Study of Electron Capture by 5-Halogenated Cytosine in Gas and Condensed Phases

Halogenated pyrimidines have been a compound of interest for a long time due to their wide range of applications in biological science. In this study, we sought to evaluate the effectiveness of halogenated cytosine as a radiosensitizer by analyzing its interaction with low-energy electrons produced during radiotherapy. Our results revealed that halogenated cytosine forms a dipole-bound state in the gas phase after electron attachment in the Franck-Condon region. We utilized a nuclear charge stabilization method and employed the regularized analytical continuation technique to determine the position and width of the π* and σ* resonance states in gas-phase halogenated cytosines. The halogenation of cytosine enhances the stability of the π* resonance. The σ* resonance of the C-Cl bond appears at 2.71 eV, whereas for the C-Br bond, it is observed at a lower energy of 1.68 eV. In the condensed phase, stable five-water clusters of halogenated nucleobases [5 - XC(H₂O)₅] were analyzed using a polarizable continuum model of implicit solvation to study the bulk effect of water. The vertically attached anions were found to be π* valence-bound in nature. From ab initio molecular dynamics simulations, dissociation in the brominated cytosine cluster was predicted, whereas no dissociative processes occurred in the chlorinated and fluorinated clusters.