455 — Effect of pH on the electroactivity of horse heart cytochrome c

The effect of pH on the electroactivity of horse heart cytochrome c has been studied by differential pulse polarography, differential pulse voltammetry at the gold electrode, cyclic voltammetry and spectro-photometry; the gold electrode has been activated by 4.4′-bipyridine. The experimental results have been interpreted on the basis of the existence of a pH-dependent equilibrium between two electroactive cytochrome c forms; in agreement with the Lambeth et al.'s scheme the existence of transient species can be postulated.From spectrophotometric and differential pulse voltammetric experiments the pK_a value of 8.1±0.1 has been calculated for the equilibrium between the neutral and alkaline cytochrome c forms: state III ? state IV+H⁺.     

An electrochemical study of the stability of cytochrome c3 from Desulfovibrio desulfuricans Norway strain

The electrochemical behaviour of cytochrome c₃ from Desulfovibrio desulfuricans, Norway strain, remains unchanged within the pH range 5–9.5. Differential pulse polarography peaks disappear at pH < 5 and two transitions are noted at alkaline pH (10.5 and 12). In denaturation experiments, guanidinium chloride has a more marked effect than urea on differential pulse polarography and cyclic voltammetry peaks.The study of absorption spectra shows that the δ band at 351 nm of the oxidized form increases above pH ∼ 10.Neither absorption spectra nor polarograms are modified after the treatment of cytochrome c₃ for 10 min at 100°C.It is concluded that hydrogen bonds must play a prominent role in maintaining the tertiary structure of the protein. Cytochrome c₃ from D. desulfuricans Norway is a particularly stable molecule as regards pH, denaturing agents, temperature and ageing.     

Electrochemical characterization of a superoxide biosensor based on the co-immobilization of cytochrome c and XOD on SAM-modified gold electrodes and application to garlic samples

This paper describes the characterization and optimization of an amperometric cytochrome c (cyt c)-based sensor for the determination of the antioxidant capacity of pure substances and natural samples. The cyt c and the xanthine oxidase (XOD) enzyme were co-immobilized on the electrode using the combination of several long-chain thiols. The self-assembled monolayer (SAM) was optimized in terms of composition and ratio between thiols. The immobilization protocol for both cyt c and XOD and the SAM formation time were evaluated through electrochemical methods, such as cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA) and impedance spectroscopy (IS). Finally, the biosensor was applied to the determination of the antioxidant capacity of pure alliin and two compounds extracted from garlic bulbs.     

Study of the electrochemical behaviour of amphetamine and its derivatives in aqueous solution

The polarographic behaviour of amphetamine-type drugs in acid and alkaline solutions was investigated. Differential pulse polarography (DPP) with a dropping mercury electrode (DME) was used to monitor these drugs in aqueous solution. DPP and cyclic voltammetry (CV) were also utilised to study the electrode process of some 2-phenylethylamines derivatised with 4-N,N-dimethylaminobenzaldehyde (Ehrlich’s reagent) and 2,4-dinitrofluorobenzene, known as Sanger’s reagent, respectively.     

Study of the electrochemical behaviour of amphetamine and its derivatives in aqueous solution

The polarographic behaviour of amphetamine-type drugs in acid and alkaline solutions was investigated. Differential pulse polarography (DPP) with a dropping mercury electrode (DME) was used to monitor these drugs in aqueous solution. DPP and cyclic voltammetry (CV) were also utilised to study the electrode process of some 2-phenylethylamines derivatised with 4-N,N-dimethylaminobenzaldehyde (Ehrlich’s reagent) and 2,4-dinitrofluorobenzene, known as Sanger’s reagent, respectively. Received: 19 March 1998 / Revised: 17 September 1998 / Accepted: 21 September 1998     

