A Triplet Mechanism for the Formation of Cyclobutane Pyrimidine Dimers in UV-Irradiated DNA

摘要

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.