Towards an effective computational tool for the study of radiation-induced lesions of DNA bases: Hydrogen addition to thymine as a test case

Radiation-induced addition of a hydrogen atom to the 5,6 double bond of thymine gives rise to the 5,6-dihydro-5-thymyl or 5,6-dihydro-6-thymyl radicals. The activation and reaction energies governing these reactions were investigated by Hartree–Fock, perturbative many-body, and Kohn–Sham methods also taking the zero-point, thermal and entropic effects into account. Among the different methods, only the B3LYP approach consistently provides reliable structural, spectroscopic and energetic characteristics both for closed- and open-shell systems. At this level, formation of the 5,6-dihydro-5-thymyl radical is preferred from both thermodynamic and kinetic points of view. This is in agreement with the predominance of products derived from this radical intermediate confirmed by several experimental studies.