Host–guest complexes of various cucurbit[n]urils with the hydrochloride salt of 2,4-diaminoazobenzene

The interaction products of normal cucurbit[n]urils (n = 7, 8; Q[7] Q[8]) and a sym- tetramethyl-substituted cucurbit[6]uril derivative (TMeQ[6]) with the hydrochloride salts of 2,4-diaminoazobenzene (g·HCl) were investigated in aqueous solution using ¹H NMR spectroscopy, electronic absorption spectroscopy, as well as single crystal X-ray diffraction. The ¹H NMR spectra analysis established a basic interaction model in which inclusion complexes with a host:guest ratio of 1:1 form for the TMeQ[6] and Q[7] cases, while they form with a host:guest ratio of 1:2 for the Q[8] case. Commonly, the hosts selectively bound to the phenyl moieties of the guests. Absorption spectrophotometric analysis in aqueous solution defined the stability of the host–guest inclusion complexes at pH 3.2. Quantitatively, at this pH, complexes with a host:guest ratio of 1:1—those with smaller hosts TMeQ[6] and Q[7]—formed with logK values between 6 and 7. That with host Q[8] and a host:guest ratio of 1:2 formed with a logK value of 10.8. Single crystal X-ray structures of the inclusion complexes TMeQ[6]–g·HCl and Q[8]–g·HCl showed the phenyl moiety of the guest inserted into the host cavity. This result supports the solution-based ¹H NMR spectroscopic study.     

Host–guest interactions of a twisted cucurbit[15]uril with paraquat derivatives and bispyridinium salts

Host–guest complexations of a twisted cucurbit[15]uril with some paraquat derivatives and bispyridinium salts in aqueous solution are investigated by nuclear magnetic resonance, UV–vis spectrometry and isothermal titration calorimetry. These complexations are mainly enthalpy-driven.     

Influence of self-assembly of amphiphilic imidazolium ionic liquids on their host–guest complexes with cucurbit[n]urils

Two symmetric amphiphilic imidazolium ionic liquids having ω-undecenyl chains form supramolecular complexes with CB[7] and CB[8] in water as revealed by ¹H NMR spectroscopy and MALDI-MS. Binding constants in the range 10⁴ to 10⁵ M^?1 were estimated from the conductivity measurements for the 1:1 complexes of these imidazolium ionic liquids with CB[7] and CB[8]. Radical initiated polymerization of these host–guest complexes at concentrations above the critical self-assembly concentration of imidazolium ionic liquids to form liposomes, destroys completely (CB[7]) or partially (CB[8]) the host–guest ionic liquid@CB[n] complex; this behaviour was proved by titration with acridine orange tricyclic dye, of CB[n]s in the colloidal solutions of the liposomes before and after performing dialysis to remove free CB[n]s. Thus, the increase in the fluorescence emission of acridine orange by CB[7] is not observed if the polymerized ionic liquid@CB[7] complex is submitted to dialysis to remove uncomplexed CB[7]. Analogous study by titration of absorbance change of acridine orange solutions caused by CB[8], reveals only a partial destruction of the host–guest complex by self-assembly of amphiphilic ionic liquid above the critical self-assembly concentration. The results obtained have been rationalized considering that the driving force for the formation of supramolecular ionic liquid@CB[n] complexes is a hydrophobic interaction between the apolar alkenyl chain and the cucurbituril interior cavity and that these hydrophobic interactions are disturbed when self-assembly leading to liposomes occurs.     

Host–guest complexes of the antituberculosis drugs pyrazinamide and isoniazid with cucurbit[7]uril

The potential use of cucurbit[7]uril (CB[7]) as an excipient in oral formulations for improved drug physical stability or for improved drug delivery was examined with the antituberculosis drugs pyrazinamide (pyrazine-2-carboxamide) and isoniazid (isonicotinohydrazide). Both drugs form 1:1 host–guest complexes with CB[7] as determined by ¹H nuclear magnetic resonance spectrometry, electrospray ionisation mass spectrometry and molecular modelling. Drug binding is stabilised by hydrophobic effects between the pyridine and pyrazine rings of isoniazid and pyrazinamide, respectively, to the inside cavity of the CB[7] macrocycle as well as hydrogen bonds between the hydrazide and amide groups of each drug to the CB[7] carbonyl portals. At pH 1.5, isoniazid binds CB[7] with a binding constant of 5.6 × 10⁵ M^?1, whilst pyrazinamide binds CB[7] at pH 7 with a much smaller binding constant (4.8 × 10³ M^?1). Finally, CB[7] prevents drug melting through encapsulation. Where previously pyrazinamide displays a typical melting point of 189 °C and isoniazid 171 °C, by differential scanning calorimetry, no melting or degradation at temperatures up to 280 °C is observed for either drug once bound by CB[7].     

