Electrochemistry of Lipophilic Redox Enzymes Associated with a Reversible Follow-up Chemical Reaction – Theoretical Consideration in Cyclic Staircase Voltammetry

Protein-film voltammetry (PFV) is established as an efficient methodology for studying the redox activity of lipophilic proteins and enzymes some 25 years ago. Yet it is still a challenge to understand specific behaviour of these systems under different voltametric conditions. In this work we apply cyclic staircase voltammetry to study theoretically the electrochemical properties of lipophilic redox enzymes, whose product of the electrochemical transformation at the working electrode is involved in a reversible chemical reaction. Attention is paid to understand how the kinetics of follow-up chemical reaction affects the features of theoretical staircase cyclo-voltammetric patterns of lipophilic redox enzymes.     

A reliable and ultrasensitive voltammetric method for the determination of molybdenum

Summary The determination of molybdenum(VI) in aquatic media by polarography, voltammetry and adsorptive stripping voltammetry using the formation of a Mo-chlorate-mandalic acid complex at +100 mV (SCE) is described. The 3 s_blank-detection limit is 0.1 nmol/l with differential pulse adsorptive stripping voltammetry (DPAdsSV, catalytic current). The calibration graphs are linear up to 200 nmol/l for the latter, 400 nmol/l for staircase voltammetry without accumulation time and 1 mg/l for staircase polarography. The accuracy of the DPAdsSV method was checked by analysis of a standard reference water material. The utility of differential pulse voltammetry was tested in different aquatic media, e.g., tap water, ground water, surface water.

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Eine zuverlässige und äußerst empfindliche Methode zur Bestimmung von Molybdän

     

Pulsed voltammetric stripping at the thin-film mercury electrode

A computer-controlled data acquisition system was used to generate comparative data for thin-film anodic stripping voltammetry with staircase, differential pulse and squarewave waveforms. Each waveform was tested for its sensitivity and speed of analysis. The square-wave form is the most sensitive, whether square wave or differential pulse measurements are used. This waveform offers the advantages of fast analysis time and discrimination against charging currents.     

A theory of single-electron non-adiabatic tunneling through a small metal nanoparticle with due account of the strong interaction of valence electrons with phonons of the condensed matter environment

A theory of electrochemical behavior of small metal nanoparticles (NPs) which is governed both by the charging effect and the effect of the solvent reorganization on the dynamic of the electron transfer (ET) is considered under ambient conditions. The exact expression for the rate constant of ET from an electrode to NP which is valid for all values of the reorganization free energy E(r), bias voltage, and overpotential is obtained in the non-adiabatic limit. The tunnel current/overpotential relations are studied and calculated for different values of the bias voltage and E(r). The effect of E(r) on the full width at half maximum of the charging peaks is investigated at different values of the bias voltage. The differential conductance/bias voltage and the tunnel current/bias voltage dependencies are also studied and calculated. It is shown that, at room temperature, the pronounced Coulomb blockade oscillations in the differential conductance/bias voltage curves and the noticeable Coulomb staircase in the tunnel current/bias voltage relations are observed only at rather small values of E(r) in the case of the strongly asymmetric tunneling contacts.     

Internal reflectance spectroscopy associated with cyclic staircase voltammetry using a microcomputer

In order to obtain extensive information on electrode-solution interfaces, a rapid-scan internal reflectance spectroscopic system is combined with cyclic staircase voltammetry; the staircase wave generated by computer software is applied to an optically transparent tin oxide electrode. The internal reflectance spectrum is measured at each step of the applied staircase voltage simultaneously with the instantaneous current.     

