Applicable Properties of Cucurbiturils

Light driven molecular lock comprises a Ru(bpy)2(hpip) complex and cucurbit[8]uril

Here, complex 1 ([Ru(bpy)2(hpip)]2+-MV2+) and CB[8] can form a stable 1 : 1 inclusion complex in aqueous solution, resembling a U-shaped conformation. Upon light irradiation, two complex 1 were reversibly locked through the formation of a MV˙+ radical dimer that is stabilized in the cavity of CB[8] with Ru complexes as blockers, in which complex 1 was transformed from a U-shaped conformation to a interlocked complex. This study provided a feasible strategy for the fabrication of a photo-driven supramolecular machine resembling a "lock".

Binding Modes of a Phenylpyridinium Styryl Fluorescent Dye with Cucurbiturils

In order to explore how cucurbituril hosts accommodate an N-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with ,',δ,δ'-tetramethyl-cucurbit[6]uril (Me4CB6) and cucurbit[7]uril (CB7) was investigated by absorption spectroscopic, fluorescence and NMR experiments. In aqueous solutions, PhSt forms 1:1 complexes with both cucurbiturils, the complex with CB7 has a higher stability constant (Ka = 6.0 × 106 M-1) than the complex with Me4CB6 (Ka = 1.1 × 106 M-1). As revealed by NMR experiments and confirmed by theoretical calculations, CB7 encapsulates the whole phenylpyridinium entity of the PhSt cation guest, whereas the cavity of Me4CB6 includes only the phenyl ring, the pyridinium ring is bound to the carbonyl rim of the host. The binding of PhSt to cucurbiturils is accompanied by a strong enhancement of the fluorescence quantum yield due to the blocking of the deactivation through a twisted intramolecular charge transfer (TICT) state. The TICT mechanism in PhSt was characterized by fluorescence experiments in polyethylene glycol (PEG) solvents of different viscosities. The PhSt-CB7 system was tested as a fluorescence indicator displacement (FID) assay, and it recognized trimethyl-lysine selectively over other lysine derivatives.

Photophysical Activity and Host-Guest Behavior of Ruthenium Polypyridyl Catalysts Encapsulated in Cucurbit[10]uril

This work outlines a strategy to combine the use of visible light and confined spaces to form a supramolecular photocatalyst system. Polypyridyl ruthenium(II) complexes [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine), [Ru(bpy)₂(bpm)]²⁺ (bpm = 2,2'-bipyrimidine), and [Ru(bpy)₂(bpz)]²⁺ (bpz = 2,2'-bipyrazine) are encapsulated in cucurbit[10]uril to form host-guest systems in aqueous solution. The photophysical properties of the complexes are altered by encapsulation, with improved emissive behavior for the heteroleptic complexes. Oxidative quenching of the photocatalyst's excited state via intermolecular charge transfer to methyl viologen can occur within the internal cavity, which acts to preorganize the reagents. The host-guest system containing [Ru(bpy)₃]²⁺ can bind suitable substrates, and essential criteria for its use as a supramolecular photocatalyst are investigated.

A simpler potentiometric method for histamine assessment in blood sera

Histamine intolerance results from a disequilibrium of accumulated histamine and the capacity for histamine degradation. An impaired histamine degradation based on reduced DAO activity and the resulting histamine excess may cause numerous symptoms mimicking an allergic reaction. For that, the determination of histamine in blood or in food products has great importance to identify risk factors. A new histamine-selective electrode is proposed using cucurbit[6]uril (CB[6]), as ionophore, in the analysis of biological samples. The selection of this smart supramolecular organic framework was based on its apparent stability constant of histamine-CB[6] (log β) of 4.33. The optimized electrode based on a polymeric membrane (PVC) combines the histamine-selective ionophore with 2-nitrophenyl octyl ether as solvent mediator and potassium tetrakis(4-chlorophenyl)borate as anionic additive. Furthermore, multi-walled carbon nanotubes particles were included in the membrane composition to partly lower the detection limit of the method, while improving stability and lowering the response drift (± 4 mV). The electrodes showed a rapid response (≃ 13 s) in the pH operational range of 2.7-5.4, with a Nernstian slope of 30.9 ± 1.2 mV/dec, a detection limit of (3.00 ± 0.61) × 10^-7 mol/L, and a lower limit of the linear range of (3.00 ± 0.00) × 10^-7 mol/L. After miniaturization, the electrode was used as a detector in a sequential-injection lab-on-valve flow setup. The optimized flow conditions were achieved for sample injection volumes of 197 μL propelled towards the cell under detection, at a flow rate of 30 μL/s during 100 s, making the analysis of 30 samples per hour possible. The developed system was used to analyze spiked blood serum samples previously cleaned by using solid-phase extraction. The sample pretreatment of the serum samples using Oasis MCX cartridges showed outstanding efficiency for histamine determination. The recovery values for three different levels of histamine concentration (1 × 10^-4 mol/L, 1 × 10^-5 mol/L, and 1 × 10^-6 mol/L) were (97 ± 6)%, (103 ± 1)%, and (118 ± 9)%, respectively, showing that this method was suitable for biological samples.

Emission enhancement of cationic tetraphenylethylene derivatives by encapsulation in a cucurbit[10]uril host in water

Encapsulation of cationic tetraphenylethylene guests in a cucurbit[10]uril host in water results in strong emission enhancement of these guests since the host–guest interaction caused restriction of intramolecular rotation.

