A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift. Issue 54 (7th August 2019)

• Record Type: Journal Article
• Authors: Yin, Hang; Cheng, Qian; Rosas, Roselyne; Viel, Stéphane; Monnier, Valérie; Charles, Laurence; Siri, Didier; Gigmes, Didier; Ouari, Olivier; Wang, Ruibing; Kermagoret, Anthony; Bardelang, David
• Is Part Of: Chemistry. Volume 25:Issue 54(2019)
• Page Start: 12552
• Page End: 12559
• Publication Date: 2019-08-07
• Subjects: 2:2 complexes -- cucurbituril -- host–guest systems -- imidazoles -- viologen; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201902057
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: A viologen derivative carrying a benzimidazole group ( V‐P‐I 2+ ; 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 p K a, by more than 1 pH unit, contrasting with the positive p K a 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 3+ 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 3+ ⋅ CB[7] and a CB[8] complex of 1‐methyl‐4‐(4‐pyridyl)pyridinium. The origin of this negative p K a 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. Abstract : Rigid ditopic guest compounds with one station designed to bind well cucurbit[8]uril (CB[8]) can trigger the formation of robust host/guest 2:2 complexes. Given their excellent binding properties in water and the increased possibilities to tune the p K a of included fragments, cucurbiturils could become attractive hosts enabling both substrate binding and catalysis, adjusted by relevant p K a shifts (see figure).