Kinetics of long-range electron transfer through alkanethiolate monolayers containing amide bonds

Non-electroactive alkanethiolate monolayers containing internal amide bonds were used as model systems for the studies of the effect of structure of the intervening medium on long-range electron transfer. The blocking properties and the kinetics of electron transfer across the monolayers immobilized on gold were studied by voltammetry with the hexachloroiridate(IV) ion as the redox probe in the solution. The electron transfer efficiency was measured over a large potential window. The three types of monolayers studied were simple octadecanethiol and two amide-containing systems with one or two amide moieties in place of selected methylene groups in the main alkyl chain. Enhanced electronic coupling between the redox probe and the metal of the electrode was found for the monolayers with internal amide bonds. We ascribed it to the contribution of a hydrogen bonded network to electron tunneling through the monolayer. In the case of monolayers formed by molecules containing two secondary amide groups, the location of amide moieties inside the monolayer was shown to play an important role in the electron transfer efficiency. The second amide moiety placed in the alkyl chain in the odd position relative to the first amide did not increase electronic coupling in the monolayer. This behavior can be explained as due to larger distances between the amide groups in the external plane of the monolayer leading to difficulty in the formation of the hydrogen bond network. The position of the amide group relative to the electrode surface may be also considered as an important factor determining the efficiency of electron tunneling through the monolayer.     

Mass transport accompanied with electron transfer between the gold electrode modified with 11-ferrocenylundecanethiol monolayer and redox species in solution — an electrochemical quartz crystal microbalance study

An electrochemical quartz crystal microbalance (EQCM) was employed to investigate mass transport during the redox reaction of the ferrocenylundecanethiolate (FcC₁₁S(H)) monolayer modified gold electrode in solution containing other redox species. The FcC₁₁S-monolayer on gold acts as a barrier for the electron transfer between a gold electrode and Fe(CN)₆^4?/3? in solution and as a mediator for the reduction of Fe³⁺ in solution. In both cases, electrochemical current responses were complicated because the observed currents were due to the redox of both the ferrocenyl group immobilized on gold and others in electrolyte solutions. The frequency change, i.e. interfacial mass change on the gold electrode surface, was observed only during the redox of ferrocenyl groups. The complex current response was deconvoluted into the current components of the redox reaction of ferrocene and that of other redox species in solution by comparing cyclic voltammograms with the current calculated from frequency changes.     

Electron transfer processes of redox proteins at inherently modified microelectrode array devices

Microelectrode array (MEA) devices were employed to investigate the direct electrochemistry of three redox proteins, namely: horse heart cytochrome c, amicyanin from Thiobacillus versutus and amicyanin from Paracoccus denitrificans. Cyclic voltammetric experiments were carried out with arrays of 15×15 square-shaped gold microdisks of 5 μm in dimension. The gold microelectrodes were defined by a photoresist or silicon nitride coating as the insulating layer. The occurrence of well-behaved, steady-state responses of the redox proteins only at MEAs fabricated with photoresist may be explained by a model which assumes that one component of the photoresist acts as a facilitator enabling electron transfer between the microelectrodes and the protein molecules. It could be shown that the photoresist facilitates the electrochemistry of positively and negatively charged proteins. The formal redox potentials of both types of amicyanin were studied over a range of solution pH. In agreement with previous studies, the formal redox potentials increased with decreasing pH because of the redox inactivity of amicyanin in the Cu^I state at low pH. These experiments demonstrated that photoresist-coated MEAs can be employed as versatile tools to gain insight into the properties of redox biomacromolecules.     

Consequences of a potential-dependent transfer coefficient in ac voltammetry and in coupled electron–proton transfer for attached redox couples

The Marcus density-of-states model for simple electron transfer predicts that the transfer coefficient is dependent on overpotential. The nature of the potential dependence is a function of the reorganization energies associated with oxidation and reduction processes. A fifth-order polynomial expression accurately yields the potential dependence of the transfer coefficient and the resulting curved Tafel plots. With this polynomial expression, the effects of the potential-dependent transfer coefficient are examined for two cases, ac voltammetry of an attached redox molecule with simple electron transfer and the kinetic behavior of the 1-electron/1-proton redox system. Simulations of ac voltammograms indicate that the effects are minor and that ac voltammetry is poorly suited for determination of the reorganization energy of the redox molecule. In the coupled electron–proton redox case, the effects are marked. As expected, the apparent standard rate constant decreases dramatically at pH values between the pK_a values of the two oxidation states. More surprisingly, the simulated Tafel plots exhibit asymmetry between the anodic and cathodic branches depending on the pH. The path of electron transfer from the oxidized to the reduced species (electron–proton or proton–electron) at a fixed pH depends on the electrode potential.     

