A triplet mechanism for the formation of cyclobutane pyrimidine dimers in UV-irradiated DNA.
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.
Zhang
Ru Bo
RB
Department of Natural Sciences and Orebro Life Science Center, Orebro University, 70182 Orebro, Sweden.
Eriksson
Leif A
LA
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
D003503:Cyclobutanes; D011743:Pyrimidines; D012997:Solvents; D004247:DNA; D013941:Thymine