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Supramolecular hydrogels gained significant attention as shear-thinning, self-healing materials. However, the introduction of non-covalent crosslinks inherently decreases the strength and stability of the hydrogel. In this work, we developed a novel supramolecular hydrogel that undergoes a reversible transformation to the corresponding covalently crosslinked hydrogel upon UV-irradiation. The supramolecular hydrogel was developed based on the ternary host–guest interaction of two anthracene moieties and one large macrocyclic host. Anthracene functionalized poly(N-acryloylmorpholine)s were synthesized by post-polymerization modification of a copolymer consisting of N-acryloylmorpholine and an activated ester comonomer, whereby two different polymers were prepared having either neutral anthracene side-chains or carrying a positive charge next to the anthracene to enhance the interaction with the host. The binding affinity of the anthracene side chains with and without an additional cationic charge were studied with two macrocyclic hosts, namely cucurbit[8]uril and γ-cyclodextrin (γ-CD) by UV-Vis titration revealing a markedly stronger binding in the presence of the cationic charge due to additional ion-dipole one of the macrocyclic hosts, cucurbit[8]uril. Subsequently, the effect of the binding affinity on the hydrogelation was investigated, indicating that the stronger binding affinity facilitated the hydrogel formation at lower concentration. Finally, the reversible transformation of the supramolecular hydrogel to a chemical hydrogel by anthracene dimerization was studied by the UV irradiation of the hydrogel at 365 nm for covalent crosslinking or at 254 nm for decrosslinking. It could be demonstrated that the dynamic nature of the hydrogel, that is responsible for the shear-thinning behavior, was indeed lost upon UV-irradiation indicative of the formation of a covalently crosslinked hydrogel. The capabilities of the formed supramolecular hydrogel that is easily processable and able to reversibly convert to a chemical hydrogel, provides potential applications in applying mechanically robust covalently crosslinked hydrogels in complex shapes and difficult to reach locations making use of the dynamic nature of the supramolecular crosslinks.

The combination of the macrocyclic hosts p‐sulfonatocalix[4]arene and cucurbit[7]uril with the fluorescent dyes lucigenin and berberine affords two label‐free enzyme assays for the detection of kinase and phosphatase activity by fluorescence monitoring. In contrast to established assays, no substrate labeling is required. Since phosphorylation is one of the most important regulatory mechanisms in biological signal transduction, the assays should be useful for identification of inhibitors and activators in high‐throughput screening (HTS) format for drug discovery., Monitoring enzyme activity: Reporter pairs composed of macrocyclic hosts and dyes afford kinase and phosphatase assays suitable for drug discovery

A convenient supramolecular strategy for constructing a ratiometric fluorescent chemosensing ensemble, consisting of a macrocyclic host (cucurbit[8]uril CB[8]), and a pyrene‐tagged amphiphilic peptide beacon (AP 1), is reported. AP 1 unfolds upon encapsulation of the pyrene termini into the hydrophobic CB[8] cavity. This changes pyrene excimer to monomer emission. Substrates with higher affinity for the CB[8] cavity can displace AP 1 from the ensemble. The released AP 1 folds again to form a pyrene excimer, which allows for the ratiometric fluorescence monitoring of the substrate. In this report, the ensemble capacity for ratiometric fluorescence monitoring of biological substrates, such as amino acid derivatives, specific peptides, and proteins, in aqueous media is demonstrated., A switching chemosensing ensemble was obtained by combining a macrocyclic host, cucurbit[8]uril (CB[8]), and a pyrene‐tagged amphiphilic peptide beacon (AP 1) for excellent ratiometric fluorescent recognition of biological substrates in aqueous media. The pyrene termini of the flexible AP 1 compete with substrates for encapsulation into the CB[8] cavity which changes pyrene excimer/monomer emission.

In this study, we investigate the influence of γ-cyclodextrin (γCD) or octakis-(2,3,6-tri-O-methyl)-γ-cyclodextrin (TMe-γCD) encapsulation on the solubility, thermal, optical, electrochemical, morphological, and electrical properties of an aromatic polyazomethine (PA). PA was prepared by oxidative C C coupling of pyrenyl groups from the monomer MA using FeCl3 as catalyst in DMF. After threading of γCD or TMe-γCD macrocycles onto the PA chains, the 1H NMR spectra were in line with the formation of the respective inclusion complexes, PA·γCD and PA·TMe-γCD, which showed distinct improvements of their solubility in common organic solvents, higher thermal stability, as well as better film-forming ability. Fluorescence spectroscopy revealed that the emission maximum shifted from 388 nm for PA to 425 nm and 428 nm for PA·γCD and PA·TMe-γCD, which is in good agreement with encapsulation of PA backbones into the hydrophobic cavities. Fluorescence lifetime measurements indicated multiexponential decays for all compounds, which is in accordance with different conformers in the excited state. Atomic force microscopy indicated that the surfaces of PA·γCD and PA·TMe-γCD in the solid state showed needle-shaped morphologies with widths of 105 ± 25 nm and 46 ± 13 nm, respectively, whereas the surface of the naked PA polymer displayed an irregular morphology with embedded particles and width of 158 ± 29 nm. The electrochemical band gaps (ΔEg) of PA·γCD and PA·TMe-γCD were lower than that of non-encapsulated PA. The electrical conductivities of the investigated compounds were about 10−11 S cm−1, which is in the range of insulating materials.

A putrescine derivative of aminomethyladamantane is established as a ditopic guest with two mutually exclusive binding sites for cucurbit[6]uril and cucurbit[7]uril. A mixture containing both hosts, the ditopic guest, and two fluorescent dyes affords a relay system with a ratiometric fluorescence response and enables a kinetic analysis of the switching mechanism.

A supramolecular host-guest FRET pair based on a carboxyfluorescein-labelled cucurbit[7]uril (CB7-CF, as acceptor) and the fluorescent dye 4',6-diamidino-2-phenylindole (DAPI, as donor) is developed for sensing of DNA. In comparison to the commercial DNA staining dye SYBR Green I, the new chemosensing ensemble offers dual-emission signals, which allows a linear ratiometric response over a wide concentration range.

