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-dime thylxanthines and 1,3,7-trimethylxanthine (caffeine) in aqueous soluti on have been studied by pulse radiolysis with optical detection. The i nitial absorption spectra of the electron adducts of all the three der ivatives show a peak around 330-345 nm with a shoulder at 305 nm. A fu rther build-up in absorbance was,observed at 305 nm with k(obs) = 2.5 x 10(5) and 1.5 x 10(5) s(-1) in the case of caffeine and 3,7-dimethyl xanthine, 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 [k(cat) = (2-5) x 10(6) dm(3) mol(-1) s(-1)]. The fully developed electron adduct spect rum of caffeine in neutral solution is identical to that obtained in t he reaction of H atoms, with molar absorptivities of 6250 and 4750 dm( 3) mol(-1) cm(-1) at 305 and 345 nm, respectively. The spectra obtaine d in the reaction of H atoms with dimethyl derivatives in acidic solut ions (pH = 1.6-1.8) are attributed to the formation of the correspondi ng radical cations. The measured yields of MV(.+) are in accord with t he rates found for the transformation of the heteroatom-protonated add ucts into the non-reducing C(8) protonated species. The spontaneous tr ansformation in neutral solutions is suggested to occur by tautomeriza tion between neutral adducts and the order found for this conversion i s guanosine > caffeine > 3,7-dimethylxanthine > adenosine x inosine.