Reaction mechanism of thymine dimer formation in DNA induced by UV light

The formation of thymine dimers in DNA is investigated by means of density functional theory (DFT) techniques. While it is found that a thermally induced [2 + 2] cycloaddition reaction proceeds via a very high energy transition state (80-88 kcal/mol above the reactant complex), the energy barrier for UV light induced formation-explored within the time-dependent DFT formalism-is only a few kilocalories per mol. As such, these results serve as an illustrative example of how UV radiation may induce DNA lesions. For the reactant complex, the calculated vertical excitation energy corresponding to the S-1 <-- S-0 transition (pi --> pi*) lies in the far-UV region, in accordance with experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.