Photoexcitation of dinucleoside radical cations: a time-dependent density functional study

The excited states of dinucleoside phosphates (dGpdG, dApdA, dApdT, TpdA, and dGpdT) in their cationic radical states were studied with time-dependent density functional theory (TD-DFT). The ground-state geometries of all the dinucleoside phosphate cation radicals considered, in their base stacked conformation, were optimized with the B3LYP/6-31G(d) method. Further, to take into account the effect of the aqueous environment surrounding the dinucleoside phosphates, the polarized continuum model (PCM) was considered and the excitation energies were computed by using the TD-B3LYP/6-31G(d) method. From this study, we find that the first transition in all the dinucleoside molecules involves hole transfer from base to base. dG*+pdG and dApdA*+ were found to have substantially lower first transition energies than others with two different DNA bases. Higher energy transitions involve base to sugar as well as base to base hole transfer. The calculated TD-B3LYP/6-31G(d) transition energies are in good agreement with previous calculations with CASSCF/CAS-PT2 level of theory. This TD-DFT work supports the experimental findings that sugar radicals formed upon photoexcitation of G*+ in gamma-irradiated DNA and suggests an explanation for the wavelength dependence found.