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  • Title: A triplet mechanism for the formation of cyclobutane pyrimidine dimers in UV-irradiated DNA.
    Author: Zhang RB, Eriksson LA.
    Journal: J Phys Chem B; 2006 Apr 13; 110(14):7556-62. PubMed ID: 16599537.
    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.
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