Cathodic reduction of 2-nitronaphthothiophen-4,9-quinone: evidence of catalysis by proton donors and its simulation

2-Nitronaphtho[2,3-b]thiophen-4,9-quinone (1) is biologically active. The reducible groups have conjugate interaction. Electrochemical experiments (cyclic voltammetry and electrolysis) were performed in order to verify possible intramolecular electron transfer or secondary redox systems and to gain insight into the redox behaviour to help in the understanding of its trypanocidal mechanism of action. Cyclic voltammograms of 1 at the Hg electrode, in DMF+TBAP or DMF+TEAP showed the presence of at least three waves, the two first related to quinone reduction and the third one relative to a catalytic process. After cathodic reduction, at potentials close to the third electron uptake, protons from adventitious water or ammonium quaternary salts can be reduced. Hydrogen formation, with the regeneration of the quinone dianion could be the cause of its catalytic nature. This effect is more pronounced with TEAP. Macroscale electrolyses reinforce the findings. This reaction can be hampered by addition of electrophiles to the medium. Simulated curves fit the experimental ones well. The fourth wave, present at fast scan rates, where the catalysis is not effective, is related to further reduction of the nitro radical anion to the hydroxylamino derivative. At the time scale of cyclic voltammetry, no intramolecular electron transfer was observed. The biological activity of 1 is, possibly, related to the very electrophilic quinone group, generating reactive radical oxygen species through redox cycling.     

Short-time preparation and electrochemical properties of a single layer of tetraoctylammonium bromide capped Au nanoparticles on dithiol self-assembled monolayer-modified Au electrode

Short time immobilization of densely packed tetraoctylammonium bromide (TOAB) stabilized gold nanoparticles (AuNPs) were established on a Au electrode modified with a self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDMT). The quartz crystal microbalance experiment showed densely packed TOAB–AuNPs single layer formation on both SAMs was achieved within 20 min. AFM images demonstrated that the immobilized TOAB–AuNPs on the SAMs were densely packed and the AuNPs film thickness was 6–7 nm. The electronic communication between the immobilized AuNPs and the underlying bulk electrode was confirmed by cyclic voltammetry and electroreflectance spectroscopy. A reversible electron transfer reaction was observed for both [Fe(CN)₆]^4−/3− and [Ru(NH₃)₆]^2+/3+ at TOAB–AuNPs immobilized on HDT (Au/HDT/AuNPs) and BDMT (Au/BDMT/AuNPs) modified electrodes. The electroreflectance spectra show a red-shifted strong positive-going plasmon resonance bands at 551 nm and 584 nm, respectively, for the Au/BDMT/AuNPs and Au/HDT/AuNPs electrodes. The observed reversible redox response for the solution redox species and red-shifted plasmon resonance bands for the immobilized AuNPs again indicated that the AuNPs were immobilized on the SAMs in a densely packed manner. An advantage of TOAB–AuNPs modified electrode prepared by short time immersion over citrate-stabilized AuNPs modified electrode was demonstrated by the enhanced oxidation of ascorbic acid (AA) at these electrodes. The oxidation of AA was shifted to 90 mV less positive potential with higher oxidation current at TOAB–AuNPs modified electrode when compared to citrate-stabilized AuNPs modified electrode.     

