Silvia H.P. Serrano

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.

On the adsorption and electrochemical oxidation of DNA at glassy carbon electrodes

Abstract New experimental results concerning the adsorption and anodic oxidation of DNA denatured in acid at glassy carbon electrodes using differential pulse, square wave and cyclic voltammetry, and impedance have been obtained, in the pH range 0–12 and as a function of adsorption potential and time of adsorption. The assumption up until now of a completely irreversible oxidation of guanine and adenine residues is shown not to be true. An explanation for these and previous results, based on desorption of the oxidation products and their diffusion away from the electrode surface, physically impeded for multilayer adsorption of DNA residues, is proposed. This situation corresponds to long adsorption times, as is the case here, and is corroborated by scanning electron microscopy.

Simultaneous voltammetric detection of ascorbic acid, dopamine and uric acid using a pyrolytic graphite electrode modified into dopamine solution.

Pyrolytic graphite electrodes (PGE) were modified into dopamine solutions using phosphate buffer solutions, pH 10 and 6.5, as supporting electrolyte. The modification process involved a previous anodization of the working electrode at +1.5 V into 0.1 mol L(-1) NaOH followed by other anodization step, in the same experimental conditions, into dopamine (DA) solutions. pH of the supporting electrolyte performed an important role in the production of a superficial melanin polymeric film, which permitted the simultaneous detection of ascorbic acid (AA), (DA) and uric acid (UA), DeltaE(AA-DA)=222 mV; DeltaE(AA-UA)=360 mV and DeltaE(DA-UA)=138 mV, avoiding the superficial poisoning effects. The calculated detection limits were: 1.4x10(-6) mol L(-1) for uric acid, 1.3x10(-5)molL(-1) for ascorbic acid and 1.1x10(-7) mol L(-1) for dopamine, with sensitivities of (7.7+/-0.5), (0.061+/-0.001) and (9.5+/-0.05) A mol(-1) cm(-2), respectively, with no mutual interference. Uric acid was determined in urine, blood and serum human samples after dilution in phosphate buffer and no additional sample pre-treatment was necessary. The concentration of uric acid in urine was higher than the values found in blood and serum and the recovery tests (92-102%) indicated that no matrix effects were observed.

Electrochemical reduction of metronidazole at a DNA-modified glassy carbon electrode☆

Abstract The electrochemical reduction of metronidazole was investigated using, for the first time, the newly developed DNA-modified glassy carbon electrode. The results are compared with reduction at bare glassy carbon electrodes. The potentials for reduction were less negative when using the DNA-modified glassy carbon electode although the mechanism was the same. A potential-pH dependence of one electron per proton was observed in acid media whereas for neutral and alkaline solution no dependence was found. The DNA-modified glassy carbon electrode enables preconcentration of the sample for chosen times on the electrode surface which is convenient for analytical applications. The limit of detection using the DNA-modified glassy carbon electrode at pH 4.5 with 2 min preconcentration is 1.67 μM, and without preconcentration it is three times higher, 3.25 μM. At a bare glassy carbon electrode, where preconcentration is not possible, the limit of detection for the same pH is 3.44 μM.

An EIS study of DNA-modified electrodes

The preparation and conditioning of DNA-modified electrodes, in which glassy carbon electrodes are covered by a thick film of double-stranded DNA and subsequently conditioned in acetate buffer, has been followed with Electrochemical Impedance Spectroscopy. Spectra were recorded at different values of applied potential in order to probe the alterations occurring in the modifier layer. The alterations in the EIS response parallel the changes in voltammetric response and show that the modifier layer is a good conductor and that the behaviour as a DNA-biosensor is essentially due to surface phenomena and interaction with single-stranded DNA in solution during conditioning.

