One-Electron Reduction of 8-Bromoisoguanosine and 8-Bromoxanthosine in the Aqueous Phase: Sequential versus Concerted Proton-Coupled Electron Routes

The reactions of hydrated electrons (e_aq⁻) with 8-bromoisoguanosine and 8-bromoxanthosine were studied by pulse radiolysis techniques and addressed computationally by means of hybrid meta DFT calculations at the B1B95/6-31+G** level. The one-electron oxidized purine derivative is formed either by oxidation of the corresponding purine with SO₄^•− or by reduction of the corresponding 8-bromopurine with e_aq⁻ at pH 7. The reactivity of 8-bromoxanthosine depends on its protonation state. In agreement with the experimental findings, DFT calculations suggest that the reaction of the e_aq⁻/H⁺ couple with 8-bromoisoguanosine and 8-bromoxanthosine (monoanion) involves sequential electron-transfer−proton-transfer (ET−PT) and concerted electron−proton-transfer (EPT) pathways, respectively.