Electrocatalytic oxidation of some amino acids on a cobalt hydroxide nanoparticles modified glassy carbon electrode

In this study we investigated the electrocatalytic oxidation of cysteine, cystine, N-acetyl cysteine, and methionine on cobalt hydroxide nanoparticles modified glassy carbon electrode in alkaline solution. Different electrochemical techniques such as cyclic voltammetry, chronoamperometry and steady-state polarization were used to track the oxidation process and its kinetics. From voltammetric studies we concluded that in the presence of amino acids the anodic peak current of Co(IV) species increased, followed by a decrease in the corresponding cathodic current peak. This indicates that amino acids were oxidized on the redox mediator which was immobilized on the electrode surface via an electrocatalytic mechanism. Using Laviron’s equation, the values of α_s and k_s for the immobilized redox species were determined as α_s,a = 0.63, α_s,c = 0.38 and k_s = 0.28 s⁻¹, respectively. The catalytic rate constants, the electron transfer coefficients and the diffusion coefficients involved in the electrocatalytic oxidation of amino acids were determined.     

Cyclic voltammetric behavior of [2 Fe—2 S] ferredoxins at a lipid bilayer modified electrode

Direct electron transfer between ferredoxins isolated from vegetative cells of the cyanobacterium Anabaena (strain 7120) and from spinach, and a gold electrode modified with a self-assembled lipid bilayer membrane, has been investigated at various ionic strengths by cyclic voltammetry. The lipid bilayers were comprised of the electrically neutral egg phosphatidylcholine (PC), either alone or in combination with varying amounts of the positively charged surfactant dimethyldioctadecylammonium chloride (DODAC⁺). Both ferredoxins were found to interact strongly enough with the lipid membrane to support an efficient electron transfer reaction with the electrode. The interaction forces could be controlled by the PC concentration, the DODAC⁺ concentration, and the ionic strength of the supporting electrolyte. The observations suggest a mechanism involving attractive electrostatic interactions between the redox protein molecules and the lipid bilayer, which lead to rapid and discriminatory electron transfer with the gold electrode.     

The reaction of cytochrome c from different species with cytochrome c oxidase immobilized in an electrode supported lipid bilayer membrane

In past work the direct electron transfer reactions of bovine cytochrome c oxidase in an electrode-supported lipid bilayer membrane have been studied. Its reaction with cytochrome c in solution was also studied and found to be consistent with previous solution studies. In this work it is shown that the electron transfer reactions of cytochrome c oxidase in this electrode-supported lipid bilayer membrane depend on the source of cytochrome c. This property has also been widely studied for solution samples. The differences in the electron transfer reaction rates correlate with the differences in the amino acid sequence for the cytochrome c molecules studied. Electrochemical results suggest that the dissociation of the cytochrome c/cytochrome c oxidase reaction complex is the rate-controlling step in this electron transfer mechanism for cytochrome c from some sources. Moreover, the electron transfer reaction mechanism exhibits biphasic reaction kinetics, which is consistent with earlier work on reactions between solubilized cytochrome c oxidase/cytochrome c samples. These results indicate that the cytochrome c oxidase modified electrodes described herein could be used to distinguish amino acid sequence variations in proteins such as cytochrome c, and this has potential relevance as a diagnostic for disease states.     

Reversal of compromised bonding to oxidized etched dentin.

The mechanism responsible for hydrogen-peroxide- or sodium-hypochlorite-induced reductions in dentin bond strength is unknown. This in vitro study tested the hypothesis that these oxidizing agents were responsible by attempting to reverse the effect with sodium ascorbate, a reducing agent. Human dentin was treated with these oxidants before or after being acid-etched and with or without post-treatment with sodium ascorbate. They were bonded with either Single Bond or Excite. Hydrogen peroxide reduced the bond strengths of both adhesives, while sodium hypochlorite produced reduction in adhesion of only Single Bond (p < 0.05). Following treatment with sodium ascorbate, reductions in bond strength were reversed. Transmission and scanning electron microscopy showed partial removal of the demineralized collagen matrix only by sodium hypochlorite. The observed compromised bond strengths cannot be attributed to incomplete deproteinization and may be related to changes in the redox potential of the bonding substrates.     

