Inclusion complex formation of ionic liquids with cucurbit[7]uril studied by competitive binding of methylene blue fluorescent probe

Dual-Color Real-Time Chemosensing of a Compartmentalized Reaction Network Involving Enzyme-Induced Membrane Permeation of Peptides

The design of synthetic systems with interrelated reaction sequences that model incipient biological complexity is limited by physicochemical tools that allow the direct monitoring of the individual processes in real-time. To mimic a simple digestion-resorption sequence, the authors have designed compartmentalized liposomal systems that incorporate extra- and intravesicular chemosensing ensembles. The extravesicular reporter pair consists of cucurbit[7]uril and methylene blue to monitor the enzymatic cleavage of short enkephalin-related peptides by thermolysin through a switch-off fluorescence response ("digestion"). Because the substrate is membrane-impermeable, but the dipeptide product is permeable, uptake of the latter into the pre-formed liposomes occurs as a follow-up process. The intravesicular chemosensing ensemble consists of i) cucurbit[8]uril, 2-anilinonaphthalene-6-sulfonic acid, and methyl viologen or ii) cucurbit[7]uril and berberine to monitor the uptake ("resorption") of the enzymatic products through the liposomal membranes by i) a switch-on or ii) a switch-off fluorescence response. The dyes are designed to allow selective optical excitation and read-out of the extra- and intravesicular dyes, which allow for dual-color chemosensing and, therefore, kinetic discrimination of the two sequential reactions.

Investigation of the Interaction of Benzo(a)Pyrene and Fluoranthene with Cucurbit[n]urils (n = 6-8): Experimental and Molecular Dynamic Study

The inclusion complexes of cucurbit[n]uril, CB[n] (n = 6-8), with poly aromatic hydrocarbon (PAH) Benzo(a)Pyrene (BaP), and fluoranthene (FLT) were investigated carefully in aqueous media. Fluorescence and ¹H NMR spectroscopy were used to characterize and investigate the inclusion complexes that were prepared in the aqueous media. The most predominant complexes of both guests with hosts were the 1:1 guest: host complexes. Stability constants of 2322 ± 547 M^-1, 7281 ± 689 M^-1, 3566 ± 473 M^-1 were obtained for the complexes of BaP with CB[6], CB[7], and CB[8], respectively. On the other hand, stability constants of 5900.270 ± 326 M^-1, 726.87 ± 78 M^-1, 3327.059 ± 153 M^-1 were obtained for the complexes of FLT with CB[6], CB[7], and CB[8], respectively. Molecular dynamic (MD) simulations were used to study the mode and mechanism of the inclusion process and to monitor the stability of these complexes in aqueous media at an atomistic level. Analysis of MD trajectories has shown that both BaP and FLT form stable inclusion complexes with CB[7] and CB[8] in aqueous media throughout the simulation time, subsequently corroborating the experimental results. Nevertheless, the small size of CB[6] prohibited the encapsulation of the two PAHs inside the cavity, but stable exclusion complex was observed between them. The main driving forces for the stability of these complexes are the hydrophobic forces, van der Waals interactions, electrostatic effect, the π····π and C-H···π interaction. These results suggest that BaP and FLT can form stable complexes with CB[n] (n = 6-8) in solution.

Encapsulation of ionic liquids inside cucurbiturils

Cucurbit[n]urils (CBn, n = 6-8) serve as molecular receptors for imidazolium-based ionic liquids (ILs) in aqueous solution. The amphiphilic nature of 1-alkyl-3-methylimidazolium guests (Cnmim), with a cationic imidazolium residue and a hydrophobic alkyl chain, enabled their complexation with CBn through a combination of the hydrophobic effect and ion-dipole interactions. 1H NMR experiments revealed that the cavity of CBn can host the hydrophobic chain of the ILs, while one of the carbonyl rims served as a docking site for the imidazolium ring. The structure of the complexes was further analyzed by molecular dynamics (MD) simulations, which indicated that the cavity of CB6 can accommodate up to 5 carbon atoms, while the larger cavity of CB7 and CB8 can encapsulate longer alkyl chains in folded conformations. Isothermal titration calorimetry (ITC) experiments provided up to micromolar affinity of ILs to CBn in aqueous solution, which was independently quantified by indicator displacement titrations.