Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China.
Phys Chem Chem Phys. 2017 May 31;19(21):13524-13533. doi: 10.1039/c7cp01511a.
In contrast to photophysics of thio-substituted nucleobases, their photoinduced cross-linking reactions with canonical nucleobases remain scarcely investigated computationally. In this work, we have adopted combined CASPT2/PCM//CASSCF and B3LYP-D3/PCM electronic structure methods to study this kind of photochemical reaction of 5-fluoro-4-thiouridine (truncated 5-fluoro-1-methyl-4-thiouracil used in calculations) and 1-methylthymine (referred to as thymine for clarity hereinafter). On the basis of CASPT2/PCM computed results, we have proposed two efficient excited-state relaxation pathways to populate the lowest T state of the complex of 5-fluoro-1-methyl-4-thiouracil and thymine from its initially populated S(ππ*) state. In the first one, the S system first hops to the S state via an S/S conical intersection, followed by a direct S → T intersystem crossing process enhanced by large S/T spin-orbit coupling. In the second path, the resultant S system first jumps to the T state, from which an efficient T → T internal conversion occurs. The T cross-linking reaction is overall divided into two phases. The first phase is a stepwise and nonadiabatic photocyclization reaction, which starts from the T complex and ends up with an S thietane intermediate. The second phase is a thermal reaction. The system first rearranges its four- and six-membered rings to form three new rings; then, an S fluorine atom transfer occurs, followed by the formation of photoproducts. Finally, the present work paves the way for studying light-induced cross-linking reactions of thionucleobases with canonical bases in DNA and RNA.
与硫代碱基的光物理性质相反，其与典型碱基的光诱导交联反应在计算上仍鲜有研究。在这项工作中，我们采用了组合 CASPT2/PCM//CASSCF 和 B3LYP-D3/PCM 电子结构方法来研究 5-氟-4-硫尿嘧啶（用于计算的 5-氟-1-甲基-4-硫代尿嘧啶的截断形式）和 1-甲基胸腺嘧啶（为清楚起见，以下简称胸腺嘧啶）的这种光化学反应。基于 CASPT2/PCM 计算结果，我们提出了两种有效的激发态弛豫途径，从其最初占据的 S(ππ*)态将 5-氟-1-甲基-4-硫代尿嘧啶和胸腺嘧啶复合物的最低 T 态占据。在第一种途径中，S 体系首先通过 S/S 锥形交叉跃迁到 S 态，随后通过大的 S/T 自旋轨道耦合增强的直接 S→T 系间窜越过程。在第二条路径中，所得的 S 体系首先跃迁到 T 态，其中发生有效的 T→T 内转换。T 交联反应总体上分为两个阶段。第一阶段是逐步的非绝热光环化反应，从 T 复合物开始，以 S 硫杂环丁烷中间体结束。第二阶段是热反应。体系首先将其四元和六元环重新排列形成三个新环；然后，发生 S 氟原子转移，随后形成光产物。最后，本工作为研究 DNA 和 RNA 中硫代碱基与典型碱基的光诱导交联反应铺平了道路。