Microcystis aeruginosa is a cyanobacterium that produces a variety of cyclic heptapeptide toxins in freshwater. The protective effects of the macromolecular container cucurbit[7]uril (CB7) were evaluated using mouse models of cyanotoxin-induced liver damage. Biochemical analysis of liver function was performed to gauge the extent of liver damage after exposure to cyanobacterial crude extract [CCE; LD50 = 35 mg/kg body weight; intraperitoneal (i.p.)] in the absence or presence of CB7 (35 mg/kg body weight, i.p.). CCE injection resulted in liver enlargement, potentiated the activities of alanine aminotransferase (ALT) and glutathione S-transferase (GST), increased lipid peroxidation (LPO), and reduced protein phosphatase 1 (PP1) activity. CCE-induced liver enlargement, ALT and GST activities, and LPO were significantly reduced when CB7 was coadministered. Moreover, the CCE-induced decline of PP1 activity was also ameliorated in the presence of CB7. Treatment with CB7 alone did not affect liver function, which exhibited a dose tolerance of 100 mg/kg body wt. Overall, our results illustrated that the addition of CB7 significantly reduced CCE-induced hepatotoxicity (P < 0.05).

Time-resolved monitoring of the permeability of analytes is of utmost importance in membrane research. Existing methods are restricted to single-point determinations or flat synthetic membranes, limiting access to biologically relevant kinetic parameters (permeation rate constant, permeation coefficients). We now use the recently introduced fluorescent artificial receptor membrane assay (FARMA) as a method to monitor, in real time, the permeation of indole derivatives through liposomal membranes of different lipid compositions. This method is based on the liposomal encapsulation of a chemosensing ensemble or "fluorescent artificial receptor", consisting of 2,7-dimethyldiazapyrenium as a fluorescent dye and cucurbit[8]uril as the macrocyclic receptor, that responds to the complexation of a permeating aromatic analyte by fluorescence quenching. FARMA does not require a fluorescent labeling of the analytes and allows access to permeability coefficients in the range from 10-8 to 10-4 cm s-1. The effect of temperature on the permeation rate of a series of indole derivatives across the phospholipid membranes was studied. The activation energies for permeation through POPC/POPS phospholipid membranes were in the range of 28-96 kJ mol-1. To study the effect of different lipid phases on the membrane permeability, we performed experiments with DPPC/DOPS vesicles, which showed a phase transition from a gel phase to a liquid-crystalline phase, where the activation energies for the permeation process were expected to show a dramatic change. Accordingly, for the permeation of the indole derivatives into the DPPC/DOPS liposomes, discontinuities were observed in the Arrhenius plots, from which the permeation activation energies for the distinct phases could be determined, for example, for tryptamine 245 kJ mol-1 in the gel phase and 47 kJ mol-1 in the liquid-crystalline phase.

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.

We introduce herein boron-dipyrromethene (BODIPY) dyes as a new class of fluorophores for the design of reporter dyes for supramolecular host–guest complex formation with cucurbit[7]uril (CB7). The BODIPYs contain a protonatable aniline nitrogen in the meso-position of the BODIPY chromophore, which was functionalized with known binding motifs for CB7. The unprotonated dyes show low fluorescence due to photoinduced electron transfer (PET), whereas the protonated dyes are highly fluorescent. Encapsulation of the binding motif inside CB7 positions the aniline nitrogen at the carbonyl rim of CB7, which affects the pKa value, and leads to a host-induced protonation and thus to a fluorescence increase. The possibility to tune binding affinities and pKa values is demonstrated and it is shown that, in combination with the beneficial photophysical properties of BODIPYs, several new applications of host–dye reporter pairs can be implemented. This includes indicator displacement assays with favourable absorption and emission wavelengths in the visible spectral region, fluorescence correlation spectroscopy, and noncovalent surface functionalization with fluorophores.

We report the formation of supramolecular complexation between cucurbit[n]urils (CBn) and an amphiphilic pyridinium-functionalized anthracene (AnPy) in aqueous solution. The CB7 cavity is capable of accommodating the pyridinium moiety, while CB8 can encapsulate the pyridinium and anthracene moieties at once. The encapsulation of AnPy by CB7 leads to the formation of nanoparticles, while the complexation of AnPy with CB8 leads to the formation of nanorods.

Cucurbit[7]uril (CB7), native α-cyclodextrin (αCD) and its tris-O-methylated derivative (TMαCD) as macrocyclic host molecules and amine-terminated poly(ethylene glycol) (PEG) as neutral guest were used for the synthesis of 3-CB7, 3-αCD and 3-TMαCD polyrotaxanes. The binding affinities of the neutral PEG guest into the CB7, αCD and TMαCD cavities were evaluated by isothermal titration calorimetry. The high value of the stability constant (1 × 104 M−1) indicates that PEG provides a preferential binding to CB7 over αCD. The polyrotaxane structure was confirmed by FT-IR and 1H NMR spectroscopy. The coverage ratio of the PEG with CB7, αCD and TMαCD macrocycles, were 63%, 71%, and 30%, respectively. The polyrotaxanes 3-CB7 and 3-αCD exhibit an enhancement in thermal stability compared with the non-rotaxane 3 counterpart. In addition, polyrotaxane 3-CB7 shows blue-shifted absorption by more than 20 nm and enhanced water solubility.

Whilst cucurbit[n]urils (CBn) have been utilized in gas encapsulation, only the smaller CBn (n = 5 and 6) have utility given their small cavity size. In this work, we demonstrate that the large cavity of CB8 can be tailored for gaseous and volatile hydrocarbon encapsulation by restricting its internal cavity size with auxiliary aryl-bisimidazolium (Bis, aryl = phenyl, naphthyl, and biphenyl) guests. The binding constants for light hydrocarbons \(C \le 4\) are similar to those measured with CB6, while larger values are obtained with Bis·CB8 for larger guests. A clear propensity for higher affinities of alkenes relative to alkanes is observed, most pronounced with the largest delocalized naphthalene residue in the auxiliary Bis guest, which provides unique evidence for sizable small-molecule \(\Pi-\Pi\) interactions.

