Calculation of spin densities of radicals of pyrimidine-type bases by density functional theory. Influence of solvent and comparison with EPR results

Abstract

Spin densities and hyperfine coupling (hfc) constants of thymine, 1-methylthymine, uracil, 1-methyluracil, 6-methyluracil, 1,3-dimethyluracil, cytosine and 1-methylcytosine radicals were calculated through the use of density functional theory (DFT) considering the influence of the surrounding solvent molecules by their dielectric properties by applying the Onsager model. The results are compared with experimental hfc constants obtained with Fourier-transform (FT) EPR experiments. The radicals were generated in aqueous solution using the triplet sensitized electron transfer from the pyrimidine-type bases to photoexcited spin-polarized anthraquinone-2,6-disulfonate. It is found that the geometry and the spin density distribution of the radical cations and their deprotonated successor radicals are influenced by a surrounding dielectric solvent. The hfc constants calculated for an aqueous medium are in good agreement with the experimental FT EPR results.