Abstract
The formation of cyclobutane thymine dimers is one of the most important DNA carcinogenic photolesions induced by ultraviolet irradiation. The long debated question whether thymine dimerization after direct light excitation involves singlet or triplet states is investigated here for the first time using nonadiabatic molecular dynamics simulations. We find that the precursor of this [2 + 2] cycloaddition reaction is the singlet doubly π2π*2 excited state, which is spectroscopically rather dark. Excitation to the bright 1ππ* or dark 1nπ* excited states does not lead to thymine dimer formation. In all cases, intersystem crossing to the triplet states is not observed during the simulated time, indicating that ultrafast dimerization occurs in the singlet manifold. The dynamics simulations also show that dimerization takes place only when conformational control happens in the doubly excited state.
Highlights
Exposure to ultraviolet (UV) radiation can damage DNA.[1]
These rules forbid a thermal dimerization in the ground state; they do not prescribe whether photochemical dimerization should proceed in the singly excited (1SE) or doubly excited (1DE) state
Our ab initio molecular dynamics simulations show that ultrafast cyclobutane thymine dimerization requires populating the singlet doubly excited state, 1DE
Summary
Exposure to ultraviolet (UV) radiation can damage DNA.[1]. The deleterious action of UV light originates from the photochemical changes that lead to biological alterations. Reaction pathways claiming the participation of the 3ππ* state have been calculated.[18,19] In addition to direct excitation, T T dimers can form under triplet photosensitization In the latter case, it is well established that the reaction proceeds via triplet states: After excitation, a photosensitizer undergoes intersystem crossing and transfers its energy to a thymine molecule, thereby promoting the latter to its triplet state. In this paper we investigate whether UV exposure triggers T T dimer formation in the singlet or triplet manifold To this aim, we report the first ab initio nonadiabatic molecular dynamics simulations of a stacked thymine dimer including all relevant singlet and triplet states. The calculations are performed in gas phase using the surface-hopping including arbitrary couplings (SHARC) scheme[23,24] coupled to multiconfigurational state-average complete active space selfconsistent field[25] (SA-CASSCF) energies, gradients, and spin−orbit couplings (see computational details in Section S1 of the Supporting Information (SI))
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