Electrochemical Oxidation Of Uric Acid Research Articles

Electrochemical oxidation of uric acid and 6-thiouric acid

Electrochemical oxidation of uric acid and 6-thiouric acid

On-line mass spectrometric investigation of the peroxidase-catalysed oxidation of uric acid

The enzymatic and electrochemical oxidation pathways of uric acid were determined on-line with thermospray-tandem mass spectrometry. Products and intermediates formed as a result of electrooxidation were monitored as the electrode potential was varied. Electrochemical results served as a model for the enzymatic studies. In fact, electrochemical studies were essential for elucidating the structures of intermediates because of the high conversion efficiencies in electrooxidation. Products and intermediates formed as a result of enzymatic oxidation of uric acid were monitored as the reaction time was varied. When the enzymatic oxidation of uric acid with peroxidase and H2O2 was studied, the same intermediates and products were observed as in the electrochemical oxidation. The tandem mass spectrometric results provide convincing evidence that the primary intermediate produced during both the enzymatic and electrochemical oxidation of uric acid has a quinonoid diimine structure. The primary intermediate can follow three distinct reaction pathways to produce the identified final products. The final enzymatic and electrochemical oxidation products observed in these studies were urea, CO2, alloxan, alloxan monohydrate, allantoin, 5-hydroxyhydantoin-5-carboxamide and parabanic acid.

725—Electrochemical and peroxidase-catalyzed oxidations of uric acid in alkaline phosphate solutions: The second voltammetric oxidation peak of uric acid

The electrochemical oxidation of uric acid at a pyrolytic graphite electrode has been studied in alkaline phosphate buffers. The first peak (I a) process is a 2 e − −2 H + reaction leading to an unstable quinonoid intermediate that hydrates and decomposes to give a single organic product, allantoin. Peak II a process may be due to electrooxidation of a phosphorylated carboxylate anion intermediate formed in the peak I a reaction. This reaction leads to two additional products, cyanuric acid and N,N′-dicarboxamide-carbodiimide. Mechanisms are proposed to account for these peak I a and peak II a products. Under conditions where voltammetric oxidation peak II a is observed, i.e. pH > 6 and high phosphate buffer concentrations, the oxidation of uric acid by H 2O 2 catalyzed by peroxidase proceeds via the same chemical reaction pathway to the same products.

Elucidation of the biological redox chemistry of purines using electrochemical techniques

Recent work concerned with the electrochemical oxidation of uric acid illustrates the way electrochemical studies provide not only mechanistic information of interest but can also give valuable insights into the chemical aspects of enzymatic and in vivo redox reaction compounds. From State-of-the-Art Symposium: Electrochemistry, ACS meeting, Kansas City, 1982.

485—Spectroelectrochemical evidence for imine-alcohol intermediate formed upon electrochemical oxidation of uric acid

Electrochemical oxidation of uric acid in phosphate-containing supporting electrolytes between pH 3–9 at a reticulated vitreous carbon electrode in a thin-layer spectroelectrochemical cell leads to formation of U.V.-absorbing intermediate species. Electrochemical reduction of the intermediate-containing solution leads to the partial regeneration of uric acid. This behavior provides compelling evidence that an imine-alcohol is one of the U.V.-absorbing intermediate species since only this compound may be expected to be reduced to a species which can regenerate uric acid.

Spectroelectrochemical evidence for imine-alcohol intermediate formed upon electrochemical oxidation of uric acid

Electrochemical oxidation of uric acid in phosphate-containing supporting electrolytes between pH 3–9 at a reticulated vitreous carbon electrode in a thin-layer spectroelectrochemical cell leads to formation of U.V.-absorbing intermediate species. Electrochemical reduction of the intermediate-containing solution leads to the partial regeneration of uric acid. This behavior provides compelling evidence that an imine-alcohol is one of the U.V.-absorbing intermediate species since only this compound may be expected to be reduced to a species which can regenerate uric acid.

ChemInform Abstract: Further Insights into the Electrochemical Oxidation of Uric Acid (Cyclovoltammetrie, Dünnschicht‐Spektroelektrochemie, Kinetik, Intermediat‐ und Produktanalyse).

