Properties and reactivity of the adenosine radical generated by radiation-induced oxidation in aqueous solution

The formation of oxidation products of DNA bases induced by hole injection into DNA duplexes is closely related to the characteristics of the radical species generated, especially those from purine bases possessing lower oxidation potentials than pyrimidines. To investigate the reactivities of adenosine base radicals generated from the radical cations of adenosine (Ado), we have conducted extensive pulse radiolysis and steady-state X-radiolysis investigations of Ado under the conditions generating oxidizing radical species such as the sulfate radical anion (SO(4)(-*)). Kinetic analysis of the transient absorption spectra demonstrated that the redox-neutral radical of adenosine [Ado(-H)(*)] decays via a homogeneous bimolecular reaction, recombines with an alkyl alcohol radical, or abstracts hydrogen from the phosphate ion, with rate constants of 2k = 4.2 x 10(8) M(-1) s(-1) and k = 6.4-8.5 x 10(8) and 10(3)-10(4) M(-1) s(-1), respectively. High-dose pulsed electron beam irradiation, which generates high concentrations of Ado(-H)(*) during the radiolysis, yielded 8-oxo-7,8-dihydroadenosine (8-oxo-Ado) with a G value of 0.018 x 10(-7) mol J(-1). It is thus proposed that the disproportionation of Ado(-H)(*) might lead to the formation of 8-oxo-Ado as a minor process, which is in addition to the widely accepted mechanism in which the addition of hydroxyl radical to Ado initiates its formation.