Excited State Dynamics of 7-Deazaguanosine and Guanosine 5'-Monophosphate.

Minorstructural modifications to the DNA and RNA nucleobases have a significanteffect on their excited state dynamics and electronic relaxation pathways. In this study, the excited statedynamics of 7-deazaguanosine and guanosine 5’-monophosphate are investigated inaqueous and in a mixture of methanol and water using femtosecond broadbandtransient absorption spectroscopy following excitation at 267 nm. The transientspectra are collected using photon densities that ensure no parasiticmultiphoton-induced signal from solvated electrons. The data can be fit satisfactorilyusing a two- or three-component kinetic model. By analyzing the results fromsteady-state, time-resolved, computational calculations, and the methanol-watermixture, the following general relaxation mechanism is proposed for bothmolecules, Lb ®La ®1ps*(ICT) ®S0, where the 1ps*(ICT) stands for an intramolecular charge transfer excitedsinglet state with significant ps*character. In general, longer lifetimes for internal conversion are obtainedfor 7-deazaguanosine compared to guanosine 5’-monophosphate. Internalconversion of the 1ps*(ICT)state to the ground state occurs on a similar time scale of a few picosecondsin both molecules. Collectively, the results demonstrate that substitution of asingle nitrogen for a methine (C-H) group at position seven of the guaninemoiety stabilizes the 1pp* Lb and La states and alter thetopology of their potential energy surfaces in such a way that the relaxationdynamics in 7-deazaguanosine are slowed down compared to those in guanosine5’-monophosphate but not for the internal conversion of 1ps*(ICT) state to the ground state.