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. Taki ng into account dynamic correlation effects (using B3LYP and MP2 calcu lations), we found that, after ionization of the parent neutral dimer, the uracil dimer cation radical dissociates spontaneously. The lack o f any activation barrier for the splitting reaction is in good agreeme nt with the extremely low stability of uracil dimer cations found expe rimentally. Hartree-Fock (HF) calculations predict two reaction pathwa ys for the splitting process. The existence of these paths is associat ed with the conformational flexibility of the intermediate in which th e two uracil rings are connected by the remaining C5-C5' bond. To gain more detailed insight into effects of dynamic correlation on the cycl oreversion reaction, all stationary points found at the HF level were also analyzed using B3LYP and MP2 methodology. Effects of a polar envi ronment on the cleavage process were explored using the self-consisten t reaction field method. In our model study, we also considered the in fluence of structural constraints in DNA on the dimer cation splitting .