Excited State Relaxation of Neutral and Basic 8‑Oxoguanine

8-Oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dGuo) is one of the most common forms of DNA oxidative damage. Recent studies have shown that 8-oxo-dGuo can repair cyclobutane pyrimidine dimers in double-stranded DNA when photoexcited, making its excited state dynamics of particular interest. The excited state lifetimes of 8-oxo-dGuo and its anion have been previously probed using transient absorption spectroscopy; however, more information is required to understand the decay mechanisms. In this work, excited state potential energy surfaces for the neutral and deprotonated forms of the free base, 8-oxoguanine (8-oxo-G), are explored theoretically using multireference methods while the nucleoside is experimentally studied using steady-state fluorescence spectroscopy. It is determined that the neutral species exhibits ultrafast radiationless decay via easy access to conical intersections. The relatively long lifetime for the anion can be explained by the existence of sizable barriers between the Franck–Condon region and two S₁/S₀ minimum energy conical intersections. A Strickler–Berg analysis of the experimentally measured fluorescence quantum yields and lifetimes is consistent with emission from <i>ππ</i>* excited states in line with theoretical predictions

Photoinduced Electron Transfer in DNA: Charge Shift Dynamics Between 8‑Oxo-Guanine Anion and Adenine

Femtosecond time-resolved IR spectroscopy is used to investigate the excited-state dynamics of a dinucleotide containing an 8-oxoguanine anion at the 5′-end and neutral adenine at the 3′-end. UV excitation of the dinucleotide transfers an electron from deprotonated 8-oxoguanine to its π-stacked neighbor adenine in less than 1 ps, generating a neutral 8-oxoguanine radical and an adenine radical anion. These species are identified by the excellent agreement between the experimental and calculated IR difference spectra. The quantum efficiency of this ultrafast charge shift reaction approaches unity. Back electron transfer from the adenine radical anion to the 8-oxguanine neutral radical occurs in 9 ps, or approximately 6 times faster than between the adenine radical anion and the 8-oxoguanine radical cation (Zhang, Y. et al. <i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>2014</b>, <i>111</i>, 11612–11617). The large asymmetry in forward and back electron transfer rates is fully rationalized by semiclassical nonadiabatic electron transfer theory. Forward electron transfer is ultrafast because the driving force is nearly equal to the reorganization energy, which is estimated to lie between 1 and 2 eV. Back electron transfer is highly exergonic and takes place much more slowly in the Marcus inverted region