Ac Polarography Research Articles

Electrochemical reduction of azidophenyl-deoxynucleoside conjugates at mercury surface

Azidophenyl-linked nucleotides were previously used for redox-labelling of DNA. Lack of information about electrochemical behavior of aromatic azides in aqueous media and more complex reduction mechanism from to-date presented straightforward two electron reduction to corresponding amines and diatomic nitrogen led us to study the electrochemical reduction process more in detail. Model potassium 4-azidophenyltrifluoroborate (AzPTFB) and the corresponding AzP-linked nucleosides, 7-(4-azidophenyl)-7-deaza-2′-deoxyadenosine (dAAzP) and 5-(4-azidophenyl)-2′-deoxycytidine (dCAzP), were studied using DC and AC polarography, cyclic voltammetry and electrochemical impedance spectroscopy at dropping mercury electrode or at hanging mercury drop electrode in aqueous buffers. Electrochemical reduction of the azidophenyl conjugates is influenced by pH. Even if electrolysis of AzPTFB and dAAzP at mercury pool confirmed consumption of two electrons per molecule, simple two electron reduction of azide group to amine proceed only in acid electrolytes (pH < 2). Preparative chromatography of the solution obtained after dAAzP electrolysis in ammonium formate-phosphate buffer pH 6.9 and potassium chloride-phosphate buffer pH 6.9 revealed also presence of formylaminophenyl- and phenyl-derivatives. Nevertheless, intermediates such as dimers of the amine compounds and/or hydroxylamine-derivatives, which yield characteristic anodic peak in CV, are transiently formed. The intermediates are finally transformed to amines via reduction at more negative potentials. According to the results, mechanism of dAAzp electrochemical reduction is proposed in this work.

Electrochemical behavior of nickel bis-(2,6-diisopropylphenyl)diazobutadiene-1,4 dibromide in an aprotic solvent

The electrochemical behavior of bis-(2,6-diisopropylphenyl)diazobutadiene-1,4 and its nickel dibromide complex on dropping mercury and glassy carbon electrodes in an aprotic solvent was studied by AC polarography and cyclic voltammetry. Both the ligand and the nickel ion complex were shown to undergo electroreduction. The electrochemical reduction of nickel bis-(2,6-diisopropylphenyl)diazobutadiene-1,4 dibromide occurred stepwise and formed intermediate nickel(I) species.

The role of early stages of dioxygen activation in the biological and catalytic activity of phenol-type natural antioxidants

The main aspect of the A.N. Bakh peroxide theory of slow oxidation is the development of concepts on the role of peroxide compounds and the early stages of activation of molecular oxygen in reactions of biological oxidation. The use of modern physicochemical methods, namely, spectroscopy, pulse radiolysis, and ac polarography in studying the catalytic and antioxidant properties of natural compounds, flavonoids, makes it possible to detect the oxidation reaction intermediates which represent the partial-charge-transfer complexes [nQrδ+…mO2∞−] and also to explain the possibility of formation of nanostructured metal particles in reverse micelles in the presence of oxygen with quercetin Qr acting as reducing agent.

Electrochemical reduction of dihydroquercetin on mercury electrode

The methods of classical and ac polarography were used to study the electrochemical behavior of dihydroquercetin on a mercury dropping electrode. This process occurs only in protonic media. The protonated and nonprotonated forms of dihydroquercetin undergo electroreduction. The amount of electrons and protons participating in electroreduction of the studied flavonoid is determined. The nature of currents measured in the processes of electrochemical reduction of dihydroquercetin is studied.