Host−guest inclusion systems of nicotine with acyclic cucurbit[n]urils for controlled heat releases

Journal of Inclusion Phenomena and Macrocyclic Chemistry - The thermal stability and slow release of nicotine are crucial to its shelf-life and applications. In this article, three kinds of acyclic...     

Interaction of host-guest complexes of cucurbit[n]urils with double probe guests

Since the structure of cucurbituril(Q[6]) has been determined in 1981[1] and its homologues cucurbit [n = 5,7,8 and 10]uril(Q[5], Q[7], Q[8] and Q[10]) have been reported in 2000[2,3], 2002[4], a series of host-guest complexes[5—7], novel supramolecular as-semblies[8—10], molecular encapsulates[11,12] and mo-lecular containers[13,14] based on Q[n] have been stud-ied extensively. All cucurbituril homologues have common char-acteristic features, i.e. hydrophobic cavity, and polar carbonyl gr…     

Host–guest interaction of hemicucurbiturils with phenazine hydrochloride salt

The host–guest interactions between phenazine hydrochloride salt (PheH⁺) and hemicucurbit[n]uril (n = 6 or 12) (HemiQ[6 or 12]) have been studied by ¹H NMR, UV–vis, IR, mass spectrometry (MS) and quantum chemistry. In ¹H NMR spectra, the broadening of proton resonances of the hosts suggests the interactions of PheH⁺ with HemiQs. The quantitative stabilities of the host–guest systems have been obtained by UV–vis titration experiments, that is, the stoichiometric interactions of PheH⁺ with HemiQ[6] have been observed with an association constant of K_a = (2.5 ± 1.2) × 10⁶ L mol^? 1, while the 2:1 ratio complexes of PheH⁺ with HemiQ[12] are formed with stepwise association constants of K₁ = (9.2 ± 2.8) × 10⁴ L mol^? 1 and K₂ = (6.4 ± 0.9) × 10⁵ L mol^? 1, respectively, which induce a total association constant of K_a = 5.9 × 10¹⁰ L² mol^? 2. Both the 1:1 and 2:1 complexes have been detected by MS. Quantum chemistry calculations have been used to understand the static structures and thermodynamic stabilities of the supramolecular assemblies.     

Theoretical investigation of the complexation,structural, and electronic properties of complexes between oseltamivir drug and cucurbit[n = 6–9]urils

Structural Chemistry - The structural geometries of cucurbit[n]uril CB[n] with n?=?6–9 and their complexes with oseltamivir (OST) drug were obtained using the density...     

Host–guest association of Morin with β-CD and C-hexylpyrogallol[4]arene: Structure of the complexes and the effect of pH

The study of the host–guest association of Morin hydrate (MO) with β-cyclodextrin (β-CD) and C-hexylpyrogllol[4]arene (C-HPA) is reported in this paper. The iInclusion complexation of MO is studied by ultraviolet-visible, steady-state fluorescence, time-resolved fluorescence, ¹H nuclear magnetic resonance (NMR), and two dimentional rotating-frame nuclear overhauser effect correlation (2D ROESY) spectroscopic techniques. The stoichiometry and the binding constant for the MO–β-CD complex are derived from the linearity of the Benesi–Hildebrand equation. The binding constant for the MO–C-HPA complex is calculated from the nonlinear curve fitting of fluorescence intensities. The effects of the acid strength on the absorption and fluorescence spectra of MO are studied in the absence and the presence of β-CD/C-HPA host molecules. The pK _a values of the ground and the excited states are reported.     

Water structuring inside the cavities of cucurbit[n]urils (n = 5–8): a quantum-chemical forecast

In this work we report findings of the quantum-chemical examination of water structuring in the cavities of cucurbit[n]urils (CB[n]), n?=?5–8 obtained within the density functional theory. The thermodynamically most stable structures of inclusion compounds (H₂O)_m@CB[n] were determined for different numbers m of H₂O molecules inside the cavities. From the viewpoint of thermodynamics, the most probable numbers m of water molecules in the CB[n] homologues are the following: m?=?2 for CB[5], m?=?4 for CB[6], m?=?8 for CB[7] and m?=?10 for CB[8]. For the case of CB[6] synthesized in aqueous solution, we compared its experimental IR spectrum with that calculated quantum-chemically for the model inclusion systems (H₂O)_m@CB[6] where m ranges from 1 to 6. The best agreement between the experimental and theoretical spectra was observed for (H₂O)₄@CB[6], in complete agreement with the conclusion made based on the thermodynamic estimations. Our results are also in good agreement with other available estimates of the most probable number of water molecules in CB[n].     

