Sugar Radical Formation by a Proton Coupled Hole Transfer in 2′-Deoxyguanosine Radical Cation (2′-dG^•+): A Theoretical Treatment

Previous experimental and theoretical work has established that electronic excitation of a guanine cation radical in nucleosides or in DNA itself leads to sugar radical formation by deprotonation from the dexoxyribose sugar. In this work, we investigate a ground electronic state pathway for such sugar radical formation in a hydrated one electron oxidized 2′-deoxyguanosine (dG^•+ + 7H₂O), using density functional theory (DFT) with the B3LYP functional and the 6-31G* basis set. We follow the stretching of the C_5′-H bond in dG^•+ to gain an understanding of the energy requirements to transfer the hole from the base to sugar ring and then to deprotonate to proton acceptor sites in solution and on the guanine ring. The geometries of reactant (dG^•+ + 7H₂O), transition state (TS) for deprotonation of the C_5′ site, and product (dG(^•C_5′, N₇−H⁺) + 7H₂O) were fully optimized. The zero point energy (ZPE) corrected activation energy (TS) for the proton transfer (PT) from C_5′ is calculated to be 9.0 kcal/mol and is achieved by stretching the C_5′−H bond by 0.13 Å from its equilibrium bond distance (1.099 Å). Remarkably, this small bond stretch is sufficient to transfer the “hole” (positive charge and spin) from guanine to the C_5′ site on the deoxyribose group. Beyond the TS, the proton (H⁺) spontaneously adds to water to form a hydronium ion (H₃O⁺) as an intermediate. The proton subsequently transfers to the N₇ site of the guanine (product). The 9 kcal/mol barrier suggests slow thermal conversion of the cation radical to the sugar radical but also suggests that localized vibrational excitations would be sufficient to induce rapid sugar radical formation in DNA base cation radicals.