EPR spectroscopic study of the radical oxidation of hydroxypurines in aqueous solution: acid–base properties of the derived radicals

Abstract

EPR spectroscopy has been employed to study the reactions of SO₄˙^– and OH˙ radicals with xanthine, 8-methylxanthine, hypoxanthine and 2-hydroxypurine.

Radicals obtained on the reaction of SO₄˙^– with these hydroxypurines resulted, depending on the pH, from one-electron oxidation of either the parent compound or its anion, followed by deprotonation. For xanthine and 8-methylxanthine the respective radical anion and radical dianion were observed, for hypoxanthine the species detected correspond to the neutral radical and radical anion, whereas for 2-hydroxypurine only its radical anion was observed.

The pK_a values of the radical anion of xanthine and 8-methylxanthine were estimated as 12.0 ± 0.1 and 12.1 ± 0.5, respectively, whereas the pK_a value of the hypoxanthine neutral radical was 7.7 ± 0.1.

In the reaction with OH˙, only the radical anion of xanthine was detected.