Voltammetric study of some macromolecule - metal complexes

The usefulness of a voltammetric model for studying macromolecule - metal complex systems is verified. Complexes formed by a metal ion (Zn(II), Cd(II), Pb(II) or Cu(II)) and a macromolecule (polymethacrylic (PMA), polyacrylic (PAA), alginic (AA) or polygalacturonic (PGA) acids) are investigated. Several voltammetric techniques have been used: sampled direct current polarography (DCP), normal pulse polarography (NPP), differential pulse polarography (DPP), reverse pulse polarography (RPP), cyclic voltammetry (CV) and differential pulse anodic stripping voltammetry (DPASV). Results confirm the validity of the model, in determining not only formation constants but also diffusion coefficients of the complexes. The behaviour of some systems agrees with that predicted by the model. Nonetheless, other systems only partially fulfil the predictions for different reasons, which are discussed.     

Polarographic Behaviour of Orotic Acid (Vitamin B13). Anodic Wave

Abstract

The electroanalytical anodic behaviour of orotic acid has been studied at several pH values, using DPP and CV techniques. The orotic acid undergoes an anodic wave in both techniques.

The best conditions for the determination of orotic acid with the mentioned techniques was studied.

The determination of Hg(II) by classic polarography, DC, was performed.     

Electrochemical and spectroscopic characterization of interaction between antimalarial drug cinchonine and human serum albumin at physiological pH

In the present study, the interaction between cinchonine (CCN) and human serum albumin (HSA) was investigated using differential pulse polarography (DPP), cyclic voltammetry (CV) and spectroscopic techniques in Britton-Robinson (B-R) buffer pH 7.4. CCN displayed a main cathodic peak at ?1.228 V (vs. Ag|AgCl|KCl_sat) on mercury working electrode. The addition of HSA into CCN sulfate solution resulted in the decrease of the main reduction peak current of CCN and no new peaks appeared. The decay in the peak current of CCN, after the addition of HSA, showed a decrease in free drug concentration and formation of a biocomplex. The peak current changes of CCN in the presence of HSA were followed by DPP to determine the binding parameters. The logarithm of binding constant and binding ratio between CCN and HSA were 6.128 and 1: 1, respectively. This interaction was also confirmed by UV-Vis and FTIR-ATR spectroscopic measurements.     

Interfacing cytochrome c to electrodes with a DNA – carbon nanotube composite film

Multi-walled carbon nanotube (MWCNT) is successfully immobilized on the surface of platinum electrode by mixing with DNA. The DNA/MWCNT modified electrodes are characterized by electrochemical impedance spectroscopy and cyclic voltammetry. Further research indicates that cytochrome c can strongly adsorbed on the surface of the modified electrode, and forms an approximate monolayer. The immobilized MWCNT can promote the redox of horse heart cytochrome c which gives reversible redox peaks with a formal potential of 81 mV vs SCE.     

A spectroelectrochemical study of carboxymethylated cytochrome-c

Direct electrochemical reduction of dicarboxymethylated cytochrome c [(Cm)-cyt c] has been carried out to investigate the effect of the displacement of methionine 80 (Met 80) from the sixth coordination position on the redox potential of heme iron. Differential pulse and cyclic voltammetry were employed in the present study. At gold microelectrodes or gold-coated RVC electrodes, the reduction process occurs in the absence of mediators. The rate of heterogeneous charge transfer for (Cm)-cyt c appears to be greater than that operative in native cytochrome c. A value of −0.218 versus s.h.e. at neutral pH was determined for the formal standard potential (U′_0,7) from spectroelectrochemical measurements.The large change in U′₀ of the modified cytochrome c with respect to the native protein (ΔU ⋍0.5 V) is in agreement with rearrangement of the tertiary structure induced by rupture of the Met 80-heme iron bond, which leads to a high degree of heme exposure to the solvent. In addition, the results support the important role of Met 80 in assuring stability, through its bond with the heme iron, to the close crevice structure.     

Electrochemical studies of berberine and jatrorubine by pulse polarography

The application of pulse polarography (PP) and differential pulse polarography (DPP) for the quantitative determination of berberine and Jatrorubine in aqueous, acidic solutions was investigated. The proposed methods could be successfully applied in the concentration range between 5·10^–7 (DPP) or 1·10^–6 (PP) and 5·10^–5 mol/dm³ of alkaloids.     