The kinetics of reduction of palladium (II) complexes with β-alanine at dropping mercury electrode and the complexes’ stability constants

By using dc and ac polarography, the kinetics of electroreduction of the palladium (II) complexes with β-alanine at a dropping mercury electrode was studied in solutions with the palladium (II) concentration from 2 × 10−5 to 2 × 10−4 M and variable β-alanine and sodium perchlorate concentrations (pH 6–12). One polarographic wave was observed in solutions with pH 9 and 10 at the β-alanine overall concentration of cβala = 1 × 10−3 to 5 × 10−2 M; two waves, at lower pH or higher cβala. It was concluded on the formation of different forms of palladium (II) complexes in the studied solutions; the complexes contained two to four β-alanine coordinated anions. Using the limiting diffusion currents for the two waves at pH 9–11 and cβala = 0.1 and 0.5 M, the stepwise stability constant for the Pd(βala)42− complex was calculated. Using two ac peaks observed at pH 7–8 and cβala = 1 × 10−2 to 0.1 M, the stepwise stability constant for the Pd(βala)3−. was calculated. The perchlorate ions adsorbed at the dropping mercury electrode, as well as βala− anions at their higher concentrations, hamper the electroreduction of the palladium (II) complexes with β-alanine.

Effect of 4-hydroxynonenal on phosphatidylethanolamine containing condensed monolayer and on its interaction with apolipoprotein A-I

4-Hydroxynonenal (4HNE), generated during polyunsaturated fatty acid oxidation, is present in atherosclerotic lesions. As 4HNE is able to react with phosphatidylethanolamine (PE), we investigated, using AC polarography, whether it may alter the physico-chemical state of a condensed PE-containing phospholipid monolayer and its interaction with apoA-I. The stability of a phospholipid monolayer relative to potential (around the potential of zero charge) is dependent on lipid composition (PE > PC > PE/PC). ApoA-I insertion into PE/PC monolayer is easier than in PC monolayer. Pre-treatment of PE/PC monolayer by 4HNE does not alter monolayer stability, but decreases apo A-I insertion into the monolayer.

Interaction of an amphitropic protein (factor Xa) with membrane models in a complex system

Phosphatidylserine (PS) plays a crucial role, in the conversion of prothrombin into thrombin by the protease, factor Xa. Physiologically, the conversion occurs in the prothrombinase complex. The question of how water-soluble proteins that normally circulate in plasma bind remains to be unambiguously determined. We previously found that the amphitropic proteins (prothrombin, factors V and Va) penetrate into phospholipid layers. AC polarography has allowed the detection for the first time of insertion of factor Xa into condensed monolayers containing phosphatidylserine (PS) and phosphatidylcholine (PC) either 100% PS or 25% PS in the presence of Ca2+. This observation demonstrates that part of factor Xa can cross the phospholipid polar headgroup/hydrocarbon chain interface. In parallel experiments, radioactive surface measurements permitted measuring binding of tritium-labeled factor Xa onto a PS monolayer and calculate an association constant, 6x10(6) M(-1). Penetration of factor Xa into PS-containing vesicles was investigated also using photoactivable 5-[125I]iodonaphthalene-1-azide, which binds selectively to the lipid embedded domains of the protein. These experiments suggest that Factor Xa penetrates preferentially by its heavy chain, an alternative mode of binding to the commonly accepted binding via its Gla domain. Interaction of factor Xa with PS vesicles also changes its apparent K(m) for S 2222.

Electrode kinetics of Eu 3+ in sodium perchlorate solutions in dimethylsulfoxide

The influence of the concentration of the sodium perchlorate supporting electrolyte on the standard electrode reaction rate constants for the one-electron reduction of Eu(CF 3SO 3) 3 on mercury in dimethylsulfoxide has been studied. The k s app-values were derived from both ac polarography on the dropping mercury electrode and from cyclic voltammetry on a hanging mercury drop in a two-electrode system. The ac data was evaluated by Randles [Trans. Faraday Soc. 48 (1952) 937] and by Sluyters [Rec. Trav. Chim. 79 (1960) 1092] methods. The standard reaction rates from cyclic voltammetry were extracted via a simulation program. Densities, viscosities, specific resistances (resistivities) and double layer capacitances of the electrolytes were measured. Correlations between electrolyte properties and the k s app-values were investigated. The standard electrode reaction rates were also treated within the Frumkin theory.

