for this particular redox mechanism (surface ECrev mechanism), and they got more pronounced at high electrode kinetics of enzymatic reaction. The features of phenomena of “split net‐SWV peak” and “quasireversible maximum”, which are typical for surface redox reactions studied in square‐wave voltammetry, are strongly affected by kinetics and thermodynamics of follow‐up chemical reaction. While we present plenty of relevant voltammetric situations useful for recognizing this particular mechanism in square‐wave voltammetry, we also propose a new approach to get access to kinetics and thermodynamics of follow‐up chemical reaction. Most of the results in this work throw new insight into the features of protein‐film systems that are coupled with chemical reactions.     

Introducing hydrophilic carbon nanoparticles into hydrophilic sol-gel film electrodes

A hydrophilic carbon nanoparticle–sol-gel electrode with good electrical conductivity within the sol-gel matrix is prepared. Sulfonated carbon nanoparticles with high hydrophilicity and of 10–20 nm diameter (Emperor 2000) are co-deposited onto tin-doped indium oxide substrates employing a sol-gel technique. The resulting carbon nanoparticle-sol-gel composite electrodes are characterized as a function of composition and salt (KCl) additive. Scanning electron microscopy and voltammetry in the absence and in the presence of a solution redox system suggest that the composite electrode films can be made electrically conducting and highly porous to promote electron transport and transfer. The effect of the presence of hydrophilic carbon nanoparticles is explored for the following processes: (1) double layer charging, (2) diffusion and adsorption of the electrochemically reversible solution redox system 1,1′-ferrocenedimethanol, (3) electron transfer to the electrochemically irreversible redox system hydrogen peroxide, and (4) electron transfer to the redox liquid tert-butylferrocene deposited into the porous composite electrode film. The extended electrochemically active hydrophilic surface area is beneficial in particular for surface sensitive processes (1) and (3), and it provides an extended solid|organic liquid|aqueous solution boundary for reaction (4). The carbon nanoparticle–sol-gel composite electrodes are optimized to provide good electrical conductivity and to remain stable during electrochemical investigation.     

Microcystin-LR and chemically degraded microcystin-LR electrochemical oxidation

Microcystins (MCs) are cyclic hepatotoxic heptapeptides produced by certain strains of freshwater cyanobacteria toxic for humans and animals. The electrochemical behaviour of microcystin-LR (MC-LR) at a glassy carbon electrode (GCE) was investigated using cyclic voltammetry (CV), square wave voltammetry (SWV) and differential pulse voltammetry (DPV). The oxidation of MC-LR is a diffusion-controlled irreversible and pH-independent process that occurs with the transfer of only one electron and does not involve the formation of any electroactive oxidation product. Upon incubation in different pH electrolytes, homogeneous degradation of MC-LR in solution was electrochemically detected by the appearance of a new oxidation peak at a lower potential. The electrochemical behaviour of chemically degraded MC-LR is an irreversible, pH-dependent process, and involves the formation of two redox products that undergo reversible oxidation. The formation of degradation products of MC-LR was confirmed by HPLC with UV detection at room temperature. Experiments were also carried out in solutions containing constituent MC-LR amino acids, which enabled the understanding of the MC-LR electron transfer reaction and degradation. An oxidation mechanism for MC-LR is proposed.     

Electrochromics by intramolecular redox switching of single bonds

Two moieties of mono- and trimethincyanines as well as those of styryl dyes were connected by a saturated alkyl tether made from compounds 3a-c, 5, 7, and 9a,b. In most cases, cyclic voltammetry and spectroelectrochemistry for these dyes together with the data for their monomeric models 4, 6, 8, and 10 reveal electrochemically irreversible transfer of two electrons but chemically reversible reaction and discoloration both on reduction and oxidation. Discoloration is interpreted as intramolecular formation of a single bond, which on redox breaking regenerates the starting colored species. Therefore, the investigated dyes exemplify a new general principle for electrochromics.     

Anodic behavior of clioquinol at a glassy carbon electrode

Clioquinol is an antifungal, antiprotozoal and an Alzheimer's disease drug with cytotoxic activity toward human cancer cells. The electrochemical behavior of clioquinol and its oxidation product was studied using cyclic, differential pulse and square-wave voltammetry over a wide pH range on a glassy carbon electrode. The results revealed that the oxidation of clioquinol is an irreversible pH-dependent process that proceeds with the transfer of one electron and one proton in an adsorption-controlled mechanism and results in the formation of a main oxidation product, which adsorbs very strongly on the glassy carbon surface. The charge transfer coefficient was calculated as 0.64. The adsorbed oxidation product presented reversible redox behavior, with two electron and two proton transfer. The electrochemical oxidation of clioquinol as a phenolic compound involves the formation of a phenoxy radical which reacts in at least two ways: in one pathway the radical initiates polymerization, the products remaining at the electrode surface, and in the other the radical is oxidized to a quinone-like structure. A mechanism for the oxidation of clioquinol is proposed.     

