State Key Laboratory of Quality Research in Chinese Medicine, Institute of Chinese Medical Sciences, University of Macau, Avenida da Universidade, Taipa, Macau, P. R. China.
Aix Marseille Univ, CNRS, Spectropole, FR 1739, Marseille, France.
Chemistry. 2019 Sep 25;25(54):12552-12559. doi: 10.1002/chem.201902057. Epub 2019 Aug 7.
A viologen derivative carrying a benzimidazole group (V-P-I ; viologen-phenylene-imidazole V-P-I) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK , by more than 1 pH unit, contrasting with the positive pK shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ⋅CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.
一种带有苯并咪唑基团的联吡啶衍生物（V-P-I；联吡啶-亚苯基-咪唑 V-P-I）可以在水中通过葫芦[8]脲（CB[8]）形成 2:2 复合物二聚化，导致客体 pK 值发生超过 1 pH 单位的负移，与通常观察到的基于 CB 的复合物的正 pK 移相 CONTRAST。尽管 NMR 无法清楚地表明 2:2 复合物的质子化，但已使用银阳离子来探测 2:2 复合物中咪唑基团的可及性。埋藏的咪唑基团的质子化能力降低，表明 CB[8]在形成 2:2 复合物时可能引发质子释放。通过 pH 计监测和有色指示剂可视化，向含有 V-P-的溶液中添加 CB[8]确实释放了质子-I。该性质用于诱导 V-P-I ⋅CB[7]和 1-甲基-4-(4-吡啶基)吡啶鎓的 CB[8]配合物之间的主体/客体交换，同时伴随质子转移。这种负 pK 移的起源被认为是理想的电荷状态，以及在两个 CB[8]内两个 pH 响应片段的位置，类似于被蛋白质吞没的残基，有利于客体分子的去质子化形式。这种由识别事件引发的质子释放类似于几种生物过程，可能为模拟生物酶催化质子转移或化学反应开辟新途径。