Digital Pulse ac Voltammetry for the Simultaneous Analysis of Electroactive and Electrosorptive Species in Flow Systems

A digital two-step and three-step pulse potential ac voltammetry system was proposed and applied for the simultaneous analysis of electrosorptive and electroactive species in flow systems. To perform the ac polarography function, a PC was interfaced to a potentiostat to mimic all the necessary hardware functions of an analog ac polarograph. From the measurement of the change of phase-selective charging current and the zero-order current, I(3)(-), Br(-) specifically adsorbed and Cd(2+), Pb(2+) reduced at a hanging mercury drop electrode can be determined simultaneously in a FIA and IC system. With the digital pulse ac voltammetry-coupled IC, detection limits as low as 5.0 microM and linear dynamic ranges from 5.0 to 100 or 200 microM with linear correlation coefficients better than 0.9990 were found for the analysis of I(3)(-), Br(-), and S(2)O(3)(2)(-).

The influence of tetraethylammonium perchlorate on the electrode kinetics of Eu 3+ in dimethylsulfoxide

The effect of different concentrations of the supporting electrolyte tetraethylammonium perchlorate on the standard electrode reaction rate constants for the one-electron reduction of Eu(CF 3SO 3) 3 on mercury in dimethylsulfoxide has been measured. Two techniques: (i) ac polarography on the dropping mercury electrode; and (ii) cyclic voltammetry (CV) in a two electrode system were employed. The ac data were evaluated both by the Randles and by the Sluyters methods. The standard reaction rates from CV were extracted via a simulation program. Viscosities, resistivities, densities and the double layer capacitances of the electrolytes are given. Correlations between electrolyte properties and the k s app-values are discussed. The standard electrode reaction rates were analyzed according to the Frumkin theory.

Kinetics and Mechanism of Reduction of Palladium(II) Complexes with Ethanolamine on a Dropping Mercury Electrode

The kinetics of electroreduction of palladium(II) complexes with ethanolamine (mea) is studied by ac and dc polarography at pH 6.2–11.5 and different ethanolamine concentrations. At cmea ≈ 0.3–1 M and pH 6.2, commensurate quantities of Pd(mea)2+4 and Pd(mea)2+3 complexes are present in solutions. The latter, as opposed to the former, contains one bidentate-coordinated ligand. The equilibrium constant for the reaction of mutual conversion of these two complexes at pH 6.2 is determined, along with the equilibrium constant for a similar reaction at pH 7.2. A diffusion nature of observed limiting currents is established. Kinetic parameters of the slow electrochemical stage are determined. The E1/2 shift in the negative direction with pH adjusted from ∼ 9.5 to 11.5 is due to the preceding reversible chemical stage of protonation of the Pd(mea)3(H–1mea)+ complexes predominant in solution in the pH interval mentioned.

Electrode kinetics of Eu3+ in dimethylsulfoxide

Standard electrode reaction rate constants for the one-electron reduction of Eu(CF 3 SO 3 ) 3 on mercury in different concentrations of the supporting electrolyte tetrabutylammonium perchlorate in dimethylsulfoxide have been measured. Two techniques (i) ac polarography on the dropping as well as on the hanging mercury electrode and (ii) cyclic voltammetry in a two electrode system were employed. The evaluation of the ac data was carried out by the Randles and the Sluyters methods. The reaction rates from cyclic voltammetry were extracted via a simulation program. Physical properties such as the viscosities, resistivities and the double layer capacitances necessary for the evaluation of the data are given. The decrease in the standard electrode reaction rates with increasing supporting electrolyte concentrations was accounted for in the Frumkin model.