Faradaic alternating current response of the adsorbed redox couple

A simple theoretical model to explain the a.c. voltammetry of the adsorbed reactant and product is developed. The theory indicates that maximum a.c. responses of the reversible redox reactions appear out of phase, whilst for the totally irreversible reaction, response appears in phase with the excitation signal. Theoretical predictions are compared with a.c. voltammograms of berberine.     

524—NAD/NADH as a model redox system: Mechanism,mediation, modification by the environment

The biologically important redox couple, β-nicotinamide adenine dinucleotide/1,4,β-dihydronicotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e^?) transfer steps coupled with (a) dimerization of an intermediate free radical product, (b) protonation—deprotonation of an intermediate product, (c) other chemical reactions, (d) adsorption of reactant, intermediate and product species, and (e) mediation by electrode surface species. Cathodic reduction of NAD⁺ proceeds through two 1 e^? steps well separated in potential; protonation of the free radical produced on the first step occurs prior to the second electron-transfer; a first-order chemical reaction coupled to the latter may involve rearrangement of an initial dihydro product to 1,4-NADH (and some 1,6-NADH). In the apparently single stage 2 e^? anodic oxidation of NADH, the initial step is an irreversible heterogeneous electron transfer, which proceeds to at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH^+?, loses a proton (first order reaction) to form a neutral radical, NAD^?, which may participate in a second heterogeneous electron transfer (ECE mechanism) or may react with NAD^+? (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD⁺.     

烟酸的电化学行为与测定

用循环伏安法研究了不同支持电解质溶液中烟酸的电化学行为。烟酸在玻碳电极表面有良好的电化学响应信号。在碱性介质中,烟酸被不可逆氧化,氧化产物具有电活性,可发生准可逆氧化还原反应;在酸性介质中,烟酸发生两步准可逆氧化还原反应。结合红外、紫外光谱分析,提出了烟酸在不同酸度的介质中的电化学反应历程。并根据－０．１３Ｖ处的氧化峰电流与烟酸浓度的关系,提出了电化学测定烟酸的新方法     

Macromolecular polyradicals with cyclic triphosphazene as a core. Spectral and electrochemical properties

Stable cyclotriphosphazenes 5 and 6, with three and four carbon radical centers, have been prepared by condensation of (4-hydroxy-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl radical (4) with tetrachloro-2,2'-dioxybiphenylcyclotriphosphazene (7). EPR studies of both polyradicals in fluid solution suggest an electronic communication through the PN multiple bonds of the cycle. EPR spectral results in frozen solutions and magnetic susceptibility measurements in the solid are consistent with very weak electron-electron dipolar interactions. Reductive cyclic voltammetry shows a single three-electron redox couple for triradical 5 and a single four-electron redox couple for tetraradical 6. Both polyradicals 5 and 6 have been chemically oxidized to a stable trication 5(3+) and a tetracation 6(4+), respectively, by electron-transfer reactions.     

Charged states of Sc3N@C68: an in situ spectroelectrochemical study of the radical cation and radical anion of a non-IPR fullerene

The redox behavior of Sc 3N@C 68 is studied systematically by means of electrochemistry, in situ ESR/Vis-NIR spectroelectrochemistry, and detailed theoretical treatment. Formation of the negatively and positively charged paramagnetic species for the same trimetallic nitride endohedral fullerene is demonstrated for the first time. The electrochemical study of Sc 3N@C 68 exhibits two electrochemically irreversible but chemically reversible reduction steps and two reversible oxidation steps. A double-square reaction scheme is proposed to explain the observed redox reaction at cathodic potentials involving the reversible dimerisation of the Sc 3N@C 68 monoanion. The spin state of the radical cation and the radical anion is probed by ESR spectroscopy, indicating that in both states, the large part of the unpaired spin is delocalized on the fullerene cage. The charged states of the non-isolated pentagon rule fullerene are characterized furthermore by in situ absorption spectroscopy. The interpretation of experimental data is supported by the density functional theory (DFT) calculations of the spin distribution in the anion and cation radicals of Sc 3N@C 68 and time-dependent DFT calculations of the absorption spectra of the charged species.     

