Quantum theoretical study of mechanism of the reaction between guanine radical cation and carbonate radical anion: Formation of 8-oxoguanine

The reaction between guanine radical cation (G ^+·) and carbonate radical anion (CO ₃ ^-·) producing the mutagenic product 8-oxoguanine that has been observed experimentally was investigated using density functional and second-order Møller-Plesset perturbation (MP2) theories. The structures of reactant, intermediate, and product complexes as well as those of the transition states were fully optimized at the B3LYP/6-31G, B3LYP/AUG-cc-pVDZ, and BHandHLYP/AUG-cc-pVDZ levels of density functional theory in gas phase. Single point energy calculations were performed at the MP2/AUG-cc-pVDZ level using the BHandHLYP/AUG-cc-pVDZ level gas-phase optimized geometries. To obtain the equivalent solvent that would approximately represent the complex medium used in the experimental study, bulk solvent effect on the stability of the reactant complex was studied by single point energy calculations in various solvent media (toluene, chlorobenzene, dichloroethane, acetone dimethylsulfoxide, and water) using the polarizable continuum model. Thus, chlorobenzene was found to represent the experimental medium fairly closely. The calculated Gibbs free barrier and released energies show that the reaction under consideration would occur efficiently.