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Issue 4, 1995
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Transformation reactions of the electron adducts of methyl derivatives of xanthine in aqueous solution by pulse radiolysis

Abstract

The reactions of hydrated electrons and H atoms with 1,3- and 3,7-dimethylxanthines and 1,3,7-trimethylxanthine (caffeine) in aqueous solution have been studied by pulse radiolysis with optical detection. The initial absorption spectra of the electron adducts of all the three derivatives show a peak around 330–345 nm with a shoulder at 305 nm. A further build-up in absorbance was observed at 305 nm with kobs= 2.5 × 105 and 1.5 × 105 s–1 in the case of caffeine and 3,7-dimethylxanthine, respectively. Though such a delayed growth was marginal with 1,3-dimethylxanthine, the catalytic effect of phosphate buffer on the rate of this reaction was seen in all the compounds [kcat=(2–5)× 106 dm3 mol–1 s–1]. The fully developed electron adduct spectrum of caffeine in neutral solution is identical to that obtained in the reaction of H atoms, with molar absorptivities of 6250 and 4750 dm3 mol–1 cm–1 at 305 and 345 nm, respectively. The spectra obtained in the reaction of H atoms with dimethyl derivatives in acidic solutions (pH = 1.6–1.8) are attributed to the formation of the corresponding radical cations. The measured yields of MV˙+ are in accord with the rates found for the transformation of the heteroatom-protonated adducts into the non-reducing C(8) protonated species. The spontaneous transformation in neutral solutions is suggested to occur by tautomerization between neutral adducts and the order found for this conversion is guanosine > caffeine > 3,7-dimethylxanthine > adenosine ≈ inosine.

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Article information


J. Chem. Soc., Faraday Trans., 1995,91, 615-621
Article type
Paper

Transformation reactions of the electron adducts of methyl derivatives of xanthine in aqueous solution by pulse radiolysis

R. R. Rao, C. T. Aravindakumar, B. S. M. Rao, H. Mohan and J. P. Mittal, J. Chem. Soc., Faraday Trans., 1995, 91, 615
DOI: 10.1039/FT9959100615

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