Computational chemistry applied to the design of chiral stationary phases for enantiomeric separation

A general computational scheme for the rational design of chiral stationary phases for the chromatographic separation of enantiomers has been established. The developed scheme was based on applying different interaction models (force field methods versus semi empirical quantum chemical methods), different docking algorithms (systematic grid search methods versus interactive methods guided by rules based on binding modes) and different levels of approximations (rigid versus flexible docking) to a representative test problem containing the 3,5-dinitrobenzoyl group. The computational methods in use covered the most sophisticated methods which could presently be applied to problems of such a size (about 80 atoms). It has been shown that the current computational approaches using rigid body approximations for the docked molecules and simple molecular mechanics (not taking pi-“effects” into account) are invalid in view of the required predictive precision of about 1–2 Kcal/mole for the differential binding energy. Another surprising result was the failure of the commonly used systematic search methods in determining the most favorable binding modes. Based on our calculations on the representative test problem we propose a new arrangement for the most stable complexes without parallel stacking of the aromatic pi-donor and the 3,5-dinitrobenzoyl pi-acceptor systems.     

General Behavior of the I–E and ΔI–E Curves Obtained when a Multistep Potential is Applied to an Electroactive Monolayer

Simple, analytical and fully explicit expressions for the I–E curves of a charge transfer process of a diffusionless system corresponding to staircase voltammetry (SCV) are presented. We have analyzed the reversibility influence and our results indicate that SCV curves obtained at different sampled times tend to coincide as the rate constant decreases. Moreover, the current–potential SCV response converts to that corresponding to linear sweep voltammetry (LSV) when the pulse amplitude ΔE→0. We have also obtained the theoretical expression for the response of differential staircase voltammetry (DSCV) which presents two peaks and a cross potential at which ΔI=0.     

Electroanalytical Study of the Antioxidant and Antitumor Agent Curcumin

Curcumin (CU) shows a wide range of pharmacological properties including antioxidant, anti‐inflammatory, and antitumor effects. In order to understand the chemical basis of different activities of curcumin, we have studied the oxidation and reduction of curcumin. Based on cyclic and differential pulse voltammetric methods, using carbon paste and hanging mercury drop electrodes, in the present study we tested different parameters to optimize the conditions for the determination of curcumin and its electrochemical characteristics. Better results were obtained via differential pulse voltammetry using carbon paste electrode. Curcumin yields well‐defined differential pulse voltammetric responses with well‐defined oxidation (in the potential region of 0.3–0.6 V, vs. Ag/AgCl) and reduction (at 0.3 V) peaks using carbon paste electrode.     

Performance of a general-purpose electrochemical instrument aided by a stand-alone microcomputer system in trace analysis

The features of a microprocessor-based data acquisition and control unit, dedicated to electrochemical experiments, are described. The menu-selectable software allows smoothing, baseline drawing and subtraction as well as differentiation, even with signal amplitudes of some tenths of nA at signal/noise ratios lower than one, without forcing the data to follow theoretical models. Concentrations of cadmium ion as low as 2 × 10^?7, 5 × 10^?8 and 5 × 10^?9 M can be measured, within 10% accuracy and precision, by using sampled d.c. polarography, staircase voltammetry and fast-sweep differential pulse voltammetry, respectively; this is a consistent improvement on literature data. Depending on the electrochemical technique used, the most significant signal parameters, including derivatives, are measured automatically, listed and used in the decision-making process for chemical characterization.     

Theoretical Analysis of a Surface Catalytic Mechanism Associated with Reversible Chemical Reaction Under Conditions of Cyclic Staircase Voltammetry

In this work, we present theoretical results in cyclic staircase voltammetry of a surface catalytic mechanism that features reversible chemical step, the so‐called “surface catalytic EC_rev’ mechanism”. We consider specific surface regenerative mechanism, in which both of the electro‐inactive substrates are present in large excess in electrochemical cell from the beginning of the experiment. The chemical reversibility brings at this mechanism more complexity in respect to the features of well‐elaborated surface catalytic EC’ mechanism coupled with chemically irreversible regenerative reaction. As we present plenty of simulated cyclic voltammograms, we also propose methods to get insight to kinetics and thermodynamics parameters relevant to chemical regenerative step. The elaboreted results can be important in analysing the kinetics and thermodynamics of many drug‐drug and drug‐DNA interactions, for example. In addition, with the results elaborated in this work we can access relevant information about the chemistry of important lipophilic enzymes studied in protein‐film voltammetry set up.     

