A.M. Oliveira Brett

Electrochemical Oxidation of 8‐Oxoguanine

Electrochemical oxidation of DNA can occur at each of the four bases and guanine is the one that can suffer the easiest oxidative damage. The occurrence of the guanine oxidation product, 8-oxoguanine, as a consequence of DNA damage caused by DNA oxidation causes important mutagenic lesions and hence it is very important to develop reliable methods for its quantification. Electrochemical study of the mechanism of oxidation of 8-oxoguanine on glassy carbon shows that it is a reversible electrode process, pH dependent, and involves several reaction products. Electroanalytical determinations of 8-oxoguanine were carried out and the detection limit was 8×10–7 M.

Detection of the damage caused to DNA by niclosamide using an electrochemical DNA-biosensor

Niclosamide is the only commercially available molluscicide recommended by the WHO for large-scale use in schistosomiasis control programs. The electrochemical reduction and oxidation mechanism of niclosamide was studied using cyclic, differential and square wave voltammetry, at a glassy carbon electrode. An indirect procedure for in situ quantification of niclosamide using batch injection analysis with electrochemical detection, possible to be used for in situ determinations in river streams and effluents, was developed. It enabled a detection limit of 8 x 10(-7) M. The investigation of the niclosamide-DNA interaction using an electrochemical DNA-biosensor showed for the first time clear evidence of interaction with DNA and suggested that niclosamide toxicity can be caused by this interaction, after reductive activation.

Electrochemical oxidation of mitoxantrone at a glassy carbon electrode

Mitoxantrone is an anthracycline used as an antitumour antibiotic for leukaemia and breast cancer treatment, due to its interaction with DNA. However, the molecular mechanism of the antitumour action is not completely understood. Using a glassy carbon electrode the electrochemical oxidation of mitoxantrone was shown to be a complex, pH-dependent, irreversible electrode process involving several metabolites. Comparison of the electrochemical oxidation behaviour of mitoxantrone, ametantrone and aminantrone enabled a deeper understanding of the mechanism and showed the relevance of electrochemical data for the understanding of the cytotoxicity of mitoxantrone. Since mitoxantrone and its oxidation products adsorb strongly on the electrode surface, causing severe problems of electrode fouling, reproducible electroanalytical determinations could only be done at very low concentrations and in an aqueous buffer supporting electrolyte containing 30% ethanol. The detection limit obtained was 10 ˇ7 M. # 1999 Elsevier Science B.V. All rights reserved.

Electrochemical studies and square wave adsorptive stripping voltammetry of the antidepressant fluoxetine

The electrochemical reduction of the antidepressant drug fluoxetine was investigated by cyclic, linear sweep, differential pulse and square wave voltammetry using a hanging mercury drop electrode in alkaline buffer solution in water and in a water/acetonitrile mixed solvent. Cyclic voltammograms in aqueous solution showed very strong adsorption of fluoxetine on the electrode with formation of a compact film. The effect of addition of different percentages of acetonitrile on the voltammetric response was evaluated. It is shown that acetonitrile protects the electrode surface, thus preventing the adsorption of fluoxetine as a compact film, although reduction occurs at more negative potentials. Adsorption was used to accumulate the drug onto the electrode surface. The adsorbed species were measured voltammetrically by reduction at -1.3 V in an aqueous 0.05 M Ringer buffer, pH 12, 20% acetonitrile v/v. Linear calibration graphs were obtained in the range 0.52-5.2 M. The quantification of fluoxetine in pharmacological formulations existing in the market was performed using adsorptive square wave cathodic stripping voltammetry. and compared with data from UV spectrophotometry. The method is simple and not time-consuming. A comparative high performance liquid chromatography assay with UV detection was performed. Recovery data for both methods are reported.

Properties of polyaniline formed at tin dioxide electrodes in weak acid solution: effect of the counterion

Polyaniline films have been formed at tin dioxide electrodes in aqueous 0.4m acetate buffer solutions of pH between 3.5 and 5.5 with added nitrate, perchlorate or halide electrolyte, in order to study the effect of the counterion on the structure and electrochemical properties of the polyaniline film. The films were characterized electrochemically by cyclic voltammetry and by impedance, and morphologically by scanning electron microscopy. Marked differences between the films are found, the highest conductivity and electrochemical response being with nitrate counterion. The films formed are generally flat and porous, with evidence of some small irregular growths. These results are compared to those obtained in sulphuric acid solution.

