Photoinduced Electron Transfer in DNA: Charge ShiftDynamics Between 8-Oxo-Guanine Anion and Adenine.

Femtosecond time-resolved IR spectroscopyis used to investigatethe excited-state dynamics of a dinucleotide containing an 8-oxoguanineanion at the 5′-end and neutral adenine at the 3′-end.UV excitation of the dinucleotide transfers an electron from deprotonated8-oxoguanine to its π-stacked neighbor adenine in less than1 ps, generating a neutral 8-oxoguanine radical and an adenine radicalanion. These species are identified by the excellent agreement betweenthe experimental and calculated IR difference spectra. The quantumefficiency of this ultrafast charge shift reaction approaches unity.Back electron transfer from the adenine radical anion to the 8-oxguanineneutral radical occurs in 9 ps, or approximately 6 times faster thanbetween the adenine radical anion and the 8-oxoguanine radical cation(Zhang, Y. et al. Proc. Natl. Acad. Sci. U.S.A.2014, 111, 11612–11617). The largeasymmetry in forward and back electron transfer rates is fully rationalizedby semiclassical nonadiabatic electron transfer theory. Forward electrontransfer is ultrafast because the driving force is nearly equal tothe reorganization energy, which is estimated to lie between 1 and2 eV. Back electron transfer is highly exergonic and takes place muchmore slowly in the Marcus inverted region. [ABSTRACT FROM AUTHOR]