Electrostatic modification of ferritin onto polypeptide-functionalized indium oxide electrode surfaces: Electrochemical and AFM studies

We demonstrated the direct electron transfer reaction of ferritin immobilized onto polypeptide-functionalized indium oxide electrodes using electrostatic interactions between polypeptides and ferritin. Polypeptides such as poly(l-lysine) and poly(l-arginine) were strongly adsorbed onto indium oxide electrode surfaces by electrostatic interactions. The modification of electrode surfaces with poly(l-lysine) was achieved by immersing indium oxide electrode into 1 mg ml^?1 poly(l-lysine) (molecular weight: 80,000 and 84,000 Da) for approximating 10 min. Ferritin molecules were fully immobilized onto poly(l-lysine)-functionalized electrodes at immersion times approximating 30 min. After accounting for the roughness for the electrode surface, the surface coverage of ferritin on the functionalized-indium oxide electrode was evaluated to be 9–13 × 10¹¹ molecules cm^?2, which indicates that ferritin molecules were densely packed like a full monolayer. Ferritin immobilized onto functionalized-electrodes showed the direct electron transfer reaction with the electrode. Potential value dependence of redox peaks on ferritin immersion times was not observed. Poly(l-arginine) (molecular weight: 94,000 Da) also acted as a modifier for immobilization of ferritin by electrostatic interactions. The electrochemical behavior of ferritin immobilized onto poly(l-ariginine)-functionalized electrodes was similar to that observed in poly(l-lysine)-functionalized systems. We obtained direct evidence for electrostatic interactions between ferritin molecules and poly(l-lysine) by tapping-mode AFM measurements; molecular ferritin binding to poly(l-lysine) molecular wires was observed.     

Mechanistic investigation of xanthene oxidation by heterogeneous and homogeneous electron transfers

Anodic oxidation of xanthene is investigated in acetonitrile at a platinum electrode by means of cyclic voltammetric and exhaustive potentiostatic electrolysis techniques. On the voltammetric scale time, the process involves two electrons and leads to xanthydrol. The corresponding mechanism is an ECE (electron–deprotonation–electron) type electrode reaction, the rate-determining step being the deprotonation of the cation radical obtained after the first electron transfer. On the other hand the analysis of the oxidation by homogeneous redox catalysis is carried out, using three organic catalysts. This allows the determination of the rate constants of the homogeneous electron transfers between xanthene and catalysts, the xanthene cation radical deprotonation rate constant and the standard potential of the xanthene cation radical/xanthene couple.     

Oxidation kinetics of NADH by heteropolyanions

A series of selected Dawson-type mixed heteropolyanions readily oxidize NADH in buffered aqueous pH 7 medium. The process was monitored by UV-visible spectroscopy, which helps to establish the 2:1 stoichiometry for the oxometalate/NADH couple. The starting system for electrochemistry consists of the one-electron reduction product of the heteropolyanions in the presence of various amounts of NADH. Cyclic voltammetry confirms unequivocally that the oxidized forms of the selected heteropolyanions are capable of catalyzing efficiently the oxidation of NADH. The kinetics were studied quantitatively by double step chronocoulometry. The logarithm of the second order rate constant was a linear function of the E^o of the first redox systems of the heteropolyanions with a slope of 16.4 V^?1. This result indicates that the oxidation of NADH proceeds by a multistep mechanism involving an initial rate-limiting one-electron transfer. An estimate of the E^o value for the one-electron NADH/NADH^·+ redox couple has been obtained.     

Electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) at a chlorogenic acid modified glassy carbon electrode

The redox response of chlorogenic acid solution at an inactivated glassy carbon electrode was investigated and an ECE mechanism was proposed for the electrode process. It has been shown that the oxidation of chlorogenic acid at an activated glassy carbon electrode leads to the formation of a deposited layer of about 4.5×10^?10 mol cm^?2 at the surface of the electrode. Cyclic voltammetry was used for the deposition process and the resulting modified electrode retains the activity of the quinone/hydroquinone group anticipated for a surface-immobilized redox couple. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were also examined. The pH dependence of the redox activity of these films was found to be 57 mV per pH unit, which is very close to the anticipated Nernstian dependence of 59 mV per pH unit. The modified electrode exhibits potent and persistent electrocatalysis for NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 430 mV and an increase in peak current. The electrocatalytic current increases linearly with NADH concentration from 0.1 to 1.0 mM. The apparent electron transfer rate constant, k_s, and the heterogeneous rate constant for electrooxidation of NADH, k_h, were also determined using cyclic voltammetry and rotating disk electrode voltammetry, respectively.     

