Naumov, S. and von Sonntag, C. Guanine-Derived Radicals: Dielectric Constant-Dependent Stability and UV/Vis Spectral Properties: A DFT Study. Radiat. Res. 169, 364–372 (2008). Upon successive deprotonation of the guanine radical cation, various neutral radicals and radical anions can be formed. Their relative stability and UV/Vis absorption spectra have been calculated by DFT in the vacuum and in aqueous solution. Good agreement with experimental data is obtained when solvent effects are taken into account. The experimental observation that in the nucleosides deprotonation of the guanine radical cation occurs at N1 (formation of N1G · ) in water and at N2 (formation of N2G · ) in single crystals is now explained by a strong effect of the dielectric constant of the environment on their stability. While SCRF=PCM and CPCM (Gaussian 03) describe the trend, SCRF=DPCM (Gaussian 98) even shows the crossover from N2G · to N1G · at high dielectric constant. A crossover of the preferred deprotonation site is also given by the nucleoside itself. While for the gas phase a deprotonation at N2 is calculated to be favored over that at N1, the reverse is found for an aqueous environment (in agreement with the experiment). The radical anions of guanine, N9N1G ·− and N9N2G ·− , are very similar in energy, but a comparison of the experimental and calculated UV/Vis spectra allows us to identify the experimentally observed intermediate clearly as N9N1G ·− .

Naumov, S. and von Sonntag, C. The Energetics of Rearrangement and Water Elimination Reactions in the Radiolysis of the DNA Bases in Aqueous Solution (e aq − and · OH Attack): DFT Calculations. Radiat. Res. 169, 355–363 (2008). DFT calculations on the relative stability of various nucleobase radicals induced by e aq − and · OH have been carried out for assessing the energetics of rearrangements and water elimination reactions, taking the solvent effect of water into account. Uracil and thymine radical anions are protonated fast at O2 and O4, whereby the O2-protonated anions are higher in energy (50 kJ mol −1 , equivalent to a 9-unit lower p K a ). The experimentally observed p K a = 7 is thus that of the O4-protonated species. Thermodynamically favored protonation occurs slowly at C6 (driving force, thymine: 49 kJ mol −1 , uracil: 29 kJ mol −1 ). The cytosine radical anion is rapidly protonated by water at N3. Final protonation at C6 is disfavored here. The kinetically favored pyrimidine C5 · OH adducts rearrange into the thermodynamically favored C6 · OH adducts (driving force, thymine: 42 kJ mol −1 ). Very similar in energy is a water elimination that leads to the Ura-5-methyl radical. Purine · OH adducts at C4 and C5 (plus C2 in guanine) eliminate water in exothermic reactions, while water elimination from the C8 · OH adducts is endothermic. The latter open the ring en route to the FAPY products, an H transfer from the C8 · OH to N9 being the most likely process.