Chemischer InformationsdienstVolume 13, Issue 20 Article ChemInform Abstract: Further Insights into the Electrochemical Oxidation of Uric Acid (Cyclovoltammetrie, Dünnschicht-Spektroelektrochemie, Kinetik, Intermediat- und Produktanalyse). R. N. GOYAL, R. N. GOYALSearch for more papers by this authorA. BRAJTER-TOTH, A. BRAJTER-TOTHSearch for more papers by this authorG. DRYHURST, G. DRYHURSTSearch for more papers by this author R. N. GOYAL, R. N. GOYALSearch for more papers by this authorA. BRAJTER-TOTH, A. BRAJTER-TOTHSearch for more papers by this authorG. DRYHURST, G. DRYHURSTSearch for more papers by this author First published: May 18, 1982 https://doi.org/10.1002/chin.198220088AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume13, Issue20May 18, 1982 RelatedInformation

A comparison of the peroxidase-catalyzed and electrochemical oxidation of uric acid

The oxidation of uric acid by hydrogen peroxide in the presence of type VIII peroxidase has been studied between pH 5.2 and 8. Intermediates generated in the reaction have been characterized in terms of their U.V. spectra and kinetics of decay. In addition at least one U.V.-absorbing intermediate has been trapped, converted to its trimethylsilyl derivative and identified by gas chromatography-mass spectrometry. This intermediate is 1-carbohydroxy-2,4,6,8-tetraza-3,7-dioxo-4-ene-bicyclo-(3, 3, 0)-octane. At pH ⩾ 7 the product is allantoin while at lower pH 5-hydroxyhydantoin-5-carboxamide is also formed as a major product. The intermediates and products formed and spectral and kinetic measurements observed during and after peroxidase-catalyzed oxidation of uric acid are virtually identical to those noted upon electrochemical oxidation. It has thus been concluded that the mechanisms of electrochemical and enzymic oxidation of uric acid are, in a chemical sense, indentical.

479—A comparison of the peroxidase-catalyzed and electrochemical oxidation of uric acid

The oxidation of uric acid by hydrogen peroxide in the presence of type VIII peroxidase has been studied between pH 5.2 and 8. Intermediates generated in the reaction have been characterized in terms of their U.V. spectra and kinetics of decay. In addition at least one U.V.-absorbing intermediate has been trapped, converted to its trimethylsilyl derivative and identified by gas chromatography-mass spectrometry. This intermediate is 1-carbohydroxy-2.4,6,8-tetraza-3,7-dioxo-4-ene-bicyclo-(3,3,0)-octane. At pH ⩾ 7 the product is allantoin while at lower pH 5-hydroxyhydantoin-5-carboxamide is also formed as a major product. The intermediates and products formed and spectral and kinetic measurements observed during and after peroxidase-catalyzed oxidation of uric acid are virtually identical to those noted upon electrochemical oxidation. It has thus been concluded that the mechanisms of electrochemical and enzymic oxidation of uric acid are, in a chemical sense, indentical.

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 2 e-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 2 e-2H + oxidation of uric acid. In high-phosphate buffers H 2PO 4 − attacks the diimine, whereas in low-phosphate buffers solvent (H 2O) 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.

Application of thin-layer spectroelectrochemistry and gas chromatography/mass spectrometry to identification of intermediates formed in the electrochemical oxidation of purines

A combination of thin-layer spectroelectrochemistry and gas-chromatography/mass spectroscopy has been used to generate, trap, derivatize and analyze for the UV-absorbing intermediate formed upon electrochemical oxidation of uric acid at physiological pH. The mass spectral data confirm the earlier speculation that the UV-absorbing intermediate is an imine-alcohol species. Enzymic oxidation of uric acid with peroxidase and H 2 O 2 gives the same intermediate species.

Application of thin-layer spectroelectrochemistry and gas chromatography/mass spectrometry to identification of intermediates formed in the electrochemical oxidation of purines

A combination of thin-layer spectroelectrochemistry and gas-chromatography/mass spectroscopy has been used to generate, trap, derivatize and analyze for the UV-absorbing intermediate formed upon electrochemical oxidation of uric acid at physiological pH. The mass spectral data confirm the earlier speculation that the UV-absorbing intermediate is an imine-alcohol species. Enzymic oxidation of uric acid with peroxidase and H 2O 2 gives the same intermediate species.

