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Joint femtosecond fluorescence upconversion experiments and theoreticalcalculations provide a hitherto unattained degree of characterization andunderstanding of the mutagenic etheno adduct 3,N4-etheno-2'-deoxycytidine($\epsilon$dC) excited state relaxation. This endogenously formed lesion isattracting great interest because of its ubiquity in human tissues and itshighly mutagenic properties. The $\epsilon$dC fluorescence is modified withrespect to that of the canonical base dC, with a 3-fold increased lifetime andquantum yield at neutral pH. This behavior is amplified upon protonation of theetheno ring ($\epsilon$dCH+). Quantum mechanical calculations show that thelowest energy state $\pi$$\pi$*1 is responsible for the fluorescence and thatthe main nonradiative decay pathway to the ground state goes through anethene-like conical intersection, involving the out-of-plane motion of the C5and C6 substituents. This conical intersection is lower in energy than the$\pi$$\pi$* state ($\pi$$\pi$*1) minimum, but a sizable energy barrier explainsthe increase of $\epsilon$dC and $\epsilon$dCH+ fluorescence lifetimes withrespect to that of dC.

the journal of physical chemistry letters

The Excited State Dynamics of a Mutagenic Cytidine Etheno Adduct  Investigated by Combining Time-Resolved Spectroscopy and Quantum Mechanical  Calculations