Self-assembly of cucurbiturils and cyclodextrins to supramolecular millstones with naphthalene derivatives capable of translocations in the host cavities

Naphthylpyridine derivatives and cavitands form inclusive complexes of different structure and stoichiometry; the guests are capable of translocations within cucurbit[7,8]uril cavities.

Performance and selectivity of lower-rim substituted calix[4]arene as a stationary phase for capillary gas chromatography

This work presents the investigation of p-tert-butyl(tetradecyloxy)calix[4]arene (C4A-C10) as stationary phase for capillary gas chromatographic (GC) separations. The statically-coated C4A-C10 capillary column showed weak polarity and column efficiency of 2566 plates per m determined by n-dodecane at 120 °C. Impressively, the C4A-C10 column exhibited extremely high resolving capability for a wide range of analytes from nonpolar to polar, including n-alkanes, esters, ketones, aldehydes, alcohols and bromoalkanes. Most importantly, the C4A-C10 column exhibited an excellent separation performance for positional, structural and cis-/trans-isomers. Among them, the column displayed advantageous resolving capability over the commercial polysiloxane stationary phase for aromatic amine isomers. Moreover, the C4A-C10 column showed good column repeatability with RSD values below 0.06% for run-to-run, 0.12–0.27% for day-to-day and 2.8–5.3% for column-to-column.

This work presents the investigation of p-tert-butyl(tetradecyloxy)calix[4]arene (C4A-C10) as stationary phase for capillary gas chromatographic (GC) separations.

Photodeamination to quinone methides in cucurbit[n]urils: potential application in drug delivery

A prodrug is encapsulated in CB[7] and is photochemically transformed into an active drug inside this supramolecular complex.

Attractive Interactions between Heteroallenes and the Cucurbituril Portal

In this paper, we report on the noteworthy attractive interaction between organic azides and the portal carbonyls of cucurbiturils. Five homologous bis-α,ω-azidoethylammonium alkanes were prepared, where the number of methylene groups between the ammonium groups ranges from 4 to 8. Their interactions with cucurbit[6]uril were studied by NMR spectroscopy, IR spectroscopy, X-ray crystallography, and computational methods. Remarkably, while the distance between the portal plane and most atoms at the guest end groups increases progressively with the molecular size, the β-nitrogen atoms maintain a constant distance from the portal plane in all homologues, pointing at a strong attractive interaction between the azide group and the portal. Both crystallography and NMR support a specific electrostatic interaction between the carbonyl and the azide β-nitrogen, which stabilizes the canonical resonance form with positive charge on the β-nitrogen and negative charge on the γ-nitrogen. Quantum computational analyses strongly support electrostatics, in the form of orthogonal dipole-dipole interaction, as the main driver for this attraction. The alternative mechanism of n → π* orbital delocalization does not seem to play a significant role in this interaction. The computational studies also indicate that the interaction is not limited to azides, but generalizes to other isoelectronic heteroallene functions, such as isocyanate and isothiocyanate. This essentially unexploited attractive interaction could be more broadly utilized as a tool not only in relation to cucurbituril chemistry, but also for the design of novel supramolecular architectures.

Light-induced piston nanoengines: ultrafast shuttling of a styryl dye inside cucurbit[7]uril

A proof of principle for an ultrafast molecular shuttle based on the light-operated movement of a styryl dye inside cucurbit[7]uril was described.

Enhanced anion binding by heteroatom replacement in bambusurils

Calculations predict that protonated aza-bambusurils would bind multiple anions along their main axis and may also function as synthetic anion channels.

Catalytic alcoholysis of epoxides using metal-free cucurbituril-based solids

Cucurbit[7]uril containing entrapped hydronium ions possesses unique acid properties as demonstrated by its ability to catalyse alcoholysis reactions under mild conditions.

Cucurbit[7]uril inclusion complexation as a supramolecular strategy for color stabilization of anthocyanin model compounds

Complete stabilization of the color of a flavylium cation by the formation of a higher order (1 : 2) complex with cucurbit[7]uril.

Cucurbiturils: from synthesis to high-affinity binding and catalysis

Major developments in the synthesis of cucurbiturils and applications related to their high-affinity binding and catalysis have recently taken place.

Determination of the kinetics underlying the pKa shift for the 2-aminoanthracenium cation binding with cucurbit[7]uril

The binding dynamics of the 2-aminoanthracenium cation (AH⁺) and 2-aminoanthracene (A) with cucurbit[7]uril (CB[7]) was studied using stopped-flow experiments. The kinetics was followed by measuring the fluorescence changes over time for AH⁺ and A, which emit at different wavelengths. The studies at various pH values showed different mechanisms for the formation of the AH⁺@CB[7] complex, with this complex formed either by the binding of AH⁺ or by the initial binding of A followed by protonation. In the latter case, it was possible to determine the protonation ((1.5 ± 0.4) × 10⁹ M⁻¹ s⁻¹) and deprotonation (89 ± 7 s⁻¹) rate constants for complexed A/AH⁺, which showed that the pK_a shift of +3.1 for A/AH⁺ in the complex is mainly due to a lower deprotonation rate constant.

New lanthanide–CB[6] coordination compounds: relationships between the crystal structure and luminescent properties

Four new isostructural complexes were obtained with Ln³⁺ and CB[6]. The compounds present interesting spectroscopic properties, not observed before in Ln–CB[n] compounds.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.