Department of Chemistry, Faculty of Science, Sultan Qaboos University, Muscat, Sultanate of Oman.
J Phys Chem A. 2011 May 5;115(17):4195-201. doi: 10.1021/jp201649z. Epub 2011 Apr 8.
Tautomerism in the ground and excited states of 7-hydroxyquinoline (7HQ) was studied in different solvents using steady-state and lifetime spectroscopic measurements, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Equilibrium between the enol and the keto/zwitterion tautomers exists in 7HQ, which is solvent-dependent. Of the solvents used in this study, only in water does the absorbance spectrum of 7HQ show absorption from both the enol and zwitterion tautomers. In addition, in aqueous media, fluorescence is observed from the zwitterion tautomer only, which is attributed to self-quenching of the enol fluorescence by energy transfer to the ground-state zwitterion tautomer and energetically favorable excited-state proton transfer. Solvation of the hydrogen bonding sites of 7HQ was studied in binary mixtures of 1,4-dioxane and water, and three water molecules were estimated to connect the polar sites and induce intermolecular proton transfer. The results are confirmed by DFT calculations showing that three water molecules are the minimum number required to form a stable solvent wire. Mapping the water density around the polar sites using MD simulations shows well-defined hydrogen bonds around the amino and hydroxyl groups of the enol tautomer and slightly less well-defined hydrogen bonds for the zwitterion tautomer. The presence of three-member water wires connecting the polar centers in 7HQ is evident in the MD simulations. The results point to the unique spectral signatures of 7HQ in water, which make this molecule a potential probe to detect the presence of water in nanocavities of macromolecules.
7-羟基喹啉（7HQ）在基态和激发态的互变异构平衡研究使用稳态和寿命光谱测量、密度泛函理论（DFT）计算和分子动力学（MD）模拟在不同溶剂中进行。7HQ 中存在烯醇和酮/两性离子互变异构体之间的平衡，这种平衡取决于溶剂。在本研究中使用的溶剂中，只有在水中，7HQ 的吸收光谱才显示出烯醇和两性离子互变异构体的吸收。此外，在水介质中，仅从两性离子互变异构体观察到荧光，这归因于烯醇荧光通过能量转移到基态两性离子互变异构体和有利的激发态质子转移而自猝灭。7HQ 的氢键结合部位在 1,4-二氧六环和水的二元混合物中的溶剂化作用进行了研究，估计有三个水分子连接极性部位并诱导分子间质子转移。DFT 计算结果证实了这一点，表明形成稳定溶剂线所需的最小水分子数为三个。使用 MD 模拟在极性部位周围绘制水分子密度图表明，在烯醇互变异构体的氨基和羟基周围存在明确的氢键，而在两性离子互变异构体周围的氢键则稍不明确。在 MD 模拟中，三原子水分子连接极性中心的存在在 7HQ 中是显而易见的。结果表明 7HQ 在水中具有独特的光谱特征，这使得该分子成为探测大分子纳米腔中水分子存在的潜在探针。