The supramolecular chemisorption of CO2 by the oligomeric chitin-acetate (CA) in DMSO as a green solvent offers a novel eco-friendly approach for CO2 capture. Since the amino groups in the sorbent material are blocked either by protonation or acetylation, the multi-armed hydroxyl based oligosaccharide captures CO2 through the formation of an organic carbonate species as confirmed by 13C NMR, in situ ATR-FTIR spectroscopy and conductivity. DFT calculations verified the formation of the CA-CO2 adduct, in which the organic carbonate group is stabilized through supramolecular ionic interaction and hydrogen bonding with the neighboring ammonium ion and hydroxyl functional group along the oligomer backbone. The use of other polar aprotic solvents (N,N-dimethylformamide (DMF), acetonitrile, and acetone) was not successful due to solubility issues.

Competitive occupation of surface-bound supramolecular hosts allows precise control of surface coverage densities on particles., We report herein the controlled surface functionalization of micro- and nanoparticles by supramolecular host–guest interactions. Our idea is to exploit the competition of two high-affinity guests for binding to the surface-bound supramolecular host cucurbit[7]uril (CB7). To establish our strategy, surface azide groups were introduced to hard-sphere (poly)methylmethacrylate particles with a grafted layer of poly(acrylic acid), and a propargyl derivative of CB7 was coupled to the surface by click chemistry. The amount of surface-bound CB7 was quantified with the high-affinity guest aminomethyladamantane (AMADA), which revealed CB7 surface coverage densities around 0.3 nmol cm–2 indicative of a 3D layer of CB7 binding sites on the surface. The potential for surface functionalization was demonstrated with an aminoadamantane-labeled rhodamine (Ada-Rho) as a second high-affinity guest. Simultaneous incubation of CB7-functionalized particles with both high-affinity guests, AMADA and Ada-Rho, revealed a simple linear relationship between the resulting surface coverage densities of the model fluorescent dye and the mole fraction of Ada-Rho in the incubation mixture. This suggests a highly modular supramolecular strategy for the stable immobilization of application-relevant molecules on particle surfaces and a precise control of their surface coverage densities.

Polyhedral carboxymethyl carborane (C2 B10 H12 ) derivatives, including mono- and disubstituted o-, m-, and p-isomers, have been synthesized. Supramolecular host-guest complexation of these derivatives with cyclodextrins (CDs; namely, α-, β-, and γ-CD) has been investigated in water. The globular structure of the carborane binding moiety and its hydrophobic character qualify it as an ideal recognition site to form stable inclusion complexes with macrocyclic host molecules in aqueous solution. The measured binding affinities for the carborane derivatives were in the millimolar range (Ka =103 -104 m-1 ) with differently sized CDs, and preferential binding to β-CD.

A poly[(2-ethyl-2-oxazoline)-ran-(2-nonyl-2-oxazoline)] random copolymer was synthesized and its thermoresponsive behavior in aqueous solution modulated by the addition of different supramolecular host molecules. The macrocycles formed inclusion complexes with the nonyl aliphatic side-chains present in the copolymer, increasing its cloud point temperature. The extent of this temperature shift was found to depend on the cavitand concentration and on the strength of the host-guest complexation. The cloud point temperature could be tuned in an unprecedented wide range of 30 K by supramolecular interactions. Since the temperature-induced breakage of the inclusion complexes constitutes the driving force for the copolymer phase transition, the shift in cloud point temperature could be utilized to estimate the association constant of the nonyl side chains with the cavitands.

A fluorescent perylene dye with two aromatic units was designed for binding to cucurbit[8]uril. The binding affinity of the complexes increased about 3 orders of magnitude compared to the dye without a secondary aromatic unit. The high affinity allows the complexes to act as fluorescent probes for detection of strong binding guests with nanomolar sensitivity.

Steroids have been identified as endocrine-disrupting agents, which are thought to impact the fertility of aquatic organisms and may even have direct effects on humans. The removal of steroids from wastewater is therefore essential, and this is most efficiently achieved by microbial treatment. We report herein a simple fluorescent method to identify microorganisms that are capable of steroid degradation and to optimize the conditions for steroid removal. The method is based on the supramolecular macrocycle cucurbit[8]uril (CB8), which can bind either the fluorescent dye berberine or a steroid in their inner cavity. In absence of steroid, the cavity is free to bind the dye, leading to a strong increase in fluorescence. In contrast, in the presence of steroid, the dye is displaced into the bulk solution. This principle affords a stable (no thermal or photodegradation was noted), fluorescent chemosensor (excitation ca. 450 nm, maximum emission at 525 nm), which can detect testosterone at concentrations > 0.7 μM. We show that this displacement principle can be applied to follow the removal of micromolar concentrations of the steroid testosterone from a bacterial culture of Buttiauxella sp. S19-1. The reliability of the chemosensor in screening applications is demonstrated by an excellent Z-factor, which was in the range of 0.52 to 0.74 for all experiments carried out with this assay. Graphical abstract Steroid depletion by bacterial cultures can be followed by fluorescence spectroscopy using a supramolecular chemosensor based on berberine and cucurbit[8]uril.