Supramolecular assemblies of cucurbit[n]urils and 4-aminopyridine controlled by cucurbit[n]uril size (n = 5, 6, 7 and 8)

The binding interactions between 4-aminopyridine (4-AP) and a series of cucurbit[n]urils (Q[5], Q[6], TMeQ[6], Q[7], Q[8]) have been studied using ¹H NMR spectroscopy, UV–vis absorption spectroscopy, isothermal titration calorimetry (ITC) and X-ray crystallography. The data indicates that the Q[5]@4-AP complex exhibits exo binding, which is not observed in the other four host-guest complexes. Furthermore, X-ray crystallography clearly reveals how the Q[n]s bind with 4-AP to form complexes, for example Q[5] forms an outer-surface complex, whilst Q[6], TMeQ[6] and Q[7] formed 1:1 host and guest type complexes, and Q[8] formed a stable 1:2 ternary complex due to its large cavity, which can accommodate two 4-AP molecules.     

Host–guest Complex of a Water-soluble Cucurbit[6]uril Derivative with the Hydrochloride Salt of 3-amino-5-phenylpyrazole

Interaction between tetramethylcucurbit[6]uril and 3-amino-5-phenylpyrazole hydrochloride in aqueous solution has been investigated by using ¹H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy, as well as by a single crystal X-ray diffraction determination. The ¹H NMR spectra analysis established a basic interaction model in which an inclusion complex with a host:guest ratio of 1:1 forms, in which the host selectively binds the phenyl moiety of the guest. Absorption spectrophotometric and fluorescence spectroscopic analysis in aqueous solution defined the stability of the host–guest inclusion complexes quantitatively as 6.8 × 10⁵ mol^? 1 L at pH 2.6; the interaction is pH dependent, decreasing as pH rises. The single crystal X-ray structure of the isolated inclusion complex shows the phenyl moiety of the guest inserted into the host cavity, which supports particularly the ¹H NMR spectroscopic study in solution. In the crystal structure of the inclusion complex, the host–guest interaction involves both inter- and intra-complex hydrogen bonding, forming 2:2 dimers that stack in one dimension as supramolecular tubes.     

Molecular photonics of inclusion complexes of cucurbit[7]uril with 3,3′-diethylthiazolinocarbocyanine iodide

The effect of cucurbit[7]uril (Cb7) on the photonics of 3,3′-diethylthiazolinocarbocyanine iodide (Car) in water was studied by spectrofluorimetry, ns-laser kinetic spectroscopy, and quantum chemistry. The formation of an inclusion complex of Car with Cb7 was established, and the binding constant and composition of the Car@Cb7 (1 : 2) complex were determined. It was shown that Car and Car@Cb7 are able to undergo trans → cis-photoisomerization. The lifetime of the cis-isomer of free Car was found to be much shorter than that of the cis-Car form localized inside the cavity of Cb7. The energies of formation of complexes Car@Cb7 were found and the structure of the complex in the lowest excited singlet state was determined according to the quantum chemical calculations by the DFT method with the PBE functional. The trans → cis-isomerization was found to proceed via the excited singlet S1 state having the “perpendicular” conformation.     

Effects of the number and placement of positive charges on viologen–cucurbit[n]uril interactions

Recent developments in the synthesis and applications of the cucurbit[n]uril family of synthetic hosts has led to an increasing interest in the detailed studies of their interactions with a wide variety of guests. This paper describes a quantitative study of the effects of the number and placement of positive charges on the binding of viologen guests to cucurbit[7]uril and cucurbit[8]uril. A series of viologen derivatives with one to four charges was characterised by isothermal titration calorimetry, ¹H NMR spectroscopy and mass spectrometry to determine the stoichiometry, affinity and mode of binding. These data show that stoichiometry can be controlled by the placement of charge, and that affinity can be increased by the addition of positive charges. This study should serve as a guide for the design of supramolecular structures built from viologens and cucurbit[n]urils.

