Electrochemical oxidation of 2-thiouracil at pyrolytic graphite electrode

2-Thiouracil has been studied in phosphate buffers of pH 1.95-11.08 using linear and cyclic sweep voltammetry, coulometry, controlled potential electrolysis and spectral studies. One well-defined oxidation peak I(a) in the pH range 1.95-11.08 was noticed. The number of electrons involved in peak I(a) was found to be four in a thin layer cell whereas under exhaustive electrolysis condition oxidation was found to involve six electrons. A reduction peak II(c) (2e, 2H(+)) is noticed in the reverse sweep. Spectral studies during oxidation were carried out at different pH. Kinetic studies indicated that the decay of the UV-absorbing intermediate is a first order reaction. The products of the electrooxidation have been characterized and a tentative EC mechanism has been suggested for the oxidation of 2-thiouracil.     

Electrochemical reduction of tartrazine at multi-walled carbon nanotube-modified pyrolytic graphite electrode

Electrochemical reduction of tartrazine on multi-walled carbon nanotube-modied pyrolytic graphite electrode is investigated. A simple, sensitive and inexpensive method for determination of tartrazine in drinks is proposed. The accuracy and reproducibility of the determination method for various known amounts of tartrazine were evaluated. This method was satisfactorily applied for the determination of tartrazine in drinks. The reduction peak currents were proportional to tartrazine concentrations over two intervals in the range from 2.0 to 70.0 mg l⁻¹ and from 70.0 to 230.0 mg l⁻¹, and the detection limit for tartrazine is 0.5 mg l⁻¹.     

Electrochemical behaviour of human adrenodoxin on a pyrolytic graphite electrode

Adrenodoxin (Adx) functions as a redox protein in the delivery of electrons to all mitochondrial cytochromes P450. In order to further characterize the human form of this protein, direct electrochemistry of human adrenodoxin (Hadx) has been observed for the first time on a pyrolytic graphite electrode (PGE) modified with poly-L-lysine. A single well-defined redox wave was observed with a midpoint potential of -448+/-3 mV vs. Ag/AgCl (sat. KCl) at scan rates of 10 mV/s and over the pH range 4.0-8.0. At slow scan rates, the reduction process was close to being electrochemically reversible whereas, at faster scan rates, only quasi-reversibility was observed. A correlation was observed between the peak separation (DeltaE) for the cyclic voltammograms and pH over a wide range of scan rates. The variation of DeltaE with pH was at a minimum (optimum reversibility) at pH 7.0 for all scan rates tested. This correlation may suggest that the direct electrochemistry method could possibly provide a means for determining protein or enzyme activity. The electron transfer rate constant, k(s), was determined to be 0.28 s(-1) at pH 7.0 and a small pH dependence was observed. The results obtained in this study demonstrate the facile nature of direct electron transfer for human adrenodoxin, and provide an estimate of the midpoint reduction potential at a pyrolytic graphite electrode via electrostatic immobilisation.     

Voltammetric oxidation of some biologically important xanthines at the pyrolytic graphite electrode

Linear and cyclic sweep voltammetry of some biologically important N-methylxanthines has been carried out. The principal and potential determining reaction for oxidation of xanthines is a 2 electron attack at the -N₉C_8⁻ double bond to give the appropriate uric acid, which is then immediately further oxidized to an intermediate 4,5-diol species. In many instances this 4,5-diol is sufficiently stable to be detected by fast sweep cyclic voltammetry as a small cathodic peak formed as a result of reduction of part of the 4,5-diol to the corresponding uric acid. The uric acid so formed can be reoxidized upon subsequent cycles at a potential close to that observed for reduction fo the 4,5-diol, so that most xanthines (and uric acids) show evidence for a reversible redox couple at about 0.4–0.55 V. Methylation at position N₇ decreases somewhat the stability of the intermediate 4,5-diol and dimethylation at N₃ and N₇ results in such a marked decreased in the stability of the diol that it cannot be observed by cyclic voltammetry.     