Flow-injection determination of catechol with a new tyrosinase/DNA biosensor

Biosensors find application in flow analysis due to their high selectivity and sensitivity. Decrease in the response during extended use, originated by degradation, inhibition or structural changes of the enzyme or leaching of active components by the flow, is the prevailing problem. As an alternative to additives and preparation techniques cited in the literature, it is proposed to use DNA as a matrix for improving preservation of the activity of a diphenol-sensor-based tyrosinase, Tyr, (EC 1.14.18.1). The Tyr‐DNA mixture was incorporated into carbon paste, CP‐DNA‐Tyr, or applied on glassy carbon, GC‐DNA‐ Tyr. The CP‐DNA‐Tyr, covered by a membrane -of Cuprophan, presented superior performance in amperometric operation under flow conditions (electroreduction of the products of the enzymatic oxidation of diphenols in the presence of O2). In comparison with paste electrodes without DNA, CP‐Tyr, a current increase of one order of magnitude was observed for catechol FIA peaks, with good repeatability during several hours of operation. The response decayed ca. 50% after every 3 to 4 days of use (with dry storage at 48C overnight). Original performance was recovered by simply substituting the used paste for a new portion of stock paste, stable for 2 months under refrigeration. Evaluation of 18 different substrates and potential interferents indicated that, at the adopted potential ofˇ0,15 V vs. Ag/AgCl, only p-cresol gives a response comparable to catechol. Flow-injection determination of catechol samples was conducted at a frequency of 30 injections/h, with linear response from the detection limit of 110 ˇ6 up to 510 ˇ5 mol l ˇ1 . # 1998 Elsevier Science B.V.

Voltammetric Behaviour of Metronidazole at Mercury Electrodes

Metronidazole is the most important drug of the group of 5-nitroimidazoles and possesses toxicity to anaerobic micro-organisms DNA being the main target for their biological action. The mechanism of biological action of metronidazole is dependent upon the nitro group reduction process. The reduction of metronidazole is pH dependent in acid medium and four electrons are involved in the complete reduction to the hydroxylamine derivative. In aprotic medium the reduction of the metronidazole occurs in two steps, the first involving one electron to form the nitro radical and the second step involving three more electrons until the formation of the hydroxylamine derivative. In this paper the mechanism of reduction of metronidazole was studied by using the voltammetric techniques: d.c. polarography, differential pulse polarography and cyclic voltammetry using the mercury drop as the working electrode.

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.

Aspectos mecanísticos da bioatividade e toxicidade de nitrocompostos

Nitrocompounds are bioactive molecules used as antibacterial, antiparasitic and antitumoral agents. In the past of years, these molecules have been broadly studied in several fields, such as medicinal chemistry, organic chemistry, biochemical, toxicology and electrochemistry. The nitrocompounds mode of action involves the biotransformation of the nitro group, releasing intermediates in the redox process. Some of those intermediates attack enzymes, membranes and DNA, providing the basis for their biological activity and adverse effects. In this report, some aspects regarding the biological activity, mechanism of action and toxicity of nitrocompounds are explored, purposing the research of new bioactive derivatives having low toxicity.

Cyclic voltammetry and computational chemistry studies on the evaluation of the redox behavior of parabens and other analogues

Parabens are antimicrobial preservatives widely used in pharmaceutical, cosmetic and food industries. The alkyl chain connected to the ester group defines some important physicochemical characteristics of these compounds, including the partition coefficient and redox properties. The voltammetric and computational analyses were carried out in order to evaluate the redox behavior of these compounds and other phenolic analogues. A strong correlation between chemical substituents inductive effects of parabens with redox potentials was observed. Using cyclic voltammetry and glassy carbon working electrode, only one irreversible anodic peak was observed around 0.8 V for methylparaben (MP), ethylparaben (EP), propylparaben (PP), butylparaben (BP), benzylparaben (BzP) and p-substituted phenolic analogues. The electrodonating inductive effect of alkyl groups was demonstrated by the anodic oxidation potential shift to lower values as the carbon number increases and, therefore the parabens (and other phenolic analogues) oxidation processes to the quinonoidic forms showed great dependence on the substituent pattern.