The absorption spectrum of guanine based radicals: a comparative computational analysis

The excited states of the three radical derivatives of guanine, i.e. guanine cation (G+) and its two main deprotonated derivatives (G-H1 and G-H2) have been characterized in the Franck-Condon region by TD-DFT, using different functionals, CASPT2, and EOM-EE-CCSD calculations. In the gas phase, all the methods provide a similar description of the main spectral features, the pictures provided by TD-DFT, with long range corrected functionals, and EOM-EE-CCSD being very close.

Solvent effects are then taken into account by a mixed discrete-continuum approach, including five water molecules of the first solvation shell and the Polarizable Continuum Model (PCM). The vibronic absorption line-width has finally been simulated at the TD-M052X level by a time dependent method within the harmonic approximation.

The resulting absorption spectra are in good agreement with their experimental counterparts, providing useful indications on the use of PCM/TD-DFT based approaches to interpret the spectra of guanine based radicals within DNA.