Mathematical model of chemical oscillations arising in a homogeneous system of cysteine and oxygenated complexes of iron(II)

A mathematical model of the homogeneous oxidation kinetics of cysteine in the presence of oxygenated complexes of iron(II) with dimethylglyoxime and cytosine expressed in the form of a system of three nonlinear differential equations is analyzed. The model is simplified by stoichiometric analysis of the suggested kinetic scheme. As determined on the basis of qualitative analysis of the system of differential equations, a single stationary state with the singular point assigned to the focus—spatial saddle type, from which a bifurcation of the Andronov-Hopf type can occur, is implemented. Upon solving numerically the system of differential equations for different initial conditions, it is found that the mathematical model has a solution in the form of a limit cycle, and the process runs in an oscillatory mode at such initial concentrations of the catalyst and cysteine that are comparable to their contents at which chemical oscillations are observed in experiments.     

Application of the superposition principle to the study of CEC, CE, EC and catalytic mechanisms in cyclic chronopotentiometry. Part III

In this paper we have obtained the general analytical equations corresponding to the response of complicated charge transfer processes with coupled homogeneous chemical reactions in cyclic chronopotentiometry by applying the superposition principle. The analysis of different cycles of this response can be used to obtain accurate and contrasted values of kinetic parameters in each complex reaction scheme analysed. These parameters are of great interest for the chemist and the biochemist. These equations are also applicable to cyclic derivative chronopotentiometry which is a very useful method since the response is obtained with peaks in a similar way to cyclic voltammetry but its mathematical treatment is simpler. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analytical application of differential staircase voltammetry

Summary Differential staircase voltammetry is a technique which utilizes the measurements of the dc-component of differential pulse voltammetry. The analytical applicability of the method was estimated using some well-known electrochemical systems. It could be shown that the detection limits of differential staircase voltammetric determinations are almost equal those obtained with differential pulse voltammetry.     

Mechanisms of electrochemically-induced retro-cyclopropanation reactions of fullerene derivatives using digital simulations

Three C(60) derivatives, 1, 2 and 3, have been studied by cyclic voltammetry (CV) under high vacuum in anhydrous tetrahydrofuran (THF). The CV behavior was essentially similar to that already observed for other cyclopropanated fullerene derivatives. After the second reduction processes all compounds undergo a chemical reaction that generates another electroactive species. This "new" chemical species is likely to be the compound with the cyclopropane ring open. Differences in CV behavior were observed for the different addends. Electrochemical data obtained at different scan rates for a given potential window, were fit with the BAS digital simulation program, DigiSim. The purpose of this study was to probe the proposed mechanisms and to obtain reliable estimations of the kinetic constants for the homogeneous chemical reactions taking place during the CV experiments. Calculations at the PM3 level lend additional support to the conclusions derived from digital simulations. The proposed mechanism is similar for all the compounds and involves two main chemical reactions in a reversible square scheme.     

Application of horizontal staircase electrophoresis in agarose minigels to the random intergenic spacer analysis of clinical samples

The random intergenic spacer analysis is a recently developed technique for the study of microbial populations. The bacterial intergenic spacer (ITS) is located between 16S rRNA and 23S rRNA genes and presents different length and sequence among bacterial species. Therefore, the amplicons can be separated by electrophoresis commonly performed at low voltage during several hours. Although this technique is especially useful for unculturable microorganisms, it has not been applied before to clinical sample analysis. As these samples have a limited number of bacterial species, the size of the gels may be reduced to facilitate their handling and to reduce the running time. To obtain maximum separation among the ITS bands, we analysed in this work different electrophoretical conditions including staircase electrophoresis, a technique based on the application of several voltage steps. The results obtained showed a different behaviour of the electrical resistance during the performance of submarine horizontal and vertical staircase electrophoresis. In the first case the resistance decreased during most of the running time whereas in the second case it increased. Here, we show that the performance of horizontal staircase electrophoresis reduces the running time more than 80% with respect to conventional electrophoresis at low voltages. This procedure was applied to the separation of ITS bands from bacterial DNA present in a tissue from a vocal cord biopsy. The sequencing of these bands allowed their identification. This new procedure may be very useful in the rapid diagnosis of bacteria present in human, animal and plant tissues.     

