Computational Study of the Radical Mediated Mechanism of the Formation of C8, C5, and C4 Guanine:Lysine Adducts in the Presence of the Benzophenone Photosensitizer.

The oxidation of guanine by triplet benzophenone in the presence of lysine has been shown to produce mono- and dilysine-substituted spiroiminodihydantion products, 8-Lys-Sp and 5,8-diLys-Sp. The potential energy surfaces for C8, C5, and C4 nucleophilic addition have been mapped out using the B3LYP/aug-cc-pVTZ//B3LYP/6-31+G(d,p) level of density functional theory with the SMD solvation model and employing methylamine as a model for the side chain of lysine. Enthalpies, barrier heights, pKa's, and reduction potentials were calculated for intermediates to find the lowest energy paths. For neutral methylamine plus guanine radical and neutral methylamine radical plus guanine, the barrier for addition at C8 is ca. 10 kcal/mol lower than that for addition at C5 and C4. The barriers for water addition at C8, C5, and C4 of guanine radical are 13-20 kcal/mol higher than that for methylamine addition at C8. Further oxidation and loss of a proton leads to 8-methylaminoguanine, the methylamino analogue of 8-oxo-7,8-dihydroguanine (8-oxoG). The barrier for the addition of a second methylamine at C5 of 8-methylaminoguanine is 4.5 kcal/mol lower than that for the corresponding addition of water. Nevertheless, if the concentration of methylamine (or lysine) is very low, water addition could be competitive with methylamine addition. This would lead to comparable fractions of 8-monosubstituted-Sp and 5-8-disubstituted-Sp, in agreement with the experimental observations.