Oxidation of 2-aminoquinoline at a stationary pyrolytic graphite electrode

The oxidation of 2-aminoquinoline was studied at a stationary pyrolytic graphite electrode in methanol-phosphate buffer at 25°C using various electroanalytical techniques. In the entire pH range (2.2–10.4), 2-aminoquinoline is oxidized and exhibits a well defined oxidation peak following a 2e^?, 2H⁺ process to give, 2,2′-azoquinoline as the major product. The linear relationship between peak current at a pyrolytic graphite electrode and concentration indicated that 2-aminoquinoline can be determined in the concentration range 0.1–1.0 mM. On the basis of cyclic voltammetry, spectral studies and controlled-potential coulometry, a mechanism of the electrode process is proposed.     

Oxidation chemistry of 2'-deoxyadenosine at pyrolytic graphite electrode

The electrochemical oxidation of 2'-deoxyadenosine has been investigated in phosphate containing supporting electrolytes in pH range 2-10 at a pyrolytic graphite electrode by cyclic sweep voltammetry, spectral studies, controlled potential electrolysis and related techniques. The oxidation of 2'-deoxyadenosine occurred in a single well-defined oxidation peak (I(a)), over the entire pH range. The electrooxidation occurred by the loss of 6.0+/-0.5 e(-) per mole over the entire pH range. The kinetics of the decay of the UV-absorbing intermediates has been studied and found to follow pseudo first order kinetics having rate constant (k) in the range (5.7-7.7)x10(-4) s(-1). The major products of electrooxidation were separated by HPLC and characterized by GC-MS/MS, (1)H NMR and a tentative mechanism for electrooxidation of 2'-deoxyadenosine has been suggested.     

Electrochemical oxidation of 4,5,6,7-tetrahydro-1H-pyrazolo[3,4-d] pyrimidine-4,6-dione (oxipurinol) at the pyrolytic graphite electrode

The electrochemical oxidation of 4,5,6,7-tetrahydro-1H-pyrazolo[3,4-d] pyrimidine-4,6-dione (oxipurinol) at the pyrolytic graphite electrode (PGE) has been studied. Oxipurinol exhibits up to three voltammetric oxidation peaks at the PGE between pH 1–12. The first pH-dependent peak (peak I_a) is proposed to be an initial, irreversible 2e-2H⁺ reaction to give 5,6-dihydro-4H-pyrazolo[3,4-d] pyrimidine-4,6-dione. This primary product further reacts by two routes. The major route, accounting for ca. 90% of the latter compound, involves a Michael addition of water followed by further electrochemical oxidation and hydrolysis to give 5,6-dihydro-5,6-dihydroxy-5-carboxy-6-diazenouracil. The minor route involves further electrochemical oxidation of 5,6-dihydro-4H-pyrazolo[3,4-d]-pyrimidine-4,6-dione in a 2e-2H⁺ reaction to give 4,5,6,7-tetrahydro-3H-pyrazolo[3,4-d]-pyrimidine-3,4,6-trione.Decomposition and, generally, additional electrochemical reactions of 5,6-dihydro-5,6-dihydroxy-5-carboxy-6-diazenouracil result in the formation of alloxan, parabanic acid, 6-diazo-isobarbituric acid and 5′-hydroxy-5-carboxy-6,6′-azouracil. The two latter compounds have never previously been reported. Decomposition of 4,5,6,7-tetrahydro-3H-pyrazolo[3,4-d]pyrimidine-3,4,6-trione results in formation of uracil-5-carboxylic acid.Detailed reaction schemes have been proposed to explain the observed electrochemistry and the formation of the observed products.     

Electrochemical oxidation of substituted 3,4-dihydro-2-pyridones at a rotating graphite electrode with a ring

Using the method of a rotating disk (graphite) electrode with a ring, UV spectroscopy, and prolonged electrolysis, it has been shown that the first stage of the electrochemical oxidation of substituted 3,4-dihydro-2-pyridones in anhydrous acetonitrile corresponds to a two-electron process taking place by an ECE mechanism and leads to the formation of the corresponding substituted 2-pyridones. The potentials of the electrochemical oxidation of the 3-cyano and 3-carbamoyl derivatives of the reduced forms and the potentials of the electroreduction of the corresponding oxidized compounds have been determined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 87–91, January, 1984.     

