Guanine-derived radicals: dielectric constant-dependent stability and UV/Vis spectral properties: a DFT study

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(*)(-).