Electrochemical and peroxidase-catalyzed redox chemistry of 9-Methyl uric acid

doi:10.1016/0368-1874(82)85145-9

Journal of Electroanalytical Chemistry and Interfacial Electrochemistry

Volume 133, Issue 2, 25 February 1982, Pages 287-297

https://doi.org/10.1016/0368-1874(82)85145-9 Get rights and content

Abstract

The electrochemical and peroxidase/H₂O₂ oxidation of 9-methyl uric acid has been studied over a wide pH range. Electrochemical, spectral, kinetic and analytical results support the view that the electrochemical and enzymic oxidations proceed by virtually identical mechanisms.

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Electrochemical and peroxidase-catalyzed oxidation of epinephrine
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It has been found that the oxidation of dopamine leads to the formation of toxic 2,3′-linked indole–indoline dimer, whereas, in the case of EPI formation of cysteinyl conjugates is reported. The peroxidase-catalyzed oxidation of purines has attracted considerable attention in the past [28–32] and it has been reported that electrochemical and peroxidase-catalyzed oxidation of simple purines follow the same pathway. Although the physiological functions and mechanism of action of EPI have been extensively analyzed, all of the details of its catabolism are still not clear even today.
Electrochemical and peroxidase-catalyzed oxidation of epinephrine (EPI) has been studied. In the electrochemical studies a single well-defined, 4e, 4H⁺, pH-dependent oxidation peak was observed in square wave and cyclic sweep voltammetry at edge plane pyrolytic graphite electrode. In the reverse sweep a redox couple was observed. The decay of the UV-absorbing intermediate generated and the first-order rate constants were calculated at different pH and were found to be ∼6.3 × 10⁻³ s⁻¹. The detection limit and sensitivity are found to be 17 × 10⁻⁸ M and 2.325 μA μM⁻¹ respectively. At pH 7.2, the electro-oxidation product was characterized using NMR and DEPT studies as leucoadrenochrome. The peroxidase-catalyzed oxidation was carried out using horseradish peroxidase and initiated by adding H₂O₂. The identical spectral changes, rate constants and product formed during electrochemical and enzymatic oxidation suggest that the same intermediate species is generated during both the oxidations. A tentative pathway for the oxidation of EPI has been suggested. It is concluded that the electrochemical and peroxidase-catalyzed oxidation of EPI proceed by an identical pathway.
Electrochemical oxidation of 2′,3′-dideoxyadenosine at pyrolytic graphite electrode
2008, Electrochimica Acta
The oxidation chemistry of 2′,3′-dideoxyadenosine (I) has been studied at a pyrolytic graphite electrode (PGE) in the pH range 2.4–10.9 and a well defined oxidation peak was noticed. The peak potential of the peak was linearly dependent on pH with dE_p/dpH as 42 mV/pH. Voltammetric, coulometric, spectral studies and product analysis indicate that the oxidation of (I) occurs in an EC reaction involving 6e⁻, 6H⁺ process at pH 7.2 to give allantoin, C–C dimer and dideoxyribose as the major products and a C–O–O–C linked dimer as a minor product. Tentative mechanisms for the formation of the products have also been suggested. A comparison of peak potential value of 2′,3′-dideoxyadenosine with adenosine and 2′-deoxyadensoine indicated that the difference is insignificant which has further been supported by the calculations of difference of energies of lowest unoccupied and highest occupied molecular orbitals.
Electrooxidation of biologically important hypoxanthine nucleosides and nucleotides at solid electrodes
2006, Electrochimica Acta
The oxidation chemistry of hypoxanthine nucleosides and nucleotides viz., inosine, 2′-deoxyinosine, inosine-5′-monophosphate and inosine-5′-triphosphate has been studied in the pH range 2.1–11.1 at pyrolytic graphite electrode. In all the four compounds oxidation occurs in 6H⁺, 6e process at physiological pH (7.2) leading to the formation of allantoin as a major product. The products dimers and tetramers formed in the secondary electrode reactions depend on the nature of sugar molecule (ribose or deoxyribose) present and number of phosphate moieties attached to the sugar moiety. A comparison of redox chemistry and formation of electrooxidised products of inosine and its derivatives has been presented in this paper.
