Binding modes of cucurbit[6]uril and cucurbit[7]uril with a series of bis-pyridinium compounds

Binding behaviors of cucurbit[6]uril (CB[6]) and cucurbit[7]uril (CB[7]) with a series of bis-pyridinium compounds N, N’-hexamethylenebis(1-alkyl-4-carbamoyl pyridinium bromide) (HBPB-n) (alkyl chain length, n = 6, 8 and 10) guests were investigated using ¹H-NMR, ESI–MS and single crystal X-ray diffraction methods. The results show that CB[6] and CB[7] can form [2]pseudorotaxanes with HBPB-n easily. When increasing the length of tail alkyl chain, the binding site of CB[6] at guest molecules changed from the tail to the middle part, while CB[7] remained located over the tail chain. As CB[6] and CB[7] were added in HBPB-8 aqueous solution, a [3]pseudorotaxane was formed by the inclusion of the internal middle site in CB[6] and the tail chain in CB[7].     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a tetracationic bis(viologen) guest

Binding behaviors of two cucurbit[n]urils (CB[n]) hosts with the [CH3bpy(CH2)6bpyCH3]4+ (bpy = 4,4'-bipyridinium) guest were investigated by 1H NMR and MALDI-TOF-MS experiments. While the CB[6] and CB[7] form [2]pseudorotaxanes with the host located over the hexamethylene chain of the guest, only the CB[7] forms a [3]pseudorotaxane with both host molecules residing over the bipyridinium groups. The initial CB[7] host vacates the inclusion of the hexamethylene chain as a result of the electrostatic and steric repulsions that would arise in simultaneous binding of adjacent aliphatic and aromatic portions of the guest.     

Cyclodextrin-driven movement of cucurbit[7]uril

The movement of cucurbit[7]uril (CB[7]) driven by alpha-cyclodextrin (alpha-CD) is investigated by various experimental techniques including NMR, ESI-MS, UV-vis, and ITC. CB[7] can form stable pseudorotaxanes with N-methyl-N'-octyl-4,4'-bipyridinium (MVO2+) and N,N'-dioctyl-4,4'-bipyridinium (OV2+) dication in aqueous solution. CB[7] shuttles between the octyl and bipyridinium moieties in MVO2+, but docks at one of the octyl moieties in OV2+. The addition of alpha-CD pushes CB[7] from the octyl moiety of MVO2+ or OV2+ to the bipyridinium moiety. Thermodynamically, the movement of CB[7] is mainly driven by exothermic enthalpy changes coming from the complexation of the octyl moiety of MVO2+ or OV2+ with alpha-CD.     

Modes of binding interaction between viologen guests and the cucurbit[7]uril host

[structure: see text] Host-guest interactions between cucurbit[7]uril (CB7) and a series of dialkyl-4,4'-bipyridinium (viologen) dicationic guests were investigated by NMR spectroscopy. CB7 includes the aromatic nucleus of short chain viologens, but the mode of interaction is different with longer chain viologens due to the favorable hydrophobic interactions between the terminal alkyl substituents and the inner cavity of the host. A new pseudorotaxane was designed and synthesized on the basis of viologen-CB7 binding interactions.     

Inclusion complexation of diquat and paraquat by the hosts cucurbit[7]uril and cucurbit[8]uril

The binding interactions in aqueous solution between the dicationic guest diquat (DQ(2+)) and the cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) hosts were investigated by (1)H NMR, UV/Vis, and fluorescence spectroscopy; mass spectrometry; single-crystal X-ray diffraction; and electrochemical techniques. The binding data were compared with previously reported results for the related paraquat guest (PQ(2+)). DQ(2+) was found to bind poorly (K=350 m(-1)) inside CB7 and more effectively (K=4.8 x 10(4) m(-1)) inside CB8. One-electron reduction led to increased binding affinity with both hosts (K(r)=1 x 10(4) m(-1) with CB7 and K(r)=6 x 10(5) m(-1) for CB8). While (1)H NMR spectroscopic data revealed that DQ(2+) is not fully included by CB7, the crystal structure of the CB8DQ(2+) complex-obtained from single-crystal X-ray diffraction-clearly establishes its inclusion nature. Overall, both diquat and its one-electron reduced radical cation are bound more effectively by CB8 than by CB7. In contrast to this, paraquat exhibits selectivity for CB7, but its radical cation forms a highly stable dimer inside CB8. These differences highlight the pronounced sensitivity of cucurbit[n]uril hosts to guest features such as charge, charge distribution and shape.     

