A Cucurbit[8]uril 2:2 Complex with a Negative p K a Shift
Résumé
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.
Domaines
Chimie
Origine : Accord explicite pour ce dépôt