Minor
structural modifications to the DNA and RNA nucleobases have a significant
effect on their excited state dynamics and electronic relaxation pathways.<b> </b>In this study, the excited state
dynamics of 7-deazaguanosine and guanosine 5’-monophosphate are investigated in
aqueous solution using femtosecond broadband transient absorption spectroscopy
following excitation at 267 nm. The transient absorption spectra are collected under
experimental conditions that eliminate the requirement to correct the data for
the formation of hydrated electrons, resulting from the two-photon ionization
of the solvent. The data is fitted satisfactorily using a two-component sequential
kinetic model, yielding lifetimes of 210 ± 50 fs and 1.80 ± 0.02 ps, and 682 ±
40 fs and 1.4 ± 0.03 ps, for 7-deazaguanosine and guanosine 5’-monophosphate, respectively.
By analyzing the results from steady-state, time-resolved, and computational
calculations, the following relaxation mechanism is proposed for 7-deazaguanosine,
S<sub>2</sub>(L<sub>b</sub>) ® S<sub>1</sub>(L<sub>a</sub>) ® S<sub>0</sub>,
whereas a S<sub>2</sub>(L<sub>b</sub>) ® S<sub>1</sub>(L<sub>a</sub>) ® S<sub>0</sub>(hot)<sub>
</sub>® S<sub>0 </sub>relaxation mechanism<sub> </sub>is
proposed for guanosine 5’-monophosphate. Interestingly, longer lifetimes for
both the L<sub>b</sub> ® L<sub>a</sub> and the L<sub>a</sub> ® S<sub>0</sub>
internal conversion pathways are obtained for 7-deazaguanosine compare to guanosine
5’-monophosphate. Collectively, the results demonstrate that substitution of a
single nitrogen for a methine (C-H) group at position seven of the guanine moiety
stabilizes the <sup>1</sup>pp* L<sub>b</sub> and L<sub>a</sub> states and alters
the topology of their potential energy surfaces in such a way that the
population dynamics of both internal conversion pathways in 7-deazaguanosine are
significantly slowed down compared to those in guanosine 5’-monophosphate.
Excited State Dynamics of 7-Deazaguanosine and Guanosine 5’ Monophosphate in Aqueous Solution