Domains of human prothrombin embedded into vesicles: relation with biological activity

In our previous works, ac polarography experiments gave evidence of penetration of prothrombin into phosphatidylserine (PS) containing monolayers and labeling of prothrombin by the highly apolar azide 5[ 125 I ]-iodonaphthalene-1-azide (INA) could be used to monitor penetration of the zymogen into phospholipid vesicles. In the present study, the photochemical labeling method was used to detect which domains in the whole prothrombin molecule are embedded in 75% PS–25% PC containing vesicles; the results are compared to those obtained on monolayers by ac polarography with isolated purified fragments of the protein. The kinetics of cleavage of prothrombin following INA labeling, in the presence of phospholipids, by factor Xa or thrombin, as well as the nature of the products formed, are reported here. The cleavages of radiolabeled INA-II and non-radiolabeled II could be independently followed by the formation of radiolabeled and non-radiolabeled products. It was found that prethrombin 2 is the main penetrating domain, as compared with prethombin 1 and fragment 1, while fragment 2 does not penetrate. Both INA-II and II were activated by factor Xa to give the expected products but INA-II was cleaved faster at Arg 271. This finding suggests that the interaction with vesicles which leads to INA-labeling of prothrombin, results in a conformational change of the protein that enhances its susceptibility to activation. In gel electrophoresis under reducing conditions, subsequent cleavage at Arg 320 resulted in the formation of radiolabeled thrombin B-chain (TB). Since thrombin is formed from radiolabeled prothrombin, this shows that the prothrombin molecules which are embedded in the membrane are biologically active.

The role of protonation, hydration, elimination, and ring opening in the electroreduction of hexazinone

The herbicide hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (lH,3H)-dione) is a 1,3,5-triazine derivative which exists in a single cyclic tautomeric form. In the presence of water its azomethine bond is covalently hydrated (Kd=0.79, in acetonitrile as a predominant co-solvent) and protonated (pKa=1.2). Following dehydration, the protonated form of the azomethine bond undergoes a two-electron reduction in wave I1. The decrease in current at pH>2.5 results from the decrease in the rate of protonation (kr=6.3×106 mol−1 s−l). The current at pH 2 to 4 is smaller than theoretical due to a competitive hydration. The two-electron reduction product formed in waves undergoes elimination of dimethylamine. The resulting azomethine bond in the product of elimination is reduced in wave I2. The oxidized form of hexazinone undergoes an acid-catalyzed ring opening at pH<1.3. The acid catalysis is attributed to a protonation of the hydrated form of the heterocycle. The formyl group of the open-chain form is reduced in a more negative wave I1a. Ac polarography indicated adsorption of the oxidized form of hexazinone, dc polarography and voltammetry show also an adsorption of its reduced form.

Adsorption behaviour of insulin and soybean trypsin inhibitor at the mercury∣solution interface

Abstract The adsorption of insulin and soybean trypsin inhibitor (STI) on the mercury electrode was investigated with phase-sensitive alternating current (ac) polarography. The investigations focused on the effect of insulin and STI concentrations on the capacitive ac component over a wide potential range at pH 8.8, when both proteins are in folded form, and at pH 1.0 when they are unfolded. Besides a pseudo-capacitance due to the faradaic process of protein –S–S– bond reduction, the presence of protein adsorption of the Frumkin type was established. From the analysis of the corresponding adsorption isotherms, the relevant thermodynamic adsorption parameters were determined. Similarities in the adsorption characteristics of both proteins studied were demonstrated, and the differences in the adsorption behaviour between native and denatured proteins were proven. The data obtained is also discussed from the aspect of protein disulphide bond interaction with mercury.

Study of Cephalexin and Cefaclor Adsorption at the Mercury/Solution Interface by AC Polarography

The adsorptive properties of Cephalexin (Cef), Cefaclor (Cfx), and 7-aminodesacetoxycephalosporanic acid (7-ADCA) were examined by applying phase sensitive ac polarography with DME in order to compare the behavior of their molecules at a mercury electrode. To find optimal conditions for the distinct adsorption effects and to determine the ionic species participating in adsorption, the influence of pH and concentration (concerning electrical double layer characteristics at the mercury/solution interface) was studied.