The synthesis and electrochemistry of 2,5-dimethylazaferrocenes with heteroaryl bridges

We report on the synthesis of complexes having two equivalent redox active 2,5-dimethylazaferrocenyl entities connected by heteroaryl (heteroaryl = thiophenyl, bithiophenyl and pyridyl) bridges. The new compounds have been investigated by various electrochemical techniques including cyclic voltammetry (CV), differential pulse voltammetry (DPV) and square wave voltammetry (SW) and were found to exhibit two consecutive reversible or partially reversible one-electron oxidations. Comproportionation constants (K_c) calculated from ΔE_1/2 values indicate that the thermodynamic stability of their monoxidized forms exceeds those of analogous ferrocenes. In this paper we also report the X-ray crystal structure and UV–Vis spectroelectrochemistry of parent 2,5-dimethylazaferrocene.     

The influence of disproportionation of the anion radical on the electrochemical reduction of 1,9-dimethyldibenzo[b,f]pentalene

The electrochemical reduction of 1,9-dimethyldibenzo[b,f]pentalene has been investigated in DMF by both cyclic voltammetry (CV) and dc polarography. The influence of the depolarizer concentration and electrode material (Pt and Hg) was studied. It was found that the first electron transfer is a reversible one under most conditions studied, whereas the second electron transfer was irreversible on Pt and quasi-reversible on the HMD. An overall EDISP.EC mechanism was suggested and discussed. It turned out that the first reduction process was accompanied by a disproportionation of the anion-radical to its parent hydrocarbon and dianion. The experimental waves were analysed on the basis of various theoretical procedures.     

Electrochemistry and biosensing activity of cytochrome c immobilized in macroporous materials

An amperometric biosensor for hydrogen peroxide (H₂O₂) has been constructed by immobilizing cytochrome c on an indium/tin oxide (ITO) electrode modified with a macroporous material. Cyclic voltammetry showed that the direct and quasi-reversible electron transfer of cytochrome c proceeds without the need for an electron mediator. A surface-controlled electron transfer process can be observed with an apparent heterogeneous electron-transfer rate constant (k_s) of 29.2?s^?1. The biosensor displays excellent electrocatalytic responses to the reduction of H₂O₂ to give amperometric responses that increase steadily with the concentration of H₂O₂ in the range from 5???M to 2?mM. The detection limit is 0.61???M at pH?7.4. The apparent Michaelis-Menten constant (K_m) of the biosensor is 1.06?mM. This investigation not only provided a method for the direct electron transfer of cytochrome c on macroporous materials, but also established a feasible approach for durable and reliable detection of H₂O₂.

Interfacial electrochemistry of cytochrome c at tin oxide,indium oxide,gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10^?2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.     

Direct electrochemical behavior of cytochrome c, and its determination on phytic acid modified electrode

Phytic acid (PA) with its unique structure was attached to a glassy carbon electrode (GCE) to form PA/GCE modified electrode which was characterized by electrochemical impedance. The electrochemical behavior of cytochrome c (Cyt c) on the PA/GCE modified electrode was explored by cyclic voltammetry and differential pulse voltammetry. The Cyt c displayed a quasi-reversible redox process on PA modified electrode pH 7.0 phosphate buffer solution with a formal potential (E ^0′) of 57 mV (versus Ag/AgCl). The peak currents were linearly related to the square root of the scan rate in the range of 20–120 mV·s^?1. The electron transfer rate constant was determined to be 12.5 s^?1. The PA/GCE modified electrode was applied to the determination of Cyt c, in the range of 5?×?10^?6 to 3?×?10^?4 M, the currents increase linearly to the Cyt c concentration with a correlation coefficient 0.9981. The detection limit was 1?×?10^?6 M (signal/noise?=?3).     