Polyamines play an important role in cell growth, differentiation, and cancer development, and the biosynthetic pathway of polyamines is established as a drug target for the treatment of parasitic diseases, neoplasia, and cancer chemoprevention. The key enzyme in polyamine biosynthesis is ornithine decarboxylase (ODC). We report herein an analytical method for the continuous fluorescence monitoring of ODC activity based on the supramolecular receptor cucurbit[6] uril (CB6) and the fluorescent dye trans-4-[4-(dimethylamino) styryl]-1-methylpyridinium iodide (DSMI). CB6 has a significantly higher binding constant to the ODC product putrescine (>10(7)M(-1)) than to the substrate L-ornithine (340 M-1). This enables real-time monitoring of the enzymatic reaction through a continuous fluorescence change caused by dye displacement from the macrocycle by the formed product, which allowed a straightforward determination of enzyme kinetic parameters (k(cat) = 0.12 s(-1) and K-M = 24 mu M) and inhibition constants of the two ODC inhibitors alpha-difluoromethylornithine (DFMO) and epigallocatechin gallate (EGCG). The potential for high-throughput screening (HTS) was demonstrated by excellent Z' factors (>0.9) in a microplate reader format, and the sensitivity of the assay is comparable to or better than most established complementary methods, which invariably have the disadvantage of not being compatible with direct implementation and upscaling to HTS format in the drug discovery process.

The intriguing dual-emission behavior of p- dimethylaminobenzonitrile (DMABN) and the identity of the associated excited states is, arguably, the most extensively investigated and also controversially discussed molecule- specific phenomenon of modern photochemistry. We have now found a new, third fluorescence band when DMABN is encapsulated within the water-soluble molecular container cucurbit[8]uril (CB8). It is centered between the previously observed emissions and assigned to the elusive excimer emission from DMABN through 1:2 CB8:DMABN complex formation. Heating of the CB8⋅(DMABN)2 complex from 0 to 100 °C results in the dissociation of the ternary complex and restoration of the dual-emission properties of the monomer. Alternatively, monomer emission can be obtained by selecting cucurbit[7]uril (CB7), a host homologue that is too small to accommodate two DMABN molecules, or by introducing ethyl instead of methyl groups at the amino terminus of the aminobenzonitrile guest.

We report the self-assembly of a series of highly charged supramolecular complexes in aqueous media composed of cyclobis(4,4'-(1,4-phenylene)bispyridine-p-phenylene)tetrakis(chloride) (ExBox) and three dicationic perylene diimides (PDIs). Efficient energy transfer (ET) is observed between the host and guests. Additionally, we show that our hexacationic complexes are capable of further complexation with neutral cucurbit[7]uril (CB[7]), producing a 3-polypseudorotaxane via the self-assembly of orthogonal recognition moieties. ExBox serves as the central ring, complexing to the PDI core, while two CB[7]s behave as supramolecular stoppers, binding to the two outer quaternary ammonium motifs. The formation of the 3-polypseudorotaxane results in far superior photophysical properties of the central PDI unit relative to the binary complexes at stoichiometric ratios. Lastly, we also demonstrate the ability of our binary complexes to act as a highly selective chemosensing ensemble for the neurotransmitter melatonin.

The design of high-affinity and analyte-selective receptors operating in aqueous solutions is an outstanding problem in supramolecular chemistry. Directing the focus toward the unique properties of water, we present here a new strategy toward this goal and support it by molecular dynamics simulations and calorimetric measurements. We illustrate the procedure in the case of self-assembled 1:1 complexes of the rigid macrocycle cucurbit[8]uril (CB8) and dicationic auxiliary guests (AG). These CB8•AG complexes contain residual water molecules whose conformational space and hydrogen-bond formation ability is restricted by the geometrically confined and hydrophobic cavity of the receptor. We show that upon inclusion of an analyte to form a 1:1:1 CB8•AG•analyte complex, these "high-energy" cavity water molecules are released to the aqueous bulk, providing a strong enthalpic driving force to the association, and resulting in binding constants of up to 10 6 M-1 for aromatic analytes. This binding model is supported by the measurements of large solvent and solvent isotope effects. The selectivity of the CB8•AG receptor can be modified or even switched toward small aliphatic analytes by a rational choice of the auxiliary guest, demonstrating the tunable recognition features of such self-assembled receptors. Furthermore, by comparison of the results to those for the extensively studied macrocyclic host cyclobis(paraquat-p-phenylene) -the so-called "blue-box" -it is shown that in aqueous solution the release of "high-energy" water molecules from the CB8•AG cavity can be more favorable than the use of direct host-guest interactions © 2013 American Chemical Society.

Poly[2,7-(9,9-dioctylfluorene)-alt-(2,7-fluorene/cucurbit[7]uril)] polyrotaxane was synthesized according to Suzuki coupling protocol in DMSO, by reacting 2,7-dibromofluorene encapsulated into the cucurbit[7]uril (CB7) cavity with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester. The Ka value (9.5 × 103 M−1) indicates that 2,7-dibromofluorene provides a preferential binding to CB7, which allows the synthesis of a polyrotaxane with a CB7/structural unit ratio of about 1/3 and higher molecular weights. The chemical structure was proven by FTIR and 1H NMR spectroscopy. The thermal, optical, electrochemical, wetting, and surface morphological properties of the polyrotaxane have been investigated and compared to those of the neat copolymer. The polyrotaxane exhibits an enhancement in the glass transition temperature, blue-shifted absorption by about 11 nm, and a subtle effect on the LUMO energy levels. The fluorescence lifetimes follow a mono-exponential decay with a value of τF = 0.7 ns. The electrochemical band gap of the polyrotaxane (3.39 eV) is smaller than that of the uncomplexed compound. Wetting properties reveal a change in the distribution of the dispersive and polar components of the surface free energy through the CB7 encapsulation. Atomic force microscopy indicated further that the polyrotaxane film is uniformly distributed over the substrate area with a regular consistency and lower roughness parameters.

In this study, we have investigated the effect of oligomerization and rotaxane formation on the optical and surface-morphological properties of pyrene–triazole azomethine based compounds. The combined effects of oligomerization and rotaxane formation improve the solubility, film forming ability, photophysical (increased fluorescence intensity and lifetime), and morphological properties of the azomethine oligomers. Upon rotaxane formation, the root-mean-square roughness of the films decreased from 3.5 to 1.0 nm (determined by AFM). On the contrary, no remarkable difference was found on the electrical conductivity of the iodine-doped rotaxane oligomer as compared to its un-doped form.