Electrochemical reduction of secnidazole and its determination in tablets

AbstractThe electrochemical reduction of secnidazole has been carried out in aqueous solution in the pH range of 2.0–12.0 by DC polarography, AC polarography and cyclic voltammetry. Two well‐defined reduction waves appear in acidic media, whereas at higher pHs the two waves overlap. The variation of E1/2 or Ep and Ilim or Ip with pH was studied. The diffusion‐controlled nature of the waves was established and the irreversibility of the electrode process was verified by different criteria. The mechanism of reduction was discussed. A method for the detemination of secnidazole by DC polarography at pH 8.5 which allows quantitation over the 1 × 10−5 M‐4 × 10−4 M range was proposed and applied to the determination of secnidazole in tablets. The mean recovery was 99.67% and the relative standard deviation was 0.89%. Excipients did not interfere in the determination.

Hydroacridines. Part 16.1 Steric effect in the polarographic reduction of N-epimeric amine oxides derived from (4aα,8aβ,9aβ,10aα)- and (4aα,8aα,9aβ,10aα)-tetradecahydro-10-methylacridine

The electrochemical reduction of (4aα,8aβ,9aβ,10β,10aα)- (1a), (4aα,8aβ,9aβ,10α,10aα)- (1e), (4aα,8aα,9aβ,10β,10aα)- (2a) and (4aα,8aα,9aβ,10α,10aα)-tetradecahydro-10-methylacridine-10-oxide (2e) has been investigated by dc and ac polarography. For both of these N-epimeric pairs of amine oxides, the polarographic behaviour is influenced in the same way by the steric orientation of the N–O bond. So, whereas the epimers with axial oxygen (1a and 2a) give normal, single-wave polarographic curves, both epimers with equatorial oxygen (1e and 2e) give unusually shaped, double-wave curves. The steric configuration of the tricyclic framework affects only the values of the half-wave potentials and not the shape of the polarographic curves. These results suggest that polarography could be used to assign the steric configurations of the N–O bonds in saturated heterocyclic amine oxides, or to support assignments achieved by other methods.

Behaviour of β2-microglobulin in the electrical double layer of the mercury/solution interface

β 2-Microglobulin ( β 2-m) is a small globular protein (12000 Da) which exists as a monomer in numerous body fluids. Data published on the electrochemical behaviour of β 2-m are scarce. In this paper an attempt was made to ge more information on the structure of the double layer at mercury/solution interface in the presence of β 2-m. Using phase selective ac polarography the capacitive current-potential ( C- E) curve was analysed as the function of various β 2-m concentration, pH of medium, temperature, denaturation agent concentration etc. The data obtained are discussed particularly from the aspect of folding/unfolding state of the protein studied.

Electrochemical behaviour of some naturally occurring hydroxy derivatives of 9,10-anthraquinone in chloroform at mercury and glassy carbon electrodes: Application of AC polarography to the analysis of rhubarb roots

Electrochemical behaviour of some naturally occurring hydroxy derivatives of 9,10-anthraquinone such as chrysophanic acid (Ch), rhein (Rh) and emodin (Em) at mercury and glassy carbon electrodes using two different supporting electrolytes in chloroform is described. In the presence of piperidinium perchlorate (0.75 M) + piperidine (0.25 M) as a suitable acid supporting electrolyte, the reduction of Ch and Rh was a reversible two-electron process without complicating chemical reactions or adsorption phenomena, but the reduction process of these compounds in the presence of tetrabutylammonium perchlorate (TBAP) 0.5 M + 5% acetic acid (AcOH) in chloroform was quasi-reversible. In both supporting electrolytes the overall reduction process of Em was irreversible. In the AC polarography and DP voltammetry at a GC electrode, the detection limit for Ch, Rh and Em was acceptably low and the relative standard deviation for the determination of 5 × 10 −6 M level, never exceeded 2%. AC polarography has been used for the determination of Ch and Em in a local rhubarb sample after extraction into chloroform and separation by column chromatography.