Supramolecular complexation of homocysteine and cysteine with cucurbit[7]uril

Supramolecular complexation of two bio-thiols, homocysteine (Hcys) and cysteine (Cys), by cucurbit[7]uril (CB[7]) has been fully investigated by ¹H NMR spectroscopy, mass spectrometry, isothermal titration calorimetry, and the results were further verified with computational investigations. NMR titration experimental results obviously indicate that the binding stoichiometry of CB[7] to Hcys is 1:1 and to Cys is 1:2 in aqueous solution. The binding constants and thermodynamic parameters associated with the complexation between CB[7] and the bio-thiols were determined by isothermal titration calorimetry. The energy-minimized structures of the supramolecular complexes of CB[7] with Hcys and Cys were determined and provide good agreement with the experimental results. The CB[7] cavity is sufficient to include the two Cys, but is unable to accommodate two Hcys due to steric hindrance. The differing binding abilities of Hcys and Cys in aqueous solution towards CB[7] host may lead to discriminate them.     

Synthesis,characterisation and properties of star polypseudorotaxanes with cucurbit[7]uril

New star polypseudorotaxanes that include 4,4′-dipyridyl-terminated 4-arm-poly(ethylene glycol) (PC1) and cucurbit[7]uril (CB[7]) were easily synthesised. The ¹H NMR, UV–vis and resonance light scattering (RLS) data show that the viologen units of the star polypseudorotaxanes could fall off from the hydrophobic cavity of CB[7] with the addition of 1-adamantylamine (ADA). The cyclic voltammograms results indicate that the star polypseudorotaxanes have the redox property with the addition of ADA compared with the non-redox property of the pure star polypseudorotaxanes. The biological toxicity experiment shows that the bactericidal activity of the star polypseudorotaxanes in Escherichia coli was aroused by adjusting CB[7] dethreading from the star polypseudorotaxanes with the addition of another competitive guest ADA, which has a very high affinity for CB[7].     

Light- and pH-regulated Water-soluble Pseudorotaxanes Comprising a Cucurbit[7]uril and a Flavylium-based Axle

A linear double pyridinium-terminated thread comprising a central chalcone moiety is shown to provide two independent binding sites with similar affinity for cucurbit[7]uril (CB7) macrocycles in water as judged from NMR, UV-Visible and fluorescence spectroscopies. Association results in [2] and [3]pseudorotaxanes, which are both pH and photosensitive. Switching from the neutral chalcone to the cationic flavylium form upon irradiation at 365 nm under acidic conditions provided an enhanced CB7 association (K_1:1 increases from 1.2×10⁵ M⁻¹ to 1.5×10⁸ M⁻¹), limiting spontaneous on-thread cucurbituril shuttling. This co-conformational change in the [2]pseudorotaxane is reversible in the dark with k_obs=4.1×10⁻⁴ s⁻¹. Threading the flavylium moiety into CB7 leads to a dramatic increase in the fluorescence quantum yield, from 0.29 in the free axle to 0.97 in the [2]pseudorotaxane and 1.0 in the [3]pseudorotaxane.     

Guest binding dynamics with cucurbit[7]uril in the presence of cations

The binding dynamics of R-(+)-2-naphthyl-1-ethylammonium cation (NpH(+)) with cucurbit[7]uril (CB[7]) was investigated. Competitive binding with Na(+) or H(3)O(+) cations enabled the reaction to be slowed down sufficiently for the kinetics to be studied by fluorescence stopped-flow experiments. The binding of two Na(+) cations to CB[7], i.e., CB[7]·Na(+) (K(01) = 130 ± 10 M(-1)) and Na(+)·CB[7]·Na(+) (K(02) = 21 ± 2 M(-1)), was derived from the analysis of binding isotherms and the kinetic studies. NpH(+) binds only to free CB[7] ((1.06 ± 0.05) × 10(7) M(-1)), and the association rate constant of (6.3 ± 0.3) × 10(8) M(-1) s(-1) is 1 order of magnitude lower than that for a diffusion-controlled process and much higher than the association rate constant previously determined for other CB[n] systems. The high equilibrium constant for the NpH(+)@CB[7] complex is a consequence of the slow dissociation rate constant of 55 s(-1). The kinetics results showed that formation of a complex between a positively charged guest with CB[n] can occur at a rate close to the diffusion-controlled limit with no detection of a stable exclusion complex.     

