title: A triplet mechanism for the formation of cyclobutane pyrimidine dimers in UV-irradiated DNA.
abstract: The reaction pathways for the photochemical formation of cyclobutane thymine dimers in DNA are explored using hybrid density functional theory techniques. It is concluded that the thymine-thymine [2 + 2] cycloaddition displays favorable energy barriers and reaction energies in both the triplet and the singlet excited states. The stepwise cycloaddition in the triplet excited state involves the initial formation of a diradical followed by ring closure via singlet-triplet interaction. The triplet mechanism is thus completely different from the concerted singlet state cycloaddition processes. The key geometric features and electron spin densities are also discussed. Bulk solvation has a major effect by reducing the barriers and increasing the diradical stabilities. The present results provide a rationale for the faster cycloreaction observed in the singlet excited states than in the triplet excited states.
authors.affiliation: Department of Natural Sciences and Orebro Life Science Center, Orebro University, 70182 Orebro, Sweden.
authors.forename: Ru Bo
authors.initials: RB
authors.lastname: Zhang
authors.forename: Leif A
authors.initials: LA
authors.lastname: Eriksson
mesh_terms: D003198:Computer Simulation; D003500:Cyclization; D003503:Cyclobutanes / Q000737:chemistry*; D004247:DNA / Q000737:chemistry* / Q000528:radiation effects*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D011743:Pyrimidines / Q000737:chemistry*; D012997:Solvents; D013941:Thymine / Q000737:chemistry; D014466:Ultraviolet Rays
chemical_list: D003503:Cyclobutanes; D011743:Pyrimidines; D012997:Solvents; D004247:DNA; D013941:Thymine