CNRS, IRAMIS, SPAM, Laboratoire Francis Perrin, URA 2453, 91191 Gif-sur-Yvette, France.
J Am Chem Soc. 2012 Sep 12;134(36):14834-45. doi: 10.1021/ja304069f. Epub 2012 Aug 31.
The study addresses interconnected issues related to two major types of cycloadditions between adjacent thymines in DNA leading to cyclobutane dimers (T<>Ts) and (6-4) adducts. Experimental results are obtained for the single strand (dT)(20) by steady-state and time-resolved optical spectroscopy, as well as by HPLC coupled to mass spectrometry. Calculations are carried out for the dinucleoside monophosphate in water using the TD-M052X method and including the polarizable continuum model; the reliability of TD-M052X is checked against CASPT2 calculations regarding the behavior of two stacked thymines in the gas phase. It is shown that irradiation at the main absorption band leads to cyclobutane dimers (T<>Ts) and (6-4) adducts via different electronic excited states. T<>Ts are formed via (1)ππ* excitons; [2 + 2] dimerization proceeds along a barrierless path, in line with the constant quantum yield (0.05) with the irradiation wavelength, the contribution of the (3)ππ* state to this reaction being less than 10%. The formation of oxetane, the reaction intermediate leading to (6-4) adducts, occurs via charge transfer excited states involving two stacked thymines, whose fingerprint is detected in the fluorescence spectra; it involves an energy barrier explaining the important decrease in the quantum yield of (6-4) adducts with the irradiation wavelength.
该研究解决了与 DNA 中相邻胸腺嘧啶之间两种主要类型的环加成相关的相互关联的问题，导致环丁烷二聚体（T<>Ts）和（6-4）加合物。通过稳态和时间分辨光光谱以及与质谱联用的 HPLC 获得了单链（dT）（20）的实验结果。使用 TD-M052X 方法并包括极化连续体模型对二核苷酸单磷酸在水中进行了计算；使用 TD-M052X 计算的可靠性通过 CASPT2 计算来检查气相中两个堆叠的胸腺嘧啶的行为。结果表明，在主吸收带的照射下，通过不同的电子激发态形成环丁烷二聚体（T<>Ts）和（6-4）加合物。T<>Ts 通过（1）ππ激子形成；[2+2]二聚化沿无势垒的路径进行，与辐照波长的恒定量子产率（0.05）一致，（3）ππ态对此反应的贡献小于 10%。导致（6-4）加合物的开环反应中间体恶唑烷的形成通过涉及两个堆叠的胸腺嘧啶的电荷转移激发态发生，其指纹在荧光光谱中被检测到；它涉及一个能量势垒，解释了（6-4）加合物的量子产率随辐照波长的重要下降。