A new coumarin derivative containing benzothiazole and piperazine substituents was synthesized. Preferential inclusion of the benzothiazole group, over the coumarin and piperazine groups, inside the cavity of the molecular container cucurbit[7]uril (CB7) was evidenced by using optical and NMR techniques. The binding constant of the new complex with CB7 is higher in its protonated forms (e.g., K = 2.8 × 106 M−1) than in its neutral forms, which led to an increase in the pKa value associated with protonation of the aza nitrogen on the benzothiazole ring of ca. 2.5 units. Such CB7-induced protonation disabled the photoinduced electron transfer (PET) in the included molecule, enhancing its coumarin fluorescence up to ca. 45-fold (pH 3.5, 410 nm). The results are discussed in the context of designing sensitive analytical tools for reversible monitoring of optically inactive analytes by competitive displacement experiments.

An analytical method has been developed for the continuous monitoring of protease activity on unlabeled peptides in real time by fluorescence spectroscopy. The assay is enabled by a reporter pair comprising the macrocycle cucurbit[7]uril (CB7) and the fluorescent dye acridine orange (AO). CB7 functions by selectively recognizing N-terminal phenylalanine residues as they are produced during the enzymatic cleavage of enkephalin-type peptides by the metalloendopeptidase thermolysin. The substrate peptides (e.g., Thr-Gly-Ala-Phe-Met-NH(2)) bind to CB7 with moderately high affinity (K ≈ 10(4) M(-1)), while their cleavage products (e.g., Phe-Met-NH(2)) bind very tightly (K10(6) M(-1)). AO signals the reaction upon its selective displacement from the macrocycle by the high affinity product of proteolysis. The resulting supramolecular tandem enzyme assay effectively measures the kinetics of thermolysin, including the accurate determination of sequence specificity (Ser and Gly instead of Ala), stereospecificity (d-Ala instead of l-Ala), endo- versus exopeptidase activity (indicated by differences in absolute fluorescence response), and sensitivity to terminal charges (-CONH(2) vs -COOH). The capability of the tandem assay to measure protease inhibition constants was demonstrated on phosphoramidon as a known inhibitor to afford an inhibition constant of (17.8 ± 0.4) nM. This robust and label-free approach to the study of protease activity and inhibition should be transferable to other endo- and exopeptidases that afford products with N-terminal aromatic amino acids.

Kinetic solvent effects on hydrogen abstractions involving C-H donors (dienes, ethers, alkylbenzenes) have been corroborated by experiment and theory (UB3LYP/6-311++G**, polarized continuum model). To single out the effect of solvent polarity, rate constants for scavenging of the cumyloxyl radical and fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene were obtained in binary aprotic mixtures of ethylacetate and acetonitrile. Polar solvents result in a selective stabilization of the reactants (see TOC graphic), which results in slower rate constants.

The steady-state as well as transient absorption spectroscopy of a series of 2-bromo-aryl ketones have been comprehensively examined to gain insights concerning (i) the transient phenomena (absorption spectral attributes as well as lifetimes), (ii) rates of C–Br homolysis, and (iii) the behavior of 2-aroylaryl radicals thus generated. The X-ray crystal structure analyses of selected ketones in which the mesomeric effects operate differently reveal that the two aryl rings are drastically twisted about the C=O bond. The twisting manifests itself in the spectral features of the transients, attributed to triplet–triplet (T–T) absorptions, such that they are not readily comparable in some cases to the transients of parent diaryl ketones that lack the 2-bromo group. By associating triplet decays with C–Br cleavage rates, the absolute rate data have been determined for diverse 2-bromoaryl ketones. With the exception of 2-bromo ketones containing meta-methoxy substituents, all other ketones are found to undergo efficient C–Br bond cleavage with rates of ca. 0.1–1.0 × 108 s–1. For m-methoxy-substituted ketones, intriguingly slower deactivation of the triplets was observed. Based on solvent-dependent variation of the lifetimes (longer lifetimes in polar solvents), intramolecular charge transfer has been proposed. The preparative photochemistry and transient phenomena permit invaluable inferences as to the reactivity of 2-aroylaryl radicals in general. Quantum yield determinations and product analyses reveal that highly electrophilic aryl radicals undergo radical recombination, in a poor hydrogen-donating solvent, almost exclusively (>90 %) in the absence of incentive for stabilization via conversion to π-conjugated hydrofluorenyl radicals. Of course, when the latter is feasible, Pschorr cyclization leads to productive photochemical outcome. Moderately electrophilic radicals that lack stabilization via conversion to hydrofluorenyl radicals lend themselves to intramolecular 1,5-hydrogen shifts in conjunction with the formation of dehalogenated diaryl ketones and cyclized fluorenones (Fls) or its analogs.

UV–vis and NMR spectroscopic techniques were employed to demonstrate the ability of the synthetic macrocyclic host cucurbit[7]uril (CB7) to solubilize and stabilize widely used fungicides and anthelmintic drugs of the benzimidazole family in water, namely, albendazole (ABZ), carbendazim (CBZ), thiabendazole (TBZ), fuberidazole (FBZ), and the parent benzimidazole (BZ). CB7 binds the protonated forms of these guests very strongly (e.g., K = 2.6 × 107 L/mol for ABZ) but their neutral forms significantly more weakly (e.g., K = 6.5 × 104 L/mol for ABZ), which reflects a complexation-induced increase of their pKa values by 2.6 units for ABZ, 2.5 units for CBZ, 4.0 units for TBZ, 3.8 units for FBZ, and 3.5 units for BZ. The absolute drug solubilities increased upon complexation from 0.003 to 0.300 mmol/L for ABZ, from 0.160 to 1.12 mmol/L for CBZ, from 0.110 to 1.11 mmol/L for TBZ, and from 0.25 to 0.75 mmol/L for FBZ (for BZ, the solubility enhancement was found to be insignificant). Complexation by CB7 further improves the photostability of the drugs and alters their photophysical properties.