     

Theoretical insights into the formation, structure, and electronic properties of anticancer oxaliplatin drug and cucurbit[n]urils n = 5 to 8

Geometries, formation and electronic properties of cucurbit[n]uril-oxaliplatin n = 5–8, host-guest complexes are investigated with DFT calculations. The formation of inclusion complexes of CB[n]-oxaliplatin are facile in CB[n] n = 6–8. In the complex, the cyclohexyl group is found to be deep inside the cavity, with the formation of a hydrogen bonding between the portal oxygen atoms and the amine nitrogen of the oxaliplatin guest. NBO analysis shows the transfer of charge from the metal center to the CB[7] unit and the existence of hydrogen bonding between the oxygen portal and amine nitrogen. The HOMO orbital is localized on the carboxylate group and the LUMO orbital are localized on the cucurbituril unit in CB[7]-oxaliplatin complex. The strength of the interaction determined here reflects the ability of CB[n] to act as a host for suitably oxaliplatin guests, even in aqueous solution.     

NMR study of host–guest complexes of disulfonated derivatives of 9, 10-diphenylanthracene and corresponding endoperoxides with cyclodextrins

Disulfonated derivatives of 9,10-diphenyl anthracene (dsDPA) are known carriers of singlet oxygen. DsDPA and corresponding endoperoxides (dsDPAO₂) form host–guest complexes with native cyclodextrins (i.e. β-CD and γ-CD). The modes of host–guest interaction were studied by ¹H NMR and 2D-NMR (ROESY). Specific inclusions of phenyl groups of dsDPA/dsDPAO₂ into the cyclodextrin cavities were found for both β-CD and γ-CD. The mode of interaction depends on the size of the CD cavity and the position of the sulfonate group.     

Kinetic and thermodynamic inclusion complexes of symmetric teramethyl-substituted cucurbit[6]uril with HCl salts of N,N′-bis(pyridylmethyl)-1,6-hexanediamine

The host–guest interaction of symmetrical α,α′,δ,δ′-tetramethyl-cucurbit[6]uril (TMeQ[6]) with the hydrochloride salts of N,N′-bis(4-pyridylmethyl)-1,6-hexanediamine (P6), N,N′-bis(3-pyridyl-methyl)-1,6-hexanediamine (M6) and N,N′-bis(2-pyridylmethyl)-1,6-hexanediamine (O6) was investigated via single crystal X-ray diffraction, ¹H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy. Single crystal X-ray diffraction showed that the hexyl moiety of P6 or M6 was incorporated in the cavity of TMeQ[6], while the two pyridylmethyl moieties of O6 were incorporated in the TMeQ[6] cavity in the solid state. The ¹H NMR results in aqueous solution revealed that the TMeQ[6]-P6 and TMeQ[6]-M6 host–guest interaction systems produce a kinetic dumbbell-shaped inclusion complex at the initial stage and then an equilibrium pseudorotaxane-shaped inclusion complex as the only product after heating. However, only the pseudorotaxane-shaped inclusion complex was observed for the TMeQ[6]-O6 host–guest interaction system. Aqueous absorption spectrophotometric analysis showed that the dumbbell-shaped inclusion complexes were stable at pH 5.6, had a host–guest ratio of 2:1 and formed quantitatively at ～10¹¹ l²/mol² for the TMeQ[6]-M6 and TMeQ[6]-O6 systems. The transformation from dumbbell to pseudorotaxane-shaped inclusion complexes for the TMeQ[6]-P6 and TMeQ[6]-M6 host–guest systems yielded activation energies of 59.35 ± 1.55 and 78.7 ± 3.45 kJ/mol, respectively. The pseudorotaxane-shaped inclusion complexes were stable at pH 5.6, had a host–guest ratio of 1:1 and formed quantitatively at ～10⁷ l/mol for the TMeQ[6]-M6 and TMeQ[6]-P6 systems.     

Recent advances of functional gels controlled by pillar[n]arene-based host–guest interactions

Functional gels fabricated from supramolecular host–guest interactions exhibit outstanding characteristics including stimuli-responsiveness, self-healing and adaptability. Pillar[n]arenes are new generation of supramolecular macrocyclic host, which displayed excellent host–guest recognition properties. In the last few years, pillar[n]arene-based gels that include both hydrogels and organogels have been attracted more and more attention. In this digest, the recent advances in this field are reviewed, with special emphasis on the multistimuli responsive pillar[n]arene gels. It is anticipated that more and more pillar[n]arenes-based gel materials with smart properties will be developed in the near future.