Electrochemical determination of adenine and adenosine: adsorption of adenine and adenosine at the pyrolytic graphite electrode

Adenine and adenosine are polarographically reducible from aqueous solution at pH 4.7 at the DME at the same E(1 2 ) and are also voltammetrically oxidizable at the PGE, but at different potentials, adenosine at higher potentials. Competitive adsorption of both compounds at the PGE results in a decrease in the scanning voltammetric oxidation peak of adenine in the presence of adenosine, reaching a constant value when the concentration of the latter is above 6 mM. In mixtures, the sum of the two is obtained by polarography at the DME. Solid adenosine is then added to the solution and the adenine is determined by voltamrnetry at the PGE.     

血红蛋白在纳米羟基磷灰石修饰的热解石墨电极上的直接电化学

采用超声辅助沉淀法合成羟基磷灰石纳米晶体,制作了以纳米羟基磷灰石(HAp)修饰的热解石墨电极(EPG)。并研究了血红蛋白在该修饰电极上的直接电化学行为。在pH6.9的磷酸盐(PBS)缓冲溶液中,得到一对可逆的血红蛋白辅基血红素Fe(Ⅲ)/Fe(Ⅱ)电对的循环伏安氧化还原特征峰,式量电位E0p′=-0.356V(vs.SCE,pH6.9),几乎不随扫速的改变而变化,电子转移数为1.041,近似一个辅基发生一个电子转移。Hb在HAp/EPG电极表面直接电子转移的速率常数为0.6074。在该纳米HAp微环境中,Hb与EPG电极之间的电子传递得到极大促进,并显示了较好的稳定性。式量电势pH3.6~9.0范围内与溶液的pH成线性关系,直线斜率为-56.0mV/pH,说明Hb的电子传递过程伴随质子的转移。探讨了Hb-HAp修饰电极对H2O2的电催经性质,为制作生物传感器打下基础。     

Cyclic voltammetric studies of a thionine coated pyrolytic graphite electrode

Cyclic voltammetry has been used to study the spontaneous adsorption and oxidative coating of thionine onto basal plane pyrolytic graphite electodes. Spontaneous adsorption leads to a monolayer of flat lying molecules together with some weakly attached upper layers. Oxidation of an adsorbed thionine monolayer results in several products which appear to include polymeric species and the sulphoxide of thionine. Similar products are obtained by an oxidative coating procedure.     

314 - Electrochemical oxidation of nucleic acids and proteins at graphite electrode. Qualitative aspects

The electrochemical oxidation of DNAs differing in the content of guanine plus cytosine (G+C) was investigated at a pyrolytic graphite electrode by means of differential pulse (DP) voltammetry. At pH 6.4 all samples of DNA studied yielded a peak G on DP voltammograms corresponding to the oxidation of the guanine residues, and a peak A corresponding to the oxidation of the adenine residues. The potentials of the peaks G and A were not influenced by the G+C content in DNA and differed by 0.28 V. It was found that the ratio of the heights of the peaks A and G was identical with great accuracy to the ratio of the contents (adenine+thymine) and (G+C). This fact was exploited for developing a new method for the determination of the G+C content in DNA.The electrochemical oxidation of proteins at a spectroscopic graphite electrode impregnated with paraffin wax (WISGE) was studied by means of linear sweep, cyclic, and DP voltammetry. It was found that proteins were electrochemically oxidizable at the WISGE. They yielded a faradaic peak on voltammograms in the vicinity of 0.7–0.8 V in a neutral medium. The voltammetric study of proteins, poly-(aminoacids), peptides of known aminoacid composition and free aminoacids revealed that the irreversible electrooxidation of tyrosine (and, contingently, of tryptophan) residues is responsible for the appearance of the protein peak at the WISGE. It is suggested that DP voltammetry at a graphite electrode might become another electrochemical method suitable for studies of conformational changes of proteins, and in particular of those not containing cystine or cysteine (e.g. histones).     

Electrochemical determination of piroxicam on the surface of pyrolytic graphite electrode modified with a film of carbon nanoparticle-chitosan

The electrochemical behavior of the anti-inflammatory drug piroxicam is studied at the surface of a plain pyrolytic graphite electrode modified with chitosan-doped carbon nanoparticles. An electroactive surface was produced by drop-casting a suspension of the modifier and characterized by atomic force microscopy. A remarkable enhancement is found in studies on the cyclic voltammetric response towards piroxicam. This is described on the basis of the thin-layer mass transport regimes within the porous films, which leads to a considerable increase in the active surface area of the electrode. The electrode shows a linear response to piroxicam in the range of 0.05–50 μM, with a detection limit of 25 nM (at S/N of 3). The electrode was successfully applied to the determination of piroxicam in pharmaceutical and clinical preparations with satisfactory accuracy and precision.     

Anodic reactions of the halides in dimethyl sulfoxide at the pyrolytic graphite electrode

The anodic reactions of the halide ions in dimethyl sulfoxide at the pyrolytic graphite electrode have been studied. The iodide ion demonstrates a 3-step oxidation; the bromide, a 2-step oxidation and chloride, a 1-step oxidation. The electrode reaction (X^-→$\text{1}\text{2}$ X₂ + e^-) is complicated by a catalytic reaction occurring after the electrode reaction. The catalytic reaction is important for only bromide and chloride causing a considerable diffusion current enhancement. The αn_a value for all 3 primary reactions is of the order of 0.5.     

Electron transport in biological processes: Electrochemical behaviour of ubiquinone Q,10 adsorbed on a pyrolytic graphite electrode

The E-pH diagram has enabled us to obtain the acid-base constant of ubiquinone and the equilibrium potential at pH = 0 for the different couples participating in the process of quinone adsorption on a pyrolytic graphite electrode in aqueous buffered media. The electrochemical behaviour found for ubiquinone may explain in part the in vivo ubiquinone processes. The molecular contour of deoxyribonucleic acid changes the stability of the semiubiquinone intermediate, as shown by chemiluminescence and binding experiments.     

The adsorption of corypalline and related compounds on a pyrolytic graphite electrode

Anodic current-time curves have been used to study the adsorption, from TEAP-acetonitrile solution, of corypalline (7-hydroxy-6-methoxy-N-methyl-1,2,3,4-tetrahydroisoquinoline), isocorypalline (6-hydroxy-7-methoxy-N-methyl-1,2,3,4-tetrahydroisoquinoline), and 1-methyl-corypalline. A comparison with analogous cathodic current-time curves obtained with methylene blue leads to the conclusion that, on a pyrolytic graphite electrode, these isoquinoline compounds are adsorbed in a “flat-on” configuration. This conclusion is in harmony with the postulation that the known steroselective, stereospecific oxidative coupling of 1-methylcorypal-line occurs through the adsorption of the reacting molecules on the electrode surface.     

Electrochemical behavior of natural and biosynthetic polynucleotides at the pyrolytic graphite electrode A new probe for studies of polynucleotide structure and reactions

The electrochemical oxidation of natural and biosynthetic polynucleotides at a pyrolytic graphite electrode (PGE) has been studied under differential pulse voltammetric conditions. Denatured DNA, ribosomal and transfer RNA give two voltammetric peaks. The first (more negative peak, peak G) corresponds to electrochemical oxidation of guanine residues where-as the second, more positive peak (peak A) corresponds to electrochemical oxidation of adenine residues. Native DNA gives rise only to a small peak A, peak G being totally absent. Denatured DNA and its voltammetric oxidation product are both strongly adsorbed at the PGE. Differential pulse voltammetric oxidation of natural and biosynthetic polynucleotides may provide a valuable technique for probing A-T and G-C regions during structural and conformational changes of these molecules and for following their interactions with other solution species.