The Excited State Dynamics of a Mutagenic Guanosine Etheno Adduct Investigated by Femtosecond Fluorescence Spectroscopy and Quantum Mechanical Calculations

Abstract

[EN] Femtosecond fluorescence upconversion experiments were combined with CASPT2 and time dependent DFT calculations to characterize the excited state dynamics of the mutagenic etheno adduct 1,N2-etheno-2'-deoxyguanosine (& varepsilon;dG). This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The & varepsilon;dG fluorescence is strongly modified with respect to that of the canonical nucleoside dG, notably by an about 6-fold increase in fluorescence lifetime and quantum yield at neutral pH. In addition, femtosecond fluorescence upconversion experiments reveal the presence of two emission bands with maxima at 335 nm for the shorter-lived and 425 nm for the longer-lived. Quantum mechanical calculations rationalize these findings and provide absorption and fluorescence spectral shapes similar to the experimental ones. Two different bright minima are located on the potential energy surface of the lowest energy singlet excited state. One planar minimum, slightly more stable, is associated with the emission at 335 nm, whereas the other one, with a bent etheno ring, is associated with the red-shifted emission.; The mutagenic etheno adduct of guanine exhibits a peculiar behavior of its lowest singlet excited state. The two independent and short-lived emission bands revealed by femtosecond upconversion fluorescence are associated with two strongly coupled bright minima of different geometry. Overall, the presence of the extra etheno ring on guanine slows down the efficient relaxation pathways, which are essential for the DNA photostability. image