Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna , Währinger Straße 17, 1090 Vienna, Austria.
J Am Chem Soc. 2016 Dec 14;138(49):15911-15916. doi: 10.1021/jacs.6b06701. Epub 2016 Oct 11.
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 ππ* excited state, which is spectroscopically rather dark. Excitation to the bright ππ* or dark nπ* 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.
环丁烷胸腺嘧啶二聚体的形成是紫外线照射诱导的最重要的 DNA 致癌光损伤之一。这里首次使用非绝热分子动力学模拟首次研究了长期以来争论的问题，即直接光激发后胸腺嘧啶二聚化是否涉及单重态或三重态。我们发现，[2 + 2]环加成反应的前体是单重态双ππ激发态，其光谱相当暗。激发到亮ππ或暗nπ*激发态不会导致胸腺嘧啶二聚体的形成。在所有情况下，在所模拟的时间内都没有观察到到系间窜跃到三重态，这表明超快二聚化发生在单重态中。动力学模拟还表明，只有在双激发态中发生构象控制时，二聚化才会发生。
