Interplay of Hydrophobic and Electrostatic Interactions in Modulation of Protonation–Deprotonation Equilibria of Two Positional Isomers in Their Complexes with Cucurbiturils

Abstract

Elucidation of host−guest chemistry of 2-(4′pyridyl)benzimidazole (4PBI) and 2-(2′-pyridyl)benzimidazole (2PBI) with cucurbiturils in aqueous solution is rather challenging. The guest molecules are capable of binding in three different states of protonation: cation C, tautomer T, and normal N. Charge distribution on the species governs the formation of the inclusion complexes. Binding modes of the guest molecules with cucurbiturils have been investigated by proton NMR spectroscopy. 4PBI binds with its benzimidazole ring pointing inward, while the binding mode is opposite for 2PBI. This difference is governed by the position of substitution of the pyridyl ring. Energetics of complexation has been studied by quantum chemical calculations for 4PBI-CB as 2PBI-CB type complexes, for each form (C, T, and N) of both the molecules. Release of high energy water from cucurbituril cavity is the main driving force for complexation. Excited state proton transfer (ESPT) in 2PBI and 4PBI are affected strongly upon complexation, as is manifested in steady state and time-resolved fluorescence experiments.

Supramolecular Luminescent Sensors

“…Cucurbiturils, by contrast, can have binding affinities as high as 10 14 or 10 15 M –1 , , which rivals the binding of avidin and biotin, which is among the strongest interactions known. , Second, cucurbiturils have a ring of carbonyls pointed at each entrance to the cavity. − These carbonyls are polarized to have negative electron charge density, and therefore cucurbiturils are much stronger cation binders than are cyclodextrins. Cucurbiturils show the strongest binding affinities in cases where they can bind molecules that are both cationic (in one portion), and hydrophobic (in a different portion), such as biogenic amines, which take full advantage of the structural features of the cucurbiturils.…”

“…1285−1287 These carbonyls are polarized to have negative electron charge density, and therefore cucurbiturils are much stronger cation binders than are cyclodextrins. Cucurbiturils show the strongest binding affinities in cases where they can bind molecules that are both cationic (in one portion), 1288 and hydrophobic (in a different portion), 1289 such as biogenic amines, 1290 which take full advantage of the structural features of the cucurbiturils.…”

Calorimetric and spectroscopic investigations of interactions between cucurbituril Q7 and gemcitabine in aqueous solutions

“…Potential applications of the complexes under discussion are determined by the type and energetics of supramolecular interactions [27]. Therefore, the stoichiometry and thermodynamics of the interactions between cucurbituril Q7 and gemcitabine hydrochloride were examined by the dynamic technique of calorimetric titration under conditions of forward system (cucurbituril titrated with the drug) and reverse system (the drug titrated with cucurbituril) in an aqueous medium.…”