Mechanisms for low-energy (0.5-30 eV) electron-induced pyrimidine ring fragmentation within thymine- and halogen-substituted single strands of DNA

Low-energy (0.5-30 eV) electron irradiation of the thymine- and halogen-substituted nonamers Au/Cy*T-6(3) and Au/Cy*(6)(FU,BrU,IU)(3), chemisorbed onto a gold surface, leads to the desorption of neutral CN and OCN from fragmentation of the DNA base ring. No neutral halogen-containing species are detected. The cross sections at 15 eV for CN and OCN desorbing from the substituted oligomers bromouracil, fluorouracil, iodoracil, and thymine are measured to be 7.5-, 4.5-, (2.5-3.0)-, and (3.0-3.5) x 10(-17) cm(2), respectively; the difference between T and BrU is about 2- to 3-fold over the investigated energy range. The decrease of neutral species produced, in the sequence BrU > FU > IU approximate to T, is observed over the entire 0.5-30 eV range. Unimolecular dissociation of resonant and nonresonant electron-induced uracil-like radicals is likely to be responsible for the formation/desorption of these neutral species. Above 20 eV incident electron energy, the initial process essentially involves nonresonant interactions, whereas within the 5-20 eV energy range, core-excited resonances are implicated. At lower energy, the neutral fragments, which are observed only for bromouracil-substituted oligonucleotides, are induced via a shape resonance. The behavior of the magnitude of the CN and OCN yields with incident electron energy, along with empirical threshold calculations for neutral fragment formation and considerations of transient anion dissociation dynamics, led us to propose possible mechanisms for pyrimidine ring fragmentation by slow electron impact.