A flexible peptide chain displays structural and dynamic properties that correspond to its folding and biological activity. These properties are mirrored in intrachain site-to-site distances and diffusion coefficients of mutual site-to-site motion. Both distance distribution and diffusion determine the extent of F&ouml, rster resonance energy transfer (FRET) between two sites labeled with a FRET donor and acceptor. The relatively large F&ouml, rster radii of traditional FRET methods (R0 &gt, 20 &Aring, ) lead to a fairly low contribution of diffusion. We introduced short-distance FRET (sdFRET) where Dbo, an asparagine residue conjugated to 2,3-diazabicyclo[2.2.2]octane, acts as acceptor paired with donors, such as naphtylalanine (NAla), tryptophan, 5-l-fluorotryptophan, or tyrosine. The F&ouml, rster radii are always close to 10 &Aring, which makes sdFRET highly sensitive to diffusional motion. We recently found indications that the FRET enhancement caused by diffusion depends symmetrically on the product of the radiative fluorescence lifetime of the donor and the diffusion coefficient. In this study, we varied this product by two orders of magnitude, using both donors of different lifetime, NAla and FTrp, as well as a varying viscogen concentration, to corroborate this statement. We demonstrate the consequences of this relationship in evaluating the impact of viscogenic coadditives on peptide dimensions.

A water-based narrow-band high-efficiency dye laser was designed by means of a supramolecular host-guest chemical approach. The lasing characteristics of rhodamine B and sulforhodamine B (Kiton Red S) dyes in aqueous solution with the macrocyclic host cucurbit[7]uril (CB7) as additive were investigated in a narrow-band dye laser setup. Significant improvements in both photostability and thermo-optical properties of the aqueous CB7-complexed dye systems were observed as compared to the uncomplexed dyes in ethanol solution. The tuning curves for the new dye-CB7-water systems were constructed by measuring the laser output at different wavelengths, which showed similar peak efficiencies and red-shifted gains compared to the ethanolic solutions of the dyes, while dye laser operation revealed comparable pump threshold energies and slope efficiencies. The combined results render the dye-CB7-water system an attractive active medium for high-repetition rate dye laser operation.

The complex stability constants (K S ) and thermodynamic parameters (ΔH° and TΔS°) for the 1:1 complexation of two water-soluble calixarenes, p-sulfonatocalix[4]arene (SC4A) and 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis(n-butyl)-calix[4]arene (SC4A-Bu), with organic ammonium cations and neutral spherical organic molecules, have been determined by means of isothermal titration calorimetry (ITC) in aqueous solutions at 298.15 K. The obtained results indicate that, upon complexation with these guests by SC4A-Bu, the enthalpy changes become less favorable, whereas the entropy changes become more favorable relative to SC4A complexation. These differences can be attributed to differential degrees of desolvation and removal of high-energy water as well as the change in conformation or conformational degrees of freedom upon complexation. The calorimetric investigations, accompanied by 1 H NMR and UV/Vis spectroscopy and X-ray crystallography provide a thermodynamic explanation for the different complexation behavior of SC4A and SC4A-Bu towards charged and neutral organic guests. Binding ability and molecular selectivity are discussed from the viewpoint of the conformational geometry and electronic properties of hosts and guests.

The effect of the macrocyclic host, cucurbit[7]uril (CB7), on the photophysical properties of the 2-(2'-hydroxyphenyl)benzimidazole (HPBI) dye have been investigated in aqueous solution by using ground-state absorption and steady-state and time-resolved fluorescence measurements. All three prototropic forms of the dye (cationic, neutral, and anionic) form inclusion complexes with CB7, with the largest binding constant found for the cationic form (K approximately 2.4x10(6) M(-1)). At pH approximately 4, the appearance of a blue emission band upon excitation of the HPBI cation in the presence of CB7 indicates that encapsulation into the CB7 cavity retards the deprotonation process of the excited cation, and hence reduces its subsequent conversion to the keto form. Excitation of the neutral form (pH approximately 8.5), however, leads to an increase in the keto form fluorescence, indicating an enhanced excited-state intramolecular proton-transfer process for the encapsulated dye. In both the ground and excited states, the two pK(a) values of the HPBI dye show upward shifts in the presence of CB7. The prototropic equilibrium of the CB7-complexed dye is represented by a six-state model, and the pH-dependent changes in the binding constants have been analyzed accordingly. It has been observed that the calculated pK(a) values using this six-state model match well with the values obtained experimentally. The changes in the pK(a) values in the presence of CB7 have been corroborated with the modulation of the proton-transfer process of the dye within the host cavity.

The paper reports on the preparation of a new 2-rotaxane monomer through an acid coupling reaction between 1-pyrenecarboxaldehyde and α-CD/3,5-diamino-1,2,4-triazole inclusion complex. Pyrenyl groups are large enough to provide a blocking effect toward cyclodextrin de-threading. The oxidative C―C coupling of 2-rotaxane in the presence of RuCl 3 catalyst afforded conjugated azomethine polyrotaxanes. The expected modifications of the solubility, morphology, film forming ability for rotaxane polymer were proved. As shown by fluorescence and UV-vis spectroscopy, a material with optical properties appropriate for use in photonics was obtained.