Enzymatic and electrochemical oxidation of uric acid Mechanism for the peroxidase catalyzed oxidation uric acid

The oxidation of uric acid has been studied by thin-layer spectroelectrochemistry using an optically transparent gold minigrid electrode and enzymatically using peroxidase. Under both enzymatic and electrochemical conditions a u.v.-absorbing intermediate (λ max =302–304 nm between pH 7–9.3) is observed. This intermediate is proposed to be an imine-alcohol species formed by rapid hydration of the primary reaction product which is a diimine. Under the conditions employed in both the enzymatic electrochemical studies the imine-alcohol intermediate is hydrated in a first order reaction with an observed rate constant of 3.5×10 −3 s −1 between pH 7–9.3. This study establishes the similarity between the electrochemical and enzymatic oxidation of uric acid and indicates that electrochemical investigations of the redox behavior of purines can provide unique insights into their biological redox properties.

Enzymatic and electrochemical oxidation of uric acid: A mechanism for the peroxidase catalyzed oxidation of uric acid

The oxidation of uric acid has been studied by thin-layer spectroelectrochemistry using an optically transparent gold minigrid electrode and enzymatically using peroxidase. Under both enzymatic and electrochemical conditions a u.v.-absorbing intermediate (λ max=302–304 nm between pH 7–9.3) is observed. This intermediate is proposed to be an imine-alcohol species formed by rapid hydration of the primary reaction product which is a diimine. Under the conditions employed in both the enzymatic electrochemical studies the imine-alcohol intermediate is hydrated in a first order reaction with an observed rate constant of 3.5×10 −3s −1 between pH 7–9.3. This study establishes the similarity between the electrochemical and enzymatic oxidation of uric acid and indicates that electrochemical investigations of the redox behavior of purines can provide unique insights into their biological redox properties.

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

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

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

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 CO 2 in neutral and weakly alkaline solution.

Electrochemical Oxidation of Uric Acid and Xanthine at the Pyrolytic Graphite Electrode: Mechanistic Interpretation of Electrochemistry

The quantitative course of electrochemical oxidation of uric acid and xanthine at the pyrolytic graphite electrode has been examined between pH 1–7, the most commonly encountered pH range in biological systems. Basically, uric acid is oxidized in a primary oxidation of the bond to give a readily reducible and highly reactive bis‐imine. The bis‐imine can be detected as a cathodic peak by fast sweep cyclic voltammetry. Complete hydration of the bis‐imine gives rise to uric acid‐4,5‐diol that breaks down to alloxan and urea at pH 1, allantoin and a small amount of urea at pH 7, and at intermediate pH values mixtures of alloxan, allantoin, urea, and occasionally traces of parabanic acid are formed. Xanthine undergoes a oxidation that proceeds by two stages. The first involves oxidation of the bond to give uric acid which, being more easily oxidized than xanthine, is immediately further oxidized to the uric acid bisimine. This then hydrates and fragments in essentially the same way as described for uric acid, the same products being observed. Mechanisms are developed which explain the observed electrochemistry and over‐all formation of products from this reaction. The nature and yields of products formed electrochemically and the effect of pH on the reaction parallels the biological (enzymatic) oxidation of these compounds.

Primary Products of Electrochemical Oxidation of Uric Acid in Aqueous and Methanolic Solution

Uric acid is oxidized in aqueous and 50% methanolic acetate buffer pH 3.7 in a two‐electron process. In aqueous solution the primary product is uric acid‐4,5‐diol, which is unstable and fragments predominantly to allantoin, but a very small fraction is further oxidized to parabanic acid. In the presence of appreciable quantities (i.e., 50% v/v) of methanol the greater nucleophilicity of methanol results in formation of 4,5‐dimethoxyuric acid, which is apparently quite stable and cannot be further oxidized to parabanic acid, and it appears to be reasonably stable to the rather drastic conditions of hydrolysis used to determine allantoin. The results presented further confirm the site and nature of the primary product of the electrochemical oxidation of uric acid.