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