Electrochemical oxidation of propanil and related N-substituted amides

The electrochemical behaviour of propanil and related N-substituted amides (acetanilide and N,N-diphenylacetamide) was studied by cyclic and square wave voltammetry using a glassy carbon electrode. Propanil has been found to have chemical stability under the established analytical conditions and showed an oxidation peak at +1.27 V versus Ag/AgCl at pH 7.5. N,N-diphenylacetamide has a higher oxidation potential than the other compounds of +1.49 V versus Ag/AgCl. Acetanilide oxidation occurred at a potential similar to that of propanil, +1.24 V versus Ag/AgCl. These results are in agreement with the substitution pattern of the nitrogen atom of the amide. A degradation product of propanil, 3,4-dichloroaniline (DCA), was also studied, and showed an oxidation peak at +0.66 V versus Ag/AgCl. A simple and specific quantitative electroanalytical method is described for the analysis of propanil in commercial products that contain propanil as the active ingredient, used in the treatment of rice crops in Portugal.

Reduction of Lapachones in Aqueous Media at a Glassy Carbon Electrode

The electrochemical reduction of -lapachone, -lapachone, -lapachone-sulfonic acid and 3-bromo--lapachone was studied by cyclic voltammetry, square-wave and differential pulse voltammetry in mixed ethanolic (20%) aqueous buffered media (pH 4.5) using a glassy carbon electrode. The reduction showed, for the ortho-quinones, well-defined pH-dependent reversible peaks corresponding to a mechanism involving the same number of electrons and protons, typical of quinones. For the para-quinone, at pH 4.5, an irreversible pair of peaks is evident, corresponding probably to an EC system. Disproportionation of the radical together with a coupled chemical reaction after the first electron transfer seems to occur. The chemical reaction is probably a dihydropyran ring cleavage, leading to intermediates, that are oxidized in its more stable forms, one of them very similar to the one presented by -lapachone. The hydrogenbonding-aided stabilization of the radicals generated during -lapachone and derivatives reduction, after electron and proton capture confers more stability to the reduced ortho-lapachones in comparison to their reduced para-isomers. At pH 7.0, the reduction of -lapachone is quasi-reversible. In all cases, the ortho-quinones are reduced easier than the para-isomers. The electrochemical method enables the study of the stability of these quinones after reduction, and allows their detection and quantification which confers to this methodology an extra advantage. For the soluble sulfonic acid the detection limit is 1.7 10 6 M.

Electrochemical behaviour of cytochrome c at electrically heated microelectrodes

The structural changes in cytochrome c with temperature have been been followed using a recently developed electrically-heated microelectrode sensor. Differential pulse voltammetry was used to perform electrochemical measurements of cytochrome c oxidation at different temperatures at heated bare gold electrodes contained in phosphate-buffered cytochrome c solution at room temperature. The voltammetric response shows the onset of unfolding and a marked dependence of the signal on electrode temperature. This augurs well for applications of heated electrodes as local probes in the study of the temperature dependence of electron transfer processes of other redox proteins, avoiding problems of bulk deterioration.

Transient measurements at the wall-jet ring disc electrode

Theory is presented for the wall-jet ring disc electrode (WJRDE) which predicts the current transient at the ring resulting from a potential step at a WJRDE disc for the case where the ring is potentiostatted so as to reverse the reaction which occurs at the disc. The shape of such transients, in which double-layer charging effects are minimal, is revealed to be very sensitive to differences in the diffusion coefficient of the electroactive species which reacts at the disc and that of the corresponding product. Accordingly such measurements are recommended for the study of complex electrode processes in order to separate mass transport parameters (diffusion coefficients) from homogeneous kinetic phenomena.

Wall-jet electrodes : the importance of radial diffusion

Existing models for mass transport to wall jet electrodes (WJE) are critically re-evaluated in the light of some new calculations and experiments which relate to steady state and transient currents observed at wall jet ring disc electrodes both in the presence and absence of homogeneous chemical kinetic complications. Specifically, it is concluded that in many cases quantitative descriptions are only realisable if transport to the electrode is described as having a significant contribution from radial diffusion in addition to the radial convection and normal diffusion that are usually only considered. This greatly increases the complexity of the mathematical solution of problems of interest and may prove an important limitation of WJEs vis-a-vis alternative hydrodynamic electrodes for other than analytical purposes.