Abstract N6-Methyladenine (6MeAde) was regarded as an important epigenetic biomarker in gene regulation. Recently, Zhou et al. observed a dual peak fluorescence emission of 6MeAde in aqueous solution (Phys. Chem. Chem. Phys., 2019, 21, 6878), in which the long-wavelength fluorescence was speculated to the isomerized intramolecular charge transfer (ICT). Nevertheless, this speculation still lacked a direct evidence and the essential mechanism of how 6MeAde was regulated by water molecules remained ambiguous. In present work, we theoretically studied the effect of water molecules on excited-state proton transfer (ESPT) process of 6MeAde based on the time-dependent DFT (TDDFT) methods. 6MeAde possessed both hydrogen acceptor and donor that formed intermolecular hydrogen bonds with water molecules. Thus, the excited-state hydrogen bonding dynamics may occur in three scenarios. The constructed potential energy surfaces provided direct evidence that proton transfer cannot occur for 6MeAde monomer and 6MeAde + H2O, but could occur spontaneously for 6MeAde+2H2O due to the coplanar closed-loop hydrogen bond structure. This result pointed out that determining the energy barrier of a potential energy surface was the key to explore the real ESPT channel. Besides, our analyses of electron spectra and electronic structures confirmed that the long-wavelength fluorescence originated from the ICT emission.

中文翻译：

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水溶液中 N6-甲基腺嘌呤的 ESPT 途径和 ICT 排放的直接证据†

摘要 N6-甲基腺嘌呤（6MeAde）被认为是基因调控中重要的表观遗传生物标志物。最近，周等人。观察到 6MeAde 在水溶液中的双峰荧光发射 (Phys. Chem. Chem. Phys., 2019, 21, 6878)，其中长波长荧光推测为异构化的分子内电荷转移 (ICT)。尽管如此，这种推测仍缺乏直接证据，6MeAde 受水分子调控的基本机制仍不明确。在目前的工作中，我们基于时间依赖的 DFT (TDDFT) 方法从理论上研究了水分子对 6MeAde 激发态质子转移 (ESPT) 过程的影响。6MeAde 具有氢受体和供体，与水分子形成分子间氢键。因此，激发态氢键动力学可能发生在三种情况下。构建的势能面提供了直接证据，表明 6MeAde 单体和 6MeAde + H2O 不能发生质子转移，但由于共面闭环氢键结构，6MeAde+2H2O 可以自发发生。该结果表明，确定势能面的能垒是探索真正的ESPT通道的关键。此外，我们对电子光谱和电子结构的分析证实，长波长荧光源自 ICT 发射。但由于共面闭环氢键结构，6MeAde+2H2O 可能自发发生。该结果表明，确定势能面的能垒是探索真正的ESPT通道的关键。此外，我们对电子光谱和电子结构的分析证实，长波长荧光源自 ICT 发射。但由于共面闭环氢键结构，6MeAde+2H2O 可能自发发生。该结果表明，确定势能面的能垒是探索真正的ESPT通道的关键。此外，我们对电子光谱和电子结构的分析证实，长波长荧光源自 ICT 发射。