Electrodeposition of Catechol on Glassy Carbon Electrode and Its Electrocatalytic Activity Toward NADH Oxidation

Catechol can be oxidized electrochemically to its corresponding o‐benzoquinone. The electrogenerated quinone can be deposited by cycling the potential at the surface of glassy carbon electrodes. We have studied the electrochemical features of films derived from catechol by cyclic voltammetry. The electrodeposited film shows stable reversible redox response, dependent on pH as anticipated for quinone/catechol functionalities. Glassy carbon electrodes covered with a film derived from catechol exhibit catalytic activity in the electrooxidation of NADH at a low potential. The catalytic current is proportional to the concentration of NADH over the range 0.02–0.34 mM.     

Electrochemically controlled chemically reversible transformation of alpha-tocopherol (vitamin E) into its phenoxonium cation

Alpha-tocopherol (alpha-TOH) can be electrochemically oxidized in CH(3)CN containing Bu(4)NPF(6) in a chemically reversible two-electron/one-proton (ECE) process to form the phenoxonium cation (alpha-TO(+)) that is stable for at least several hours at 243 K. In the presence of up to approximately 1% CF(3)SO(3)H, alpha-TO(+) exists in equilibrium with the alpha-tocopherol cation radical (alpha-TOH(+)(*)), whereas at concentrations between approximately 1-3% CF(3)SO(3)H the electrochemical oxidation of alpha-TOH occurs by close to one-electron to form alpha-TOH(+)(*).alpha-TOH(+)(*) can be further oxidized in a one-electron process to form the alpha-tocopherol dication (alpha-TOH(2+)). The identity and stability of the phenolic cationic compounds were determined by a combination of electrochemical (cyclic voltammetry and controlled potential electrolysis) and in situ spectroscopic (UV-vis-NIR, FTIR, EPR, and NMR) analysis.     

Theoretical Aspects of a Surface Electrode Reaction Coupled with Preceding and Regenerative Chemical Steps: Square‐wave Voltammetry of a Surface CEC’ Mechanism

Large number of lipophilic substances, whose electrochemical transformation takes place from adsorbed state, belong to the class of so‐called “surface‐redox reactions”. Of these, especially important are the enzymatic redox reactions. With the technique named “protein‐film voltammetry” we can get insight into the chemical features of many lipophilic redox enzymes. Electrochemical processes of many redox adsorbates, occurring at a surface of working electrode, are very often coupled with chemical reactions. In this work, we focus on the application of square‐wave voltammetry (SWV) to study the theoretical features of a surface electrode reaction coupled with two chemical steps. The starting electroactive form Ox_(ads) in this mechanism gets initially generated via preceding chemical reaction. After undergoing redox transformation at the working electrode, Ox_(ads) species got additionally regenerated via chemical reaction of electrochemically generated product Red_(ads) with a given substrate Y. The theory of this so‐called surface CEC’ mechanism is presented for the first time under conditions of square‐wave voltammetry. While we present plenty of calculated voltammograms of this complex electrode mechanism, we focus on the effect of rate of regenerative (catalytic) step to simulated voltammograms. We consider both, electrochemical reactions featuring moderate and fast electron transfer. The obtained voltammetric patterns are very specific, having sometime hybrid‐like features of voltammograms as typical for CE, EC and EC’ mechanisms. We give diagnostic criteria to recognize this complex mechanism in SWV, but we also present hints to access the kinetic and thermodynamic parameters relevant to both chemical steps, and the electrochemical reaction, too. Indeed, the results presented in this work can help experimentalists to design proper experiments to study chemical features of important lipophilic systems.     

Reversible sequence of intramolecular associative and dissociative electron-transfer reactions in hydrotris(pyrazolylborate) complexes of rhodium

The one-electron chemically reversible oxidation of four neutral [RhLL'(kappa(2)-Tp(Me2))]complexes [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate], which leads to kappa(3)-Tp(Me2) bonding in the corresponding monocations, has been studied by cyclic voltammetry (CV) and other electrochemical methods. The CV behavior of [Rh(CO)[P(OPh)(3)]Tp(Me2)] (1) and [Rh(CO)(PPh(3))Tp(Me2)] (2) is quasi-nernstian at slow CV scan rates, with heterogeneous charge-transfer rates, k(s), of 0.025 cm s(-1) and 0.015 cm s(-1) (at 273 K), respectively. By contrast, [Rh(CO)(PCy(3))Tp(Me2)] (3, Cy = cyclohexyl) and [Rh(PPh(3))(2)Tp(Me2)] (4) display electrochemically irreversible CV curves that arise from rate-limiting slow electron-transfer reactions. Both the oxidation of 3 (or 4) and the rereduction of 3(+) (or 4(+)) have two-step (EC-type) mechanisms in which the electron transfer (e.t.) process is followed by a separate structural change, leading to an overall square scheme with irreversible charge-transfer kinetics. Homogeneous redox catalysis was used to determine the E(1/2) value of the oxidation of 3 to an intermediate 3C(+) which is postulated to have a pseudo-square pyramidal structure. Digital simulations gave k(s) = 9 x 10(-3) cm s(-1) for the heterogeneous charge-transfer rate of 3/3C(+). The close-to-nernstian CV behavior of 1 is ascribed to the fact that, unlike the sterically constrained derivatives 3 and 4, the third pyrazolyl ring in 1 is already in a configuration which favors formation of the Rh-N(2) bond in 1(+). The overall redox mechanism for this series of compounds involves an associative oxidative e.t. reaction followed by a dissociative reductive e.t. process.     

