Splitting of cyclobutane-type uracil dimer cation radicals. Hartree-Fock, MP2, and density functional studies

The cation pathway of the cyclobutane-type uracil dimer cycloreversion process has been studied using various quantum chemical methods. Taking into account dynamic correlation effects (using B3LYP and MP2 calculations), we found that, after ionization of the parent neutral dimer, the uracil dimer cation radical dissociates spontaneously. The lack of any activation barrier for the splitting reaction is in good agreement with the extremely low stability of uracil dimer cations found experimentally. Hartree-Fock (HF) calculations predict two reaction pathways for the splitting process. The existence of these paths is associated with the conformational flexibility of the intermediate in which the two uracil rings are connected by the remaining C5-C5′ bond. To gain more detailed insight into effects of dynamic correlation on the cycloreversion reaction, all stationary points found at the HF level were also analyzed using B3LYP and MP2 methodology. Effects of a polar environment on the cleavage process were explored using the self-consistent reaction field method. In our model study, we also considered the influence of structural constraints in DNA on the dimer cation splitting.