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A Cucurbit[8]uril 2:2 Complex with a Negative p K a Shift

Abstract : A viologen derivative carrying a benzimidazole group (VP-I 2+) 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 pKa, by more than 1 pH unit, contrasting with the positive pKa shift usually observed for CB-based complexes. While 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 pKa 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 catalysing proton transfer or chemical reactions.
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https://hal-amu.archives-ouvertes.fr/hal-02355140
Contributor : Emilie Pepe <>
Submitted on : Friday, November 8, 2019 - 10:10:38 AM
Last modification on : Thursday, July 9, 2020 - 1:12:03 PM

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Hang Yin, Qian Cheng, Roselyne Rosas, Stéphane Viel, Valérie Monnier, et al.. A Cucurbit[8]uril 2:2 Complex with a Negative p K a Shift. Chemistry - A European Journal, Wiley-VCH Verlag, 2019, 25 (54), pp.12552-12559. ⟨10.1002/chem.201902057⟩. ⟨hal-02355140⟩

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