Product distribution in preparative scale electrolysis: Part II. EC reaction schemes followed by competition between first-order chemical reaction and further electron transfer. One-electron systems

In many organic electrochemical reactions, the initially formed ion radical undergoes a rapid and irreversible reaction leading to an electrochemically and chemically reactive secondary radical. In the cases where this secondary radical is easier to reduce (or to oxidize) than the starting compound and at the same time undergoes a chemical reaction yielding an electroinactive species, a three-cornered ECC-ECE-Disp competition occurs involving the concurrent formation of two final products. The product distribution is determined as a function of the intrinsic-rates, rate ratios, diffusion coefficient- and operational-concentration, diffusion layer thickness and current density-parameters of the system. Two limiting situations are of particular interest, each corresponding to a two-term competition between chemical reaction and electron transfer, the latter process occurring predominantly at the electrode (ECE) or in the solution (Disp) respectively. Zone diagrams are drawn which provide a quick and convenient view of the effect of all the intrinsic and operational parameters on the ECC-ECE-Disp competition in terms of product distribution.     

Electrochemical behaviour of some polyoxo clusters

We describe the redox behaviour in non-aqueous solvents of some cyclopentadienyl(oxo)titanium derivatives. The derivative [Ti₄{η⁵-C₅H₄(SiMe₃)}₄(μ-O)₆] shows an electrochemically and chemically reversible le reduction process, followed by a multi-electron, chemically complicated reduction at a fairly cathodic potential. On the basis of the overall electrochemical features and the comparison with the redox behaviour of the quasi-planar compound [[Ti{η⁵-C₅H₄(SiMe₃)}Cl(μ-O)]₄] we propose an EECCEE mechanism for the first derivative, where the second electron-transfer induces a cascade of chemical reactions giving rise to irreversible cluster breakdown. The electrochemically induced fragmentation can be viewed as a retrosynthetic pathway. The heterometallic derivative [{Ti(η⁵-C₅H₄Me)₂(μ₂-MoO₄)₂}₂] shows two consecutive reduction processes; the first is chemically reversible, and the second quasi-reversible. The molybdate bridges apparently increase the stability of the electrogenerated anions. However none of these poly-oxo clusters can be considered as good models of electron ‘sinks’.     

Sorbic Acid and Its Degradation Products: Electrochemical Characterization

The electrochemical redox behavior of sorbic acid (SA), an important food preservative, was investigated at a glassy carbon electrode using cyclic, differential pulse, and squarewave voltammetry over a wide pH range. The oxidation of SA is an irreversible, diffusion-controlled, and pH-independent process that occurs with the transfer of only one electron and does not involve the formation of any electroactive oxidation product. Adsorption of SA at GCE electrodes was also observed. Following incubation in different pH electrolytes, the degradation of SA was electrochemically detected by the appearance of a new oxidation peak at lower potential value. The degradation products, formed homogenously in solution, undergo irreversible oxidation and lead to the formation of two oxidation products that strongly adsorb on the electrode surface and are reversibly oxidized. SA degradation was also confirmed using HPLC and UV-Vis spectrophotometry. A mechanism for oxidation of SA and its degradation products in aqueous solutions was proposed.     

A novel multi-electrochromic polymer based on selenophene and benzotriazole via electrochemical and chemical polymerization

In this study, a novel donor-acceptor type monomer was designed based on selenophene and benzotriazole with a bulky pendant group and synthesized through Stille coupling reaction. The monomer was polymerized electrochemically by using cyclic voltammetry and also chemically by oxidation in the presence of FeCl₃. Both polymers were then compared in terms of their optical properties, electrochemical and spectroelectrochemical behaviors, kinetic and colorimetric properties and surface morphologies. Independent of the polymerization method, both electrochemically (E-PSeBTz) and chemically polymerized (C-PSeBTz) coatings showed quite similar properties. Both polymers have p-doping character and multichromic properties in their oxidized states. The polymers can be fully switched between their oxidized and neutral states in fairly short times with acceptable optical contrast at different wavelengths. Both polymers exhibit a λ_max of 505?nm and the optical band gaps of the materials were found to be 1.85?eV and 1.80?eV for E-PSeBTz and C-PSeBTz, respectively.