Li Chaozheng, Yang Yonggang, Li Donglin, Liu Yufang
College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China.
Phys Chem Chem Phys. 2017 Feb 8;19(6):4802-4808. doi: 10.1039/c6cp07716a.
The excited-state double proton transfer (ESDPT) mechanism in a model DNA base pair, 7-azaindole (7AI) dimer, has been debated over the years. Recently, Otero and coworkers concluded that the stepwise mechanism is not possible and the concerted mechanism dominates the dynamics (Chem. Sci., 2015, 6, 5762). In this work, the potential energy surfaces of the 7AI dimer in the ground state (S) and the lowest energy excited singlet state (S) were constructed. After vertical excitation to the S state, the single proton transfer can occur. The second proton transfer process in the stepwise mechanism is blocked by a high potential barrier (36.4 kcal mol), which is consistent with the result proposed by Otero and coworkers. However, the single proton transfer process is compatible with the concerted mechanism and we show that the single proton transfer process rather than the concerted mechanism dominates the dynamics. The concerted process is unfavorable in the S state compared with the barrierless single proton transfer process. In addition, the proton transfer process in the S state is revealed. The single proton transfer tautomer in the S state returns to the S state and transfers the second proton via a barrierless process. Finally, the double proton transfer tautomer in the S state can recover to the normal dimer through the reverse proton transfer reaction.
多年来，模型DNA碱基对7-氮杂吲哚（7AI）二聚体中的激发态双质子转移（ESDPT）机制一直存在争议。最近，奥特罗及其同事得出结论，逐步机制是不可能的，协同机制主导了动力学过程（《化学科学》，2015年，第6卷，5762页）。在这项工作中，构建了7AI二聚体在基态（S）和最低能量激发单重态（S）的势能面。垂直激发到S态后，单质子转移可以发生。逐步机制中的第二个质子转移过程被一个高势能垒（36.4千卡/摩尔）阻断，这与奥特罗及其同事提出的结果一致。然而，单质子转移过程与协同机制是兼容的，并且我们表明单质子转移过程而非协同机制主导了动力学过程。与无势垒的单质子转移过程相比，协同过程在S态是不利的。此外，还揭示了S态中的质子转移过程。S态中的单质子转移互变异构体回到S态，并通过无势垒过程转移第二个质子。最后，S态中的双质子转移互变异构体可以通过逆质子转移反应恢复到正常二聚体。