A combination of moderately selective host-guest binding with the impressive specificity of enzymatic transformations allows the real-time monitoring of enzymatic reactions in a homogeneous solution. The resulting enzyme assays ("supramolecular tandem assays") exploit the dynamic binding of a fluorescent dye with a macrocyclic host in competition with the binding of the substrate and product. Two examples of enzymatic reactions were investigated: the hydrolysis of arginine to ornithine catalyzed by arginase and the oxidation of cadaverine to 5-aminopentanal by diamine oxidase, in which the substrates have a higher affinity to the macrocycle than the products ("substrate-selective assays"). The depletion of the substrate allows the fluorescent dye to enter the macrocycle in the course of the enzymatic reaction, which leads to the desired fluorescence response. For arginase, p-sulfonatocalix[4]arene was used as the macrocycle, which displayed binding constants of 6400 M(-1) with arginine, 550 M(-1) with ornithine, and 60,000 M(-1) with the selected fluorescent dye (1-aminomethyl-2,3-diazabicyclo[2.2.2]oct-2-ene); the dye shows a weaker fluorescence in its complexed state, which leads to a switch-off fluorescence response in the course of the enzymatic reaction. For diamine oxidase, cucurbit[7]uril (CB7) was used as the macrocycle, which showed binding constants of 4.5 x 10(6) M(-1) with cadaverine, 1.1 x 10(5) M(-1) with 1-aminopentane (as a model for the thermally unstable 1-aminopentanal), and 2.9 x 10(5) M(-1) with the selected fluorescent dye (acridine orange, AO); AO shows a stronger fluorescence in its complexed state, which leads to a switch-on fluorescence response upon enzymatic oxidation. It is demonstrated that tandem assays can be successfully used to probe the inhibition of enzymes. Inhibition constants were estimated for the addition of known inhibitors, i.e., S-(2-boronoethyl)-L-cysteine and 2(S)-amino-6-boronohexanoic acid for arginase and potassium cyanide for diamine oxidase. Through the sequential coupling of a "product-selective" with a "substrate-selective" assay it was furthermore possible to monitor a multistep biochemical pathway, namely the decarboxylation of lysine to cadaverine by lysine decarboxylase followed by the oxidation of cadaverine by diamine oxidase. This "domino tandem assay" was performed in the same solution with a single reporter pair (CB7/AO).

A variety of differently substituted 6- and 7-arylchromenes such as that depicted undergo photoinduced CO bond cleavage to yield colored o-quinonoid intermediates. A combined analysis of μs–ms (laser flash) and real-time kinetic data show that the o-quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating. A ready synthetic accessibility of a series of 6- and 7-arylchromenes via Pd0-catalyzed Suzuki coupling protocol has permitted a comprehensive investigation of the thermal decay behavior of a broad set of photogenerated o-quinonoid reactive intermediates. It is shown that substantial mesomeric effect between the benzopyran nucleus and the aryl ring at C6 or C7 position of the former renders significant absorption beyond 350 nm such that they are readily photoactivated to yield colored o-quinonoid intermediates. The absorption spectra of the latter are found to be strongly influenced by the substituents on C2-, C6- and C7-aryl rings; indeed the colored absorptions can be conveniently tuned by appropriate choice of substituents. The thermal decay (bleaching phenomenon), which is important from the point of view of their application in ophthalmic lenses, was investigated in each case by μs–ms as well as real-time absorption spectroscopy. By careful experimentation, we have extracted the decay rate constants for Z,E and E,E o-quinonoid isomers of all 6- and 7-arylchromenes in an attempt to establish a correlation between the electronic attributes with their thermokinetic behavior. From a combined analysis of μs–ms (laser flash) and real-time kinetic data, it is shown that the colored o-quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. In the same vein, the decay was found to occur slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron-donating. Thus, any substituent on the C2-aryl ring that contributes mesomerically to the development of charge on the quinonoid oxygen, and any substituent on the C6-/C7-aryl ring that exerts −I effect appear to expedite thermal decay. Furthermore, evidence is obtained for the first time from μs time-resolved laser-flash spectroscopy for the formation and characterization of the trans,cis (E,Z) o-quinonoid isomer, which has heretofore eluded spectral characterization in the photochromic phenomena of pyrans.

The effect of complexation of a poly[2,7-(9,9-dioctylfluorene- alt -2,7-fluorene)] copolymer into the inner cavity of γ-cyclodextrin on the morphology, the optical and the electrical properties was investigated. The fluorescence spectra of the rotaxane copolymer and of a reference, non complexed copolymer exhibit typical well resolved blue emission bands arising from the fluorene chromophore units. The complexation of the fluorene copolymer chain increases the fluorescence lifetime from 0.54 to 0.59 ns and decreases the activation energy of the electrical conduction from 2.41 to 1.49 eV. The rotaxane copolymer presents higher semiconducting properties as compared to the reference copolymer.

The derivatization of 3-amino-9-ethylcarbazole with a diamino-alkyl anchor affords a fluorescent dye suitable for indicator displacement from cucurbituril macrocycles. The novel compound 1 shows, due to a complexation-induced pKa shift, a large and predictable dual fluorescence response (100-fold increase at 375 nm and 9-fold decrease at 458 nm) upon supramolecular encapsulation and a strong affinity for cation-receptor macrocycles, in particular cucurbit[6]uril (CB6). A direct application is presented by monitoring the enzymatic activity of lysine decarboxylase.

We have investigated a mixture consisting of p-sulfonatocalix[4]arene (CX4), beta-cyclodextrin (beta-CD), and 2,3-diazabicyclo[2.2.2]oct-2-ene (1) and its bridgehead-substituted derivative (2) in the absence and presence of Zn(2+). In the absence of Zn(2+), four equally populated host-guest complexes exist in solution, as projected from their comparable binding constants (ca. 1000 M(-1)). However, upon the addition of Zn(2+), the formation of a ternary complex, CX4 x 1 x Zn(2+), is induced by a synergy of three supramolecular interactions (Coulombic, hydrophobic, and weak metal-ligand bonding). Concomitantly, the CX4 x 2 complex is destabilized by competitive binding, which drives the system toward a state where only two complexes predominate: namely, CX4 x 1 x Zn(2+) and beta-CD x 2. Known binding constants for the multiple equilibria were used to model the complex system, and the results were consistent with experimental data obtained from 1D and 2D NMR as well as induced circular dichroism (ICD) spectroscopy. The combined results demonstrate how a subtle interplay between cooperative and competitive binding can be exploited to design a complex multicomponent sorting system.