Product Inhibited Enzymatic Reactions on the Rotating Disk Electrodes

A model of cyclic staircase voltammetry of enzymatic reactions on the rotating disk electrode is developed and the inhibition by the product is investigated. The responses are mostly time – dependent current – potential curves with maxima in the forward but not in the backward branch. If the maxima appear in both branches, the inhibition is potential – dependent either directly or indirectly. The source of the second maximum is explained.     

Understanding the unusual regioselectivity in the nucleophilic ring-opening reactions of gem-disubstituted cyclic sulfates. Experimental and theoretical studies

The regioselectivity of the nucleophilic ring-opening reactions of three gem-disubstituted cyclic sulfates with sodium azide has been studied from both experimental and theoretical viewpoints. It is found that, depending on the substituent present in the cyclic sulfate, the reaction displays reversed regioselectivity, which allows one or another regioisomer to be obtained with selectivities greater than 4:1. The theoretical calculations show that, contrary to previous understanding, the intrinsic preference in all cases is azide attack at the less-substituted C(beta) position, a consequence of similar stereoelectronic effects in the three sulfates considered. The observed preference for C(alpha) attack in the case of the ester sulfate is explained in terms of differential solvent effects, which are in turn due to subtle differences in the charge transfer in the different transition structures.     

Differential electrochemical behaviour of phytofabricated and chemically synthesized silver nanoparticles towards hydrogen peroxide sensing**

Silver nanoparticles (AgNPs) are one of the most widely used nanomaterials for biomedical applications. However, the impact of its synthesis by chemical and plant-mediated routes on its differential electrochemical behaviour has not been examined till date. Here, we report for the first time the differential study of the electrochemical behaviour of the AgNPs synthesized by different routes. First, the AgNPs were obtained by different routes (chemical and phytofabrication) and extensively characterized to compare their physical properties. Thereafter, a comparison of electron transfer kinetics between chemically synthesized (Ag−C) and phyto-fabricated (Ag-Phy) nanoparticles (NPs) has been studied by electrochemical techniques such as potentiodynamic cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). To further investigate the electrocatalytic properties of both types of AgNPs, we have used the peroxide moieties (H₂O₂), and the Ag−C NPs-based sensor probe has been reported to have four times better sensitivity than the Ag−Phy NPs-based sensor. The AgNPs modified sensor probes have also been tested in real-world environments to explore the consistency of their performance in complex matrices by using clinical urine samples, where we found comparable sensitivity to the standard conditions.     

Study of the Effects of Nonlinear Potential Sweeps on Voltammetry

Voltammetric methods use a constant sweep rate during the course of a scan. This paper reports a study of the influence of a nonconstant sweep rate on the voltammetric response. In this approach, either continuously increasing or continuously decreasing potential scan rates can be employed, unlike presently available methods which rely on a constant sweep rate. The voltammetric response of potassium ferrocyanide at a glassy carbon electrode was used as a model system to test the new method. The responses obtained using traditional staircase voltammetry (linear staircase voltammetry) and the new approach (nonlinear staircase voltammetry) were compared by experiment and by simulation. The new approach offers capability for signal enhancement, whereby enhanced current or enhanced peak shape can be obtained by choice of appropriate waveform parameters.     

BSSE‐free description of the formamide dimers

The different configurations (linear, zig‐zag, and cyclic) of formamide dimers have been studied at the level of both Hartree–Fock (HF) and second order Møller–Plesset perturbation theory (MP2). The widely used a posteriori Boys–Bernardi “counterpoise” (CP) correction scheme has been compared with our a priori methods utilizing the “chemical Hamiltonian approach” (CHA). The appropriate interaction energies have been calculated in six different basis sets (6‐31G, 6‐31G**, DZV, DZP, TZV, and cc‐pVDZ). © 2001 John Wiley & Sons, Inc. Int J Quant Chem, 2001