Polarographic study on the evolution of the diphenylamine as stabiliser of the solid propellants

Differential pulse polarography (DPP) and square wave voltammetry (SWV) were investigated, in order to know the stability of solid propellants which contain diphenylamine. The simultaneous determination of N-nitrousdiphenylamine (NnDPA), 2-nitrodiphenylamine (2nDPA), 4-nitrodiphenylamine (4nDPA) and 2,4-dinitrodiphenylamine (2,4dnDPA) by DPP and SWV was proposed due to these nitro-derivatives appear during the stabilisation process from degradation of diphenylamine (DPA) used as stabiliser in propellant compositions. The proposed methods were successfully applied to the simple base solid propellant (with nitrocellulose as the only active component), with its stabiliser DPA. In all cases and with both the techniques, detection limits ≤0.01 ppm were obtained.When the usual LC procedure was applied to the real sample, no significant differences were found between the obtained results and those given by the electroanalytical techniques. In addition, the detection limits reached by the electrochemical methods were better than those obtained by LC.Moreover, the proposed procedure can be considered an objective test that would avoid the use of the classic stability tests and would allow one to determine the stability of propellants accurately, faster and cheaper than LC methods.     

Electrochemistry and biosensing activity of cytochrome c immobilized on a mesoporous interface assembled from carbon nanospheres

We report on an amperometric biosensor for hydrogen peroxide. It is obtained via layer-by-layer assembly of ordered mesoporous carbon nanospheres and poly(diallyldimethylammonium) on the surface of an indium tin oxide (ITO) glass electrode and subsequent adsorption of cytochrome c. UV–vis absorption spectroscopy was applied to characterize the process of forming the assembled layers. Cyclic voltammetry revealed a direct and quasi-reversible electron transfer between cytochrome c and the surface of the modified ITO electrode. The surface-controlled electron transfer has an apparent heterogeneous electron-transfer rate constant (k _s ) of 5.9?±?0.2?s^?1 in case of the 5-layer electrode. The biosensor displays good electrocatalytic response to the reduction of H₂O₂, and the amperometric signal increase steadily with the concentration of H₂O₂ in the range from 5?μM to 1.5?mM. The detection limit is 1?μM at pH 7.4. The apparent Michaelis-Menten constant (K _m ) of the sensor is 0.53?mM. We assume that the observation of a direct electron transfer of cytochrome c on mesoporous carbon nanospheres may form the basis for a feasible approach for durable and reliable detection of H₂O₂.

The Use of Differential Pulse and D.C. Polarography in the Analysis of Solutions Containing Surfactants

Abstract

Peak currents in differential pulse polarography (DPP) are often used for quantitative analysis. In the presence of surfactants, peak currents depend not only on the concentration of the electroactive species, but also on that of the surfactant. In the presence of surfactants of biological origin, limiting currents obtained by d.c. polarography are often less sensitive to the effects of surfactants and are more suitable for quantitative analysis than DPP peak currents. Effects of surfactants on DPP curves were demonstrated for 4-nitrobenzoic acid in the presence of oligomers resulting from washing lignin preparations with water. In the study of adsorption of nitro compounds on lignin, d.c. polarographic curves yielded reliable adsorption isotherms whereas DPP peak currents indicated erroneously much stronger adsorption of nitro compounds on lignin.     

Electrochemical determination of rate constants and product yields for the spontaneous dediazoniation of p‐nitrobenzenediazonium tetrafluoroborate in acidic aqueous solution

We have examined the kinetics and mechanisms of the dediazoniation of p‐nitrobenzenediazonium tetrafluoroborate in acidic aqueous solutions by employing differential pulse polarography (DPP) and differential pulse voltammetry (DPV) on a glassy carbon electrode combined with the use of a coupling reaction to quench unreacted p‐nitrobenzenediazonium ion. These electrochemical techniques show an effective sensitivity and selectivity for detecting arenediazonium ions and arenedediazoniation products under the appropriate experimental conditions (pH, solvent, electrolyte), which allows simultaneous monitoring of the rates of arenediazonium ion loss and product formation and determination of product yields. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 419–430, 2000