Kinetic solvent effects on hydrogen abstractions, namely, acceleration in nonpolar solvents, have been presumed to be restricted to O-H hydrogen donors. We demonstrate that also abstractions from C-H and even Sn-H bonds by cumyloxyl radicals and n,pi*-excited 2,3-diazabicyclo[2.2.2]oct-2-ene are fastest in the gas phase and nonpolar solvents but slowest in acetonitrile. Accordingly, solvent effects on hydrogen abstractions are more general, presumably due to stabilization of the reactive oxygen or nitrogen species in polar solvents.

A water-soluble supramolecular sensing assembly, composed of an imidazolium-substituted calix[4]arene and a fluorescent aminodiacetate derivative of 1,8-naphthalimide, was studied. Addition of citrate led to a large fluorescence enhancement, while tartrate, acetate, as well as selected inorganic anions gave smaller effects. The sensing principle and selectivity for citrate rely on the formation of a ternary fluorophore−host−anion complex and complexation-induced pKa shifts of an amino group attached to the fluorophore. The complexation of citrate induces a protonation of the amino group, which switches off intramolecular photoinduced electron transfer as the fluorescence quenching pathway, leading to an enhancement of the optical output signal. The intricate sensor principle was corroborated by pH titrations, binding constants, and structural information as obtained by 1H NMR spectroscopy.

The potential of cucurbiturils, water-soluble macrocyclic host molecules composed of glycoluril units, for tuning the properties of fluorescent dyes and advancing new applications is illustrated. Cucurbit[7]uril (CB7), which presents a particularly attractive derivative due to its intermediary size and high water solubility, has been shown to display a variety of advantageous effects on fluorescent dyes, which include increased fluorescence intensity and brightness, enhanced photostability, protection towards fluorescence quenchers, solubilization, and deaggregation. Particularly noteworthy is the prolongation of the fluorescence lifetimes of different dyes, which can be traced back to the low polarizability of the host cavity. In addition, the host serves as cation receptor, which causes a considerable shift of protonation equilibria and assists the protonation of fluorescent dyes. The latter effect can be exploited in the design of protolytic fluorophore displacement assays. The perspective of cucurbitu...

Artificial photosynthetic systems for solar energy conversion exploit both covalent and supramolecular chemistry to produce favorable arrangements of light-harvesting and redox-active chromophores in space. An understanding of the interplay between key processes for photosynthesis, namely light-harvesting, energy transfer, and photoinduced charge separation and the design of novel, self-assembling components capable of these processes are imperative for the realization of multifunctional integrated systems. We report our investigations on the potential of extended tetracationic cyclophane/perylene diimide systems as components for artificial photosynthetic applications. We show how the selection of appropriate heterocycles, as extending units, allows for tuning of the electron accumulation and photophysical properties of the extended tetracationic cyclophanes. Spectroscopic techniques confirm energy transfer between the extended tetracationic cyclophanes and perylene diimide is ultrafast and quantitative, while the heterocycle specifically influences the energy transfer related parameters and the acceptor excited state.

Cucurbiturils (CB6 and CB7) were shown to inhibit the enzymatically catalyzed restriction of plasmids and linear DNA. This effect can be inverted by supramolecular masking of the macrocycles through competitive complexation with polyamines. These experiments provide supramolecular control of biocatalytic processes. Spanish MINECO [CTQ2011-28390]; FEDER; COST [CM1005]; DFG [NA-686/5]; Portuguese FCT [SFRH/BD/81628/2011, PEst-OE/EQB/LA0023/2013]

The fluorescent azoalkane, 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), forms inclusion complexes with p-sulfonatocalix[4]arene (CX4). The binding constants are on the order of 103 M–1 in water. The addition of CX4 to DBO solutions results in an efficient fluorescence quenching (up to 90 %). This supramolecular system can be used as a truly water-soluble sensor system to signal the binding of organic ammonium ions over a large pH range. Addition of choline and carnitine derivatives and tetraalkylammonium ions results in regeneration of this fluorescence, from which the binding constants (KC = 103–105 M–1) are calculated by means of a competitive complexation model. Electrostatic effects are observed, namely, a more-than-one order of magnitude weaker binding of the carnitines in neutral solution.

[reaction: see text] The complexation of p-sulfonatocalix[4]arene (CX4) with 2,3-diazabicyclo[2.2.1]hept-2-ene (1), 2,3-diazabicyclo[2.2.2]oct-2-ene (2), 2,3-diazabicyclo[3.2.2]non-2-ene (3), 1-methyl-4-isopropyl-2,3-diazabicyclo[2.2.2]oct-2-ene (4), and 1-phenyl-2,3-diazabicyclo[2.2.2]oct-2-ene (5) was studied in D2O at pD 7.4 by 1H NMR spectroscopy. The formation of deep inclusion complexes was indicated by large upfield 1H NMR shifts of the guest protons (up to 2.6 ppm), which were also used to assign, in combination with 2D ROESY spectra, a preferential inclusion of the isopropyl group of 4 and a dominant inclusion of the azo bicyclic residue for 5. The bicyclic azoalkanes 1-3 showed exceptionally high binding constants on the order of 1000 M(-1), 1-2 orders of magnitude larger than for previously investigated noncharged organic guest molecules. The strong binding was attributed to the spherical shape complementarity between the guest and the conical cavity offered by CX4. Interestingly, although the derivatives 4 and 5 are more hydrophobic, they showed a 2-3 times weaker binding, which was again attributed to the deviation from spherical shape in these bridgehead-substituted derivatives. The preferential inclusion of the hydrophilic but spherical bicyclic residue of 5 rather than the hydrophobic aromatic phenyl group provides a unique observation in aqueous host-guest chemistry and corroborates the pronounced spherical shape affinity of CX4.