Glenn Dryhurst

Electrochemical oxidation of uric acid and xanthine: An investigation by cyclic voltammetry, double potential step chronoamperometry and thin-layer spectroelectrochemistry

Abstract The electrochemical oxidation of uric acid and xanthine has been studied at various graphite electrodes and at a gold electrode by a variety of techniques including cyclic voltammetry, double potential step chronoamperometry, and thin-layer spectroelectrochemistry. It has been confirmed that the primary electrooxidation product formed from uric acid or xanthine is a diimine species. This diimine is very unstable in solution and is rapidly hydrated to an imine-alcohol in a first order reaction having an apparent rate constant of ca. 30 s−1 at pH 8.0. Both below and above pH 8.0 the rate of the latter reaction appears to increase. Although the diimine primary electrooxidation product is very unstable in homogeneous solution, having a half-life at pH 8.0 of ca. 23 ms, a voltammetric reduction peak due to reduction of the diimine may be observed on cyclic voltammetry of uric acid or xanthine at quite slow sweep rates (e.g., 200 mV s−1 at pH 8.0) at graphite electrodes provided that the electrode has a rough surface (large surface area). This is so because the diimine is strongly adsorbed at the rough electrode surface and in the adsorbed state is appreciably more stable than in solution. The imine-alcohol, formed by partial hydration of the diimine, may be observed as a further, unstable, u.v.-absorbing intermediate upon electrochemical oxidation of uric acid or xanthine at pH 7–9 by means of thin-layer spectroelectrochemical studies. The imine-alcohol is hydrated in a first order reaction to give uric acid-4,5-diol, characterized by an observed rate constant of 8×10−3 s−1 between pH7 and 9. Uric acid-4,5-diol further decomposes to allantoin and CO2 in neutral and weakly alkaline solution.

Electrochemical Oxidation of Adenine: Reaction Products and Mechanisms

The electrochemical oxidation of adenine (6‐aminopurine), which gives a single well‐defined voltammetric wave at the pyrolytic graphite electrode (PGE), was investigated by macroscale controlled electrode potential at the PGE in aqueous 1M acetic acid solution (pH 2.3) with exhaustive isolation, identification, and determination of reaction products and intermediates. The electrochemical oxidation of adenine appears to follow initially the same path as the enzymatic oxidation, but further oxidation and fragmentation of the purine ring system occur. Thus, adenine is oxidized in a process involving a total of 6 electrons per adenine molecule to give as the primary product a dicarbonium ion intermediate, which, being unstable, undergoes a further series of reactions: (a) electrochemical oxidation to parabanic acid (some of which is further hydrolyzed to oxaluric acid), urea, carbon dioxide, and ammonia; (b) electrochemical reduction to give ultimately 4‐aminopurpuric acid, carbon dioxide, and ammonia; and (c) hydrolysis to allantoin, carbon dioxide, and ammonia.

Electrochemical oxidation-reduction paths for pyrimidine, cytosine, purine and adenine: Correlation and application

In order to evaluate the striking discrepancy between the experimental ease of polarographic reduction of adenine and cytosine, and that predicted by molecular orbital calculation, the electrochemical oxidation-reduction behaviour of pyrimidine, cytosine, purine, adenine and related compounds was investigated at both mercury and graphite electrodes. Information was obtained on the specific adsorption of reactant and product species on the electrode, the reversibility of the energy-controlling electron-transfer step, and accompanying chemical reactions. Triangular sweep voltammetry, a.c. and d.c. polarography, and electrocapillary data, in particular, were utilized. The first three techniques were critically examined for their potential analytical utility. The results were compared with previously obtained electrochemical data and the sequence of electron-transfer and various non-electron transfer steps was more firmly established. It became clear that in order validly to correlate quantum mechanically calculated data for the energy required to add or remove an electron to or from the outermost electron level of each molecule (in the gas phase), with electrochemical redox potentials (in solution), the effects of adsorption, electron-transfer reversibility and solvation energy must be considered.

Redox chemistry of guanine and 8-oxyguanine and a comparison of the peroxidase-catalyzed and electrochemical oxidation of 8-oxyguanine

Abstract The electrochemical oxidation of guanine and 8-oxyguanine has been studied over a wide pH range in aqueous solution. Guanine is initially oxidized in a

Inhibitors of mitochondrial respiration, iron (II), and hydroxyl radical evoke release and extracellular hydrolysis of glutathione in rat striatum and substantia nigra : Potential implications to Parkinson's disease

Abstract : In this investigation, microdialysis has been used to study the effects of 1‐methyl‐4‐phenylpyridinium (MPP+), an inhibitor of mitochondrial complex I and α‐ketoglutarate dehydrogenase and the active metabolite of the dopaminergic neurotoxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), on extracellular concentrations of glutathione (GSH) and cysteine (CySH) in the rat striatum and substantia nigra (SN). During perfusion of a neurotoxic concentration of MPP+ (2.5 mM) into the rat striatum or SN, extracellular concentrations of GSH and CySH remain at basal levels (both ~2 μM). However, when the perfusion is discontinued, a massive but transient release of GSH occurs, peaking at 5,000% of basal levels in the striatum and 2,000% of basal levels in the SN. The release of GSH is followed by a slightly delayed and smaller elevation of extracellular concentrations of CySH that can be blocked by the γ‐glutamyl transpeptidase (γ‐GT) inhibitor acivicin. Low‐molecular‐weight iron and extracellular hydroxyl radical (OH·) have been implicated as participants in the mechanism underlying the dopaminergic neurotoxicity of MPTP/MPP+. During perfusion of Fe2+ (OH·) into the rat striatum and SN, extracellular levels of GSH also remain at basal levels. When perfusions of Fe2+ are discontinued, a massive transient release of GSH occurs followed by a delayed, small, but progressive elevation of extracellular CySH level that again can be blocked by acivicin. Previous investigators have noted that extracellular concentrations of the excitatory/excitotoxic amino acid glutamate increase dramatically when perfusions of neurotoxic concentrations of MPP+ are discontinued. This observation and the fact that MPTP/MPP+ causes the loss of nigrostriatal GSH without corresponding increases of glutathione disulfide (GSSG) and the results of the present investigation suggest that the release and γ‐GT/dipeptidase‐mediated hydrolysis of GSH to glutamate, glycine, and CySH may be important factors involved with the degeneration of dopamine neurons. It is interesting that a very early event in the pathogenesis of Parkinsons disease is a massive loss of GSH in the SN pars compacta that is not accompanied by corresponding increases of GSSG levels. Based on the results of this and prior investigations, a new hypothesis is proposed that might contribute to an understanding of the mechanisms that underlie the degeneration of dopamine neurons evoked by MPTP/MPP+, other agents that impair neuronal energy metabolism, and Parkinsons disease.

