Wang Hongjuan, Chen Xuebo
Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Department of Chemistry, Beijing Normal University, Xin-wai-da-jie No. 19, Beijing 100875, China.
Institute of New Energy Materials and Low Carbon Technology, School of Material Science and Engineering, Tianjin University of Technology, Tianjin 300384, China.
Molecules. 2016 Dec 31;22(1):60. doi: 10.3390/molecules22010060.
The photodimerization reaction between the two adjacent thymine bases within a single strand has been the subject of numerous studies due to its potential to induce DNA mutagenesis and possible tumorigenesis in human skin cells. It is well established that the cycloaddition photoreaction takes place on a picosecond time scale along barrierless or low barrier singlet/triplet pathways. However, the observed dimerization quantum yield in different thymine multimer is considerable lower than might be expected. A reasonable explanation is required to understand why thymine in DNA is able to survive ultrafast dimerization damage. In this work, accurate quantum calculations based on the combined CASPT2//CASSCF/AMBER method were conducted to map the excited state relaxation pathways of the thymine monomer in aqueous solution and of the thymine oligomer in DNA. A monomer-like decay pathway, induced by the twisting of the methyl group, is found to provide a bypass channel to ensure the photostability of thymine in single-stranded oligomers. This fast relaxation path is regulated by the conical intersection between the bright S(¹ππ*) state with the intra-base charge transfer character and the ground state to remove the excess excitation energy, thereby achieving the ground-state recovery with high efficiency.
单链中两个相邻胸腺嘧啶碱基之间的光二聚反应因其可能在人类皮肤细胞中诱导DNA诱变和引发肿瘤而受到众多研究。众所周知，环加成光反应沿着无势垒或低势垒的单重态/三重态路径在皮秒时间尺度上发生。然而，在不同胸腺嘧啶多聚体中观察到的二聚化量子产率远低于预期。需要一个合理的解释来理解为什么DNA中的胸腺嘧啶能够在超快二聚化损伤中存活下来。在这项工作中，基于组合的CASPT2//CASSCF/AMBER方法进行了精确的量子计算，以描绘水溶液中胸腺嘧啶单体和DNA中胸腺嘧啶寡聚体的激发态弛豫路径。发现由甲基扭转诱导的类似单体的衰变路径提供了一条旁路通道，以确保单链寡聚体中胸腺嘧啶的光稳定性。这条快速弛豫路径由具有碱基内电荷转移特征的明亮S(¹ππ*)态与基态之间的锥形交叉点调节，以去除多余的激发能，从而高效地实现基态恢复。
