Cucurbituril-Based Supramolecular Assemblies: Prospective on Drug Delivery, Sensing, Separation, and Catalytic Applications.

Precise control over the stimuli-responsive noncovalent interactions operative in a complex molecular system has emerged as a convenient way to realize applications in the detection and sensing of trace analytes, metal ion separation, uptake-release, in situ nanoparticle synthesis, and catalytic activity. This feature article focuses on the attributes and advantages of noncovalent host-guest interactions involving cucurbituril homologues (CBs) with a wide range of organic and inorganic guests, starting from organic dyes to drugs, proteins, surfactants, metal ions, and polyoxometalates. The unique structural features of CBs provide interaction sites for cations at the portals, polyanions at the periphery, and hydrophobic groups in its cavity. The facile complexation and consequent compositional and geometrical arrangements of guests such as naphthalenediimides, coumarins, porphyrins, and triphenylpyrylium ions with the host CBs led to remarkable changes in many molecular properties, especially aggregation, the proton binding and release affinity, and novel emissive dimers, and each of such spectroscopic signatures have been appropriately channeled to drug delivery and activation to improve the antibacterial efficacy and shelf life of drugs by increasing their photostability. Several technological advantages have also been extracted from the interaction of CBs with inorganic guests as well. The interaction of CB7 with the heptamolybdate anion resulted in the precipitation of a hybrid complex material which enabled a convenient separation methodology for the use of clinically pure radioactive 99mTc in diagnostic applications. Certain cucurbituril-based hybrid materials have been developed for enhanced SO2 adsorption at low pressures, high-efficiency hydrogen production, and reversible catalytic systems. Thus, this feature article provides a glimpse of the vast potential of cucurbituril homologues with organic and inorganic guests and calls for a dedicated effort to explore supramolecular strategies for better sensors, therapeutics, smart drug delivery modules, and facile devices.

Redox-mediated Negative Differential Resistance (NDR) Behavior in Perylenediimide Derivative: A Supramolecular Approach.

Deaggregated perylenediimide (PDI) derivatives exhibit exceptionally high quantum yields, photostability and appropriate molecular features for organic electronics. This work demonstrates a metal-dye-metal framework with a large and stable negative differential resistance (NDR) at ambient conditions, built using a supramolecular strategy. The deaggregation achieved through the encapsulation of the bay-substituted phenyl groups of aggregated (l/d)-Phe-PDI dyes by the ß-CD macrocyclic host is validated through detailed spectroscopic and imaging techniques. The host-guest interaction resulted in a dramatic enhancement in the emission yield from 0.28 to 0.90. In the thin film deposits, the ß-CD/(l/d)-Phe-PDI complex displayed well-connected sheet-like morphology, whereas the uncomplexed (l/d)-Phe-PDI dye remained as scattered lumps. The large and reversible I-V characteristics displaying strong NDR behavior is attributed to the oxidation/reduction processes involving the rigid π-rich PDI core and is stable at least for about six months at ambient conditions, a promising system for organic electronics applications.

Supramolecular Assembly Induced Emission of Thiazole Orange with Sulfobutylether ß-cyclodextrin: A Stimuli-Responsive Fluorescence Sensor for Tyramine.

Modulation and control of stimuli responsive features of molecular assemblies in organized assemblies/cavitand macrocycles have received immense attention in many areas. In this study, we have established the formation of a discrete molecular assembly of thiazole orange (TO) dyes at the portals of the sulfobutylether ß-cyclodextrin (SBE7 ßCD) macrocycle leading to the evolution of a strong and distinct emission band from aggregated TO. The supramolecular assembly promoted portal aggregation of TO in its 1 : 4 (SBE7 ßCD : TO) composition, characterized by absorption, fluorescence, circular dichroism, ITC and 1 H NMR measurements, was probed to be selectively responsive to tyramine among other biogenic amines/neurotransmitters. For the first time, the different extent of emission quenching of SBE7 ßCD : TO assembly in the presence of biogenic amines/neurotransmitters is translated to achieve a selective on-off fluorescence sensor for the detection of tyramine against other neurotransmitters with a limit-of-detection (LOD) as low as ∼575 nM (79 ppb). The emission features of the assembly with changes in temperature is found to be highly reproducible even after several temperature cycles and is promising to design an optical supramolecular thermometer in the ambient temperature range.

Ultra-Bright Rhodamines with Sulfobutylether-ß-Cyclodextrin: A Viable Supramolecular Dye Laser in Aqueous Medium.

Although aqueous dye lasers are much sought after, they have been of no practical use, as laser dyes show a strong tendency for aggregation in water, thus diminishing their optical output. Contributing towards this shortcoming, we studied the noncovalent interactions of two prominent laser dyes, namely, rhodamine 6G and rhodamine B, with a water soluble macrocyclic host, sulfobutylether-ß-cyclodextrin (SBE7 ßCD). Spectral changes in the absorption and fluorescence behavior of dyes in presence of the SBE7 ßCD host indicated adequate complex formation between dye and host (K∼104  M-1 ). A combination of various photophysical parameters evaluated from measurements such as Job plot, changes in the fluorescence lifetime/anisotropy values, and favorable thermodynamic parameters from isothermal titration calorimetric measurements adjudicated a 1 : 1 stoichiometric complex formation between dye and SBE7 ßCD host. Consequently, SBE7 ßCD prevents dye aggregation/adsorption and present rhodamine dyes in their monomeric forms with enhanced fluorescence yield and brightness. These vital parameters were utilized to optimize and demonstrate cost-effective supramolecular broad-band and narrow-band aqueous dye laser systems with improved lasing efficiencies (∼25 % higher for the SBE7 ßCD : RhB system and ∼10 % higher for SBE7 ßCD : Rh6G system), better beam profile, and enhanced durability compared to the respective dyes in optically matched ethanol solutions.

Molecular-recognition-assisted pKa shifts and metal-ion-induced fluorescence regeneration in p-sulfonatocalix[6]arene-encapsulated acridine.

The host-guest interactions of cationic (AcH(+) ) and neutral (Ac) forms of the dye acridine with the macrocyclic host p-sulfonatocalix[6]arene (SCX6) were investigated by using ground-state absorption, steady-state and time-resolved fluorescence, and NMR measurements. The cationic form undergoes significant complexation with SCX6 (Keq =2.5×10(4) M(-1) ), causing a sharp decrease in the fluorescence intensity and severe quenching in the excited-state lifetime of the dye. The strong binding of the AcH(+) form of the dye with SCX6 is attributed to ion-ion interactions involving the sulfonato groups (SO3 (-) ) of SCX6 