Diffusionless electron transfer of microperoxidase-11 on gold electrodes

Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete protein, it would seem that electron transfer, ET, between immobilized MP-11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1000 s^?1, that reported previously for immobilized MP-11 does not exceed 20 s^?1. This work attempts to understand this difference in measured ET rates. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s^?1; (B) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s^?1; (C) physisorbed MP-11 on gold electrodes with a self assembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s^?1 for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a multilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation.     

Thiol groups and reduced acidogenicity of dental plaque in the presence of metal ions in vivo

Metal ions are known to influence the cariogenicity of dental plaque. Inhibition of acid metabolism in plaque may be of importance in this respect. Metal ions inhibit the acidogenicity of dental plaque to a different extent and it has been suggested that an enzyme inhibition based on oxidation of thiol groups may explain this observation. The aim of the present study was to evaluate the significance of oxidation of thiol groups in the inhibition of acid production in plaque by silver, tin and zinc salts. Nine subjects with 3-d sucrose induced plaque received topical applications of the metal ions. Cysteine or gilutathione, which are known to reverse thiol oxidations, were then applied in one side of the mouth. Plaque pH measurements, in the presence of sucrose, were performed prior to and up to 2 h after treatment. The results showed that the acid production inhibited by the metal ions was reactivated by cysteine or glutathione. Iodoacetamide and ρ-chloromercuribenzoate were also shown to inhibit acid formation in dental plaque. The high affinity silver, tin and zinc have for SH groups, the observed inhibitory effect of these metals, the reactivation of the metabolism by monothiols and the fact that organic sulfhydryl reagents inhibit acid formation in plaque indicate that oxidation of thiol groups may be the mechanism by which these metals exert their effect.     

Growth and electrochemical activity of the poly-l-lysine|poly-l-glutamic acid thin layer films: an EQCM and electrochemical study

Quartz crystal microbalance (QCM) measurement and cyclic voltammetry were used to study the layer-by-layer growth of poly-l-lysine (p-Lys) and poly-l-glutamic acid (p-Glu) thin films and their ability to act as support for redox relays between metal electrodes and redox species in organic electrolytes. The mass of the film grown from buffered solution at constant pH depends on the composition of the buffer used. The growth proceeds in several stages with different mass increments for each poly-l-lysine and poly-l-glutamic acid layer. Ferro/ferricyanide confined into the polypeptide films can play the role of a redox relay between the electrode surface and the organic electrolyte. This was demonstrated on the impregnated polypeptide film|1,2-dichloroethane interface with decamethylferrocene dissolved as redox species in the organic phase. The transport of the ferro/ferricyanide in the polypeptide film is the rate limiting step of the electron transfer process. The diffusion coefficient of the ferro/ferricyanide determined from voltammetric experiments was of the order of 10^?11–10^?12 cm² s^?1. The bimolecular rate constant of the electron transfer reaction was found to be 0.2 cm s^?1 M^?1.     

Peroxidases: past and present

The history of peroxidases spans nearly two centuries. Our knowledge has developed from early phenomenological observations of the colored products of peroxidase-catalyzed reactions, to our present understanding of many of the steps in the complex peroxidation reaction mechanism. Peroxidases are ubiquitous in plant and animal tissues and occur in diverse structural forms. Collectively, they are able to catalyze the hydroperoxide oxidation of many different kinds of organic and inorganic compounds. In spite of the great diversity of structures and functions, mechanisms of heme-containing peroxidases have several common features: (i) the transfer of the oxidizing equivalents of the hydroperoxidase to the enzyme to form Compound I, (ii) the reduction of Compound I by the transfer of electrons from donor molecules, (iii) the inactivation of Compound I by excess hydroperoxide. Rate constants for these and other steps in the peroxidation mechanism, as well as redox potentials, have been reported for many peroxidases. The molecular basis for the donor specificity of peroxidases has not yet been elucidated. Today, much interest is directed towards the biological functions of peroxidases and their reaction products.