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During the early evolution of life, 8-oxo-7,8-dihydro-2'-deoxyguano-sine (O) may have functioned as a proto-flavin capable of repairing cyclobutane pyrimidine dimers in DNA or RNA by photoinduced electron transfer using longer wavelength UVB radiation. To investigate the ability of O to act as an excited-state electron donor, a dinucleotide mimic of the FADH2 cofactor containing O at the 5-end and 2-deoxyadenosine at the 3-end was studied by femtosecond transient absorption spectroscopy in aqueous solution. Following excitation with a UV pulse, a broadband mid-IR pulse probed vibrational modes of ground-state and electronically excited molecules in the double-bond stretching region. Global analysis of time-and frequency-resolved transient absorption data coupled with ab initio quantum mechanical calculations reveal vibrational marker bands of nucleobase radical ions formed by electron transfer from O to 2-deoxyadenosine. The quantum yield of charge separation is 0.4 at 265 nm, but decreases to 0.1 at 295 nm. Charge recombination occurs in 60 ps before the O radical cation can lose a deuteron to water. Kinetic and thermodynamic considerations strongly suggest that all nucleobases can undergo ultrafast charge separation when n-stacked in DNA or RNA. Interbase charge transfer is proposed to be a major decay pathway for UV excited states of nucleic acids of great importance for photostability as well as photoredox activity. [ABSTRACT FROM AUTHOR]