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| Title
| - A Triplet Mechanism for the Formation of Cyclobutane Pyrimidine Dimers inUV-Irradiated DNA
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| Abstract
| - The reaction pathways for the photochemical formation of cyclobutane thymine dimers in DNA are exploredusing 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 excitedstates. The stepwise cycloaddition in the triplet excited state involves the initial formation of a diradicalfollowed by ring closure via singlet−triplet interaction. The triplet mechanism is thus completely differentfrom the concerted singlet state cycloaddition processes. The key geometric features and electron spin densitiesare also discussed. Bulk solvation has a major effect by reducing the barriers and increasing the diradicalstabilities. The present results provide a rationale for the faster cycloreaction observed in the singlet excitedstates than in the triplet excited states.
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