The 1:1 host-guest complexation between cucurbit[7]uril and styryl dye

The photophysical properties of aqueous solution of styryl dye, 4-[(E)-2-(3,4-dimethoxyphenyl)ethenyl]-1-ethylpyridinium perchlorate (dye 1), in the presence of cucurbit[7]uril (CB[7]) was studied by means of fluorescence spectroscopy methods. The production of 1:1 host-guest complexes in the range of CB[7] concentrations up to 16 μM with K = 1.0 × 10(6) M(-1) has been observed, which corresponds to appearance of the isosbestic point at 396 nm in the absorption spectra and a 5-fold increase in fluorescence intensity. The decay of fluorescence was found to fit to double-exponential functions in all cases; the calculated average fluorescence lifetime increases from 145 to 352 ps upon the addition of CB[7]. Rotational relaxation times of dye 1 solutions 119 ± 14 ps without CB[7] and 277 ± 35 ps in the presence of CB[7] have been determined by anisotropy fluorescence method. The comparison of the results of quantum-chemical calculations and experimental data confirms that in the host cavity dye 1 rotates as a whole with CB[7].     

Characterization of inclusion complexation of various tetraalkylammonium chlorides with cucurbit[7]uril by external high-pressure studies

The inclusion constants of tetraalkylammonium salts (ASs) with cucurbit[7]uril (CB[7]) were determined at high-pressures using methylene blue as a chemical indicator. CB[7] formed 1:1 inclusion complexes with ASs. We found a characteristic dependence of the alkyl chain substituents in ASs on the inclusion equilibrium, which was responsible for the formation of clathrate hydrates of the AS in bulk. Furthermore, we examined the effects of external high pressures on the CB[7] inclusion complexations of ASs having different alkyl chain lengths. Pressure dependence experiments allowed us to calculate the reaction volume (ΔV) upon inclusion; i.e., the volume change (ΔV_desolv) accompanying desolvation around the guest molecules and the volume change (ΔV_repel) caused by the water molecules repelled from the CB[7] cavity. The volumetric study for the inclusion of ASs with CB[7] showed that in the tetraheptylammonium ion, one arm (alkyl chain) was encapsulated in the CB[7] cavity, while in other ASs, two arms were located in the cavity. NMR measurements of the complexes and pressure-dependent studies provided unique information on the structures of the inclusion complexes of ASs with CB[7].     

The inverted cucurbit[n]uril family

We report the isolation, characterization, and recognition behavior of iCB[6] and iCB[7], which are diastereomers of CB[6] and CB[7], respectively, containing a single inverted glycoluril unit. Product resubmission experiments establish that these inverted CB[n] are intermediates in the mechanism of CB[n] formation. As a consequence of the inverted glycoluril ring, these inverted cucurbiturils possess a permanent dipole moment, are slightly smaller than their diastereomers, show distinctive selectivity in their recognition behavior, and report directly on the contents of their hydrophobic cavity.     

Examination of cucurbit[7]uril and its host-guest complexes by diffusion nuclear magnetic resonance

The self-diffusion of cucurbit[7]uril (CB[7]) and its host-guest complexes in D2O has been examined using pulsed gradient spin-echo nuclear magnetic resonance spectroscopy. CB[7] diffuses freely at a concentration of 2 mM with a diffusion coefficient (D) of 3.07 x 10(-10) m(2) s(-1). At saturation (3.7 mM), CB[7] diffuses more slowly (D = 2.82 x 10(-10) m(2) s(-1)) indicating that it partially self-associates. At concentrations between 2 and 200 mM, CsCl has no effect on the diffusion coefficient of CB[7] (1 mM). Conversely, CB[7] (2 mM) significantly affects the diffusion of 133Cs+ (1 mM), decreasing its diffusion coefficient from 1.86 to 0.83 x 10(-9) m(2) s(-1). Similar changes in the rate of diffusion of other alkali earth metal cations are observed upon the addition of CB[7]. The diffusion coefficient of 23Na+ changes from 1.26 to 0.90 x 10(-9) m(2) s(-1) and 7Li+ changes from 3.40 to 3.07 x 10(-9) m(2) s(-1). In most cases, encapsulation of a variety of inorganic and organic guests within CB[7] decreases their rates of diffusion in D2O. For instance, the diffusion coefficient of the dinuclear platinum complex trans-[[PtCl(NH3)2}2mu-dpzm](2+) (where dpzm is 4,4'-dipyrazolylmethane) decreases from 4.88 to 2.95 x 10(-10) m(2) s(-1) upon encapsulation with an equimolar concentration of CB[7].     

