Excited States of One-Electron Oxidized Guanine-Cytosine Base Pair Radicals: A Time Dependent Density Functional Theory Study.

One-electron oxidized guanine (G) in DNA generates several short-lived intermediate radicals via proton transfer reactions resulting in the formation of neutral guanine radicals. The identification of these radicals in DNA is of fundamental interest to understand the early stages of DNA damage. Herein, we used time-dependent density functional theory (TD-ωB97XD-PCM/6-31G(3df,p)) to calculate the vertical excitation energies of one-electron oxidized G and G-cytosine (C) base pair in various protonation states: G, G(N1-H), and G(N2-H), as well as G-C, G(N1-H)-(H)C, G(N1-H)-(N4-H)C), G(N1-H)-C, and G(N2-H)-C in aqueous phase. The calculated UV-vis spectra of these radicals are in good agreement with the experiment for the G radical species when the calculated values are red-shifted by 40-70 nm. The present calculations show that the lowest energy transitions of proton transfer species (G(N1-H)-(H)C, G(N1-H)-(N4-H)C, and G(N1-H)-C) are substantially red-shifted in comparison to the spectrum of G-C. The calculated spectrum of G(N2-H)-C shows intense absorption (high oscillator strength), which matches the strong absorption in the experimental spectra of G(N2-H) at 600 nm. The present calculations predict the lowest charge transfer transition of C → G is π → π* in nature and lies in the UV region (3.4-4.3 eV) with small oscillator strength.