Controlled potential electrolysis of inosine: Dependence of the selected potential on the nature of the electrooxidised products
2006, Journal of Electroanalytical Chemistry
The electrochemical oxidation of inosine has been studied in the phosphate buffers of different pH at pyrolytic graphite electrode (PGE). Voltammetric, coulometric, spectral studies and product analysis indicate that the oxidation occurs in an EC reaction involving a total of 6e, 6H⁺ per mole of inosine. As the E_p of inosine is close to background discharge, the controlled potential electrolysis was carried out at two potentials (at 100 mV higher and at peak potential) to avoid water oxidation reactions. Free radical pathways are more pronounced at higher potential leading to the formation of two tetramers, whereas, oxidation at peak potential in CPE leads to two dimers, two tetramers and an allantoin. The products of electrooxidation at exact peak potential were separated by HPLC and characterized by using GC–Mass, FT-IR, ¹H NMR, m.p. A comparison of redox behaviour under two conditions is presented and tentative mechanism for the formation of products has also been suggested.
Confirmation of 5-hydroxy-7-methyl-Δ<sup>4</sup>,<sup>9</sup>-isouric acid as an intermediate in the electrochemical oxidation of 7-methyluric acid
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5-Chloro-7-methyl-Δ^4,9-isouric acid (11) has been chemically synthesized and shown to be rapidly converted to 5-hydroxy-7-methyl-Δ^4,9-isouric acid (14) in aqueous buffers at ca. pH 7. The physical and chemical properties of 14 in solution have been elucidated. Electrochemical oxidation of 7-methyluric acid (10) at its first voltammetric oxidation peak forms a UV-absorbing intermediate. This intermediate exhibits physical and chemical properties which are identical to those of 14. These observations provide the first unequivocal evidence for the chemical structure of a UV-absorbing intermediate formed upon electrochemical oxidation of any purine.
Electrochemical oxidation of 1,3,7,9-tetramethyluric acid
1986, Journal of Electroanalytical Chemistry
The electrochemical oxidation of 1,3,7,9-tetramethyluric acid at the pyrolytic graphite electrode has been studied in aqueous solution over a wide pH range. At all pH values a single voltammetric oxidation peak is observed at potentials considerably more positive than are observed for other uric acid derivatives. The pH-independent voltammetric peak is due to a primary 2 e⁻¹ oxidation of tetramethyluric acid to a very unstable quinonoid dication. Rapid attack by water leads to formation of the 4,5-epoxide of tetramethyluric acid. This then slowly hydrates further giving the corresponding 4,5-diol which in turn rapidly fragments into numerous products. At pH < 3 the latter diol decomposes to an interesting spiro compound, 3,1′,3′-trimethyloxazolidine-2,4-dione-5-spiro-5-hydantoin. The 4,5-diol also appears to undergo two other reactions. At pH > 3 it cleaves across the C(4)N(3) bond to yield, ultimately, 2,4-dimethyl-5-hydroxyhydantoin-5-N-methylcarboxamide. The remaining products formed between pH 2 and 9 suggest that the 4,5-diol undergoes a ring contraction reaction leading to 1-hydroxy-5-carbo-hydroxy-2,4,6,8-tetraaza-2,4,6,8-tetramethyl-3,7-dioxo-bicyclo-(3.3.0)-octane which then decomposes to tetramethylallantoin (pH > 3), 2,4-dimethylalloxanic acid, 2,4-dimethyl-5-carboxy-5-N-methylaminohydantoin, 2,4-dimethyl-5-hydroxyhydantoin, dimethylurea and N-methylcarbamic acid.

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¹: Present address: Department of Chemistry, University of Maine, Orono, ME, U.S.A.

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Electrochemical and peroxidase-catalyzed redox chemistry of 9-Methyl uric acid

Abstract

J. Electroanal. Chem.

Bioelectrochem. Bioenerg.

J. Electroanal. Chem.

Biochim. Biophys. Acta