Disrupting aggregation of tethered rhodamine B dyads through inclusion in cucurbit[7]uril

Hexano- and dodecano-tethered diesters of rhodamine B were prepared. The absorption and fluorescence spectra of these flexibly tethered dyads were compared with those of the rhodamine 3B ethyl ester. Increased J- and H-type dimer formation and decreased fluorescence emission were observed for the tethered dyads. Complexation of the cationic chromophoric units in cucurbit[7]uril (CB7) hosts decreased H-dimer aggregation, especially for the dodecano-tethered dyad. The monomeric dye and both dye dyads exhibited enhanced fluorescence upon addition of CB7.     

The cucurbit[n]uril family

In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.     

Interconversion between [5]pseudorotaxane and [3]pseudorotaxane by pasting/detaching two axle molecules

An acceptor-donor-acceptor-type linear molecule 1(2+) containing one electron-rich naphthoxy (NP) unit and two monocharged viologen (MCV) units was synthesized. Through the noncovalent interaction of cucurbit[8]uril (CB[8]) with one NP and one MCV in 1(2+), we first obtained a [2]pseudorotaxane ([1(2+)]?CB[8]), and the excess CB[8] included simultaneously the two bare MCV units of two [2]pseudorotaxanes to form a [5]pseudorotaxane ([1(2+)](2)?[CB[8]](3)). Its transformation to [3]pseudorotaxane was achieved through detaching the two axle molecules in the presence of acid, and then the addition of base may result in a reversible switch between two different pseudorotaxanes. This novel methodology elongating reversibly linear molecules by noncovalent interactions will benefit the development of stimuli-responsive functional molecular devices.     

pH-Triggered dethreading-rethreading and switching of cucurbit[6]uril on bistable [3]pseudorotaxanes and [3]rotaxanes

A series of water-soluble [3]rotaxanes-(n+2) and [3]pseudorotaxanes-(n+2) with short (propyl, n=1) and long (dodecyl, n=10) aliphatic spacers have been prepared in high yields by a 1,3-dipolar cycloaddition reaction catalyzed by cucurbit[6]uril (CB6). The pH-triggered dethreading and rethreading of CB6 on these pseudorotaxanes was monitored by 1H NMR spectroscopy. A previously reported [3]rotaxane-12 that is known to behave as a bistable molecular switch has two recognition sites for CB6, that is, the diaminotriazole moieties and the dodecyl spacer. By changing the pH of the system, it is possible to observe more than one state in the shuttling process. At low pH values both CB6 units are located on the diaminotriazole moieties owing to an ion-dipole interaction, whereas at high pH values both of the CB6 units are located on the hydrophobic dodecyl spacer. Surprisingly, the CB6 units shuttle back to their initial state very slowly after reprotonation of the axle. Even after eighteen days at room temperature, only about 50 % of the CB6 units had relocated back onto the diaminotriazole moieties. The rate constants for the shuttling processes were measured as a function of temperature over the range from 313 to 333 K and the activation parameters (enthalpy, entropy, and free energy) were calculated by using the Eyring equation. The results indicate that this [3]rotaxane behaves as a kinetically controlled molecular switch. The switching properties of [3]rotaxane-3 have also been studied. However, even under extreme pH conditions this rotaxane has not shown any switching action, which confirms that the propyl spacer is too short to accommodate CB6 units.     

Recognition properties of cucurbit[7]uril self-assembled monolayers studied with force spectroscopy

Specific (host-guest) and unspecific (substrate-guest) interactions between self-assembled monolayers of cucurbit[7]uril (CB[7]) on gold (Au) substrates and neutral adamantyl (Ad) guests were resolved by studying these interactions at the single molecule level using dynamic force spectroscopy. The dissociation rate constants of the Ad-Au and the Ad-CB[7] interactions were 0.3 s(-1) and 0.03 s(-1), respectively, indicating that the specific binding is more stable. The probability of observing a specific interaction (40 ± 9%) is similar to the reported surface coverages of CB[7] monolayers on Au substrates. The higher strength and stability of the Ad-CB[7] interactions explains why, although presenting an imperfect coverage, CB[n] monolayers can be used successfully as a platform for surface immobilization.     

瓜环准轮烷分子晶体结构及切割DNA研究

报道了瓜环准轮烷分子晶体结构及DNA的切割.     

Defection-selective solubilization and chemically-responsive solubility switching of single-walled carbon nanotubes with cucurbit[7]uril

Single-walled carbon nanotubes (SWCNTs) were suspended in aqueous media with cucurbit[7]uril (CB7), while SWCNTs were insoluble with cucurbit[5]uril (CB5). Moreover, defection-selective solubilization of SWCNTs with CB7 was demonstrated.