Interfacial behavior of adenine and its nucleosides and nucleotides

The adsorption of adenine, adenosine and AMP has been studied by surface electrochemical methods at pH 8.0. All compounds exhibit an initial or dilute adsorption region where they are probably adsorbed with the base flat on the electrode surface. It is proposed that adenosine and AMP adopt the syn conformation so that the sugar or sugar phosphate moiety, respectively, is largely rotated out of the plane of the electrode and the base residues can pack together almost as closely as the free bases. At potentials centered at −0.5 V and bulk solution concentrations≥ ca. 3 mM adenine appears to undergo a surface reorientation and adopts a perpendicular stance. In this new orientation it is proposed that adenine is bound to the electrode through its amino group hydrogen atoms forming a Download full-size image |-electrode hydrogen bond. Adenosine appears to form two types of perpendicular layers. At small positive electrode charges it is suggested that it adopts an anti conformation and is adsorbed with the negative end of its permanent dipole directed towards the electrode. At more negative potentials it is proposed that adenosine retains the anti conformation but is adsorbed with the positive end of its dipole directed towards the electrode

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

Abstract 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.

Interfacial behavior of uracil at the mercury-solution interface: Evidence for surface reorientation processes

Abstract The adsorption and related interfacial behavior of uracil at a mercury electrode/electrolyte solution interface has been studied by differential capacitance and maximum bubble pressure methods in 0.5 M NaF plus 0.01 M Na2HPO4 buffer pH 8.0. At concentrations below 24 mM uracil is adsorbed in a flat orientation on the electrode surface and occupies an area of 63 A2. At higher concentrations and at potentials close to −0.5 V the adsorbed uracil undergoes a reorientation and adopts a perpendicular stance on the electrode surface where it occupies an area of 39 A2. In this perpendicular stance uracil undergoes a strong intermolecular stacking interaction with its neighbors similar to that observed between adjacent pyrimidines in nucleic acids.

Futher insights into the electrochemical oxidation of uric acid

The electrochemical oxidation of uric acid has been studied between pH 1.5 and 9.5 in phosphate buffers using thin-layer spectroelectrochemistry to generate and study UV-absorbing intermediates. Some intermediates and all important products have been separated and analyzed by gas chromatography—mass spectrometry. It is concluded, on the basis of this and preceding studies, that uric acid is initially oxidized in a 2e-2H+ reaction to a very unstable quinonoid diimine (half-life \ 22 ms). At pH ⩾ 6 the anion of the latter species is attacked by water to give an anionic imine-alcohol that undergoes a ring contraction reaction to give 1-carbohydroxy-2,4,6, 8-tetraaza-3,7-dioxo-4-ene-bicyclo-(3,3,0)-octane (BCA). This then decomposes to allantoin. At pH 3–5.6 a neutral quinonoid diimine is generated upon 2e-2H+ oxidation of uric acid. In high-phosphate buffers H2PO4− attacks the diimine, whereas in low-phosphate buffers solvent (H2O) attacks the diimine. In high-phosphate buffers analysis of absorbance vs. time curves obtained following oxidation of uric acid in a thin-layer cell allows three intermediate species to be inferred. In low-phosphate buffers only two intermediates may be inferred. Mechanisms are advanced to rationalize these observations and to account for the end products formed, i.e. allantoin, 5-hydroxyhydantoin-5-carboxamide and, at pH 3, alloxan.

Adsorption of guanine and guanosine at the pyrolytic graphite electrode : Implications for the determination of guanine in the presence of guanosine

Abstract Guanine and guanosine are both electrochemically oxidized at the pyrolytic graphite electrode in aqueous solution. D.c. voltammetric concentration and scan rate studies and alternating current voltammetry have shown that both guanine and guanosine are adsorbed at this electrode. Adsorption of the compounds causes non-linear concentration vs. peak current curves, an increase in the voltammetric peak current function with scan rate and pronounced depressions of the alternating base current for pure solutions of both compounds. In the presence of guanosine, adsorbed guanine is displaced from the electrode surface, resulting in a decrease in the guanine D.c. voltammetric peak current. The extent of this decrease depends upon the relative concentrations of guanine and guanosine. Complete replacement of adsorbed guanine by guanosine appears to occur when greater than a 5-fold amount of guanosine is present. The electrooxidation of guanine then becomes diffusion-controlled. A simple and rapid determination of guanine in the presence of guanosine has been developed ; a large amount of guanosine is added to the mixture, and the current for the guanine voltammetric oxidation peak is measured.