Towards an effective computational tool for the study of radiation-inducedlesions 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 thy mine gives rise to the 5,6-dihydro-5-thymyl or 5,6-dihydro-6-thymyl radical s. The activation and reaction energies governing these reactions were inve stigated by Hartree-Fock, perturbative many-body, and Kohn-Sham methods als o taking the zero-point, thermal and entropic effects into account. Among t he different methods, only the B3LYP approach consistently provides reliabl e structural, spectroscopic and energetic characteristics both for closed- and open-shell systems. At this level, formation of the 5,6-dihydro-5-thymy l radical is preferred from both thermodynamic and kinetic points of view. This is in agreement with the predominance of products derived from this ra dical intermediate confirmed by several experimental studies. (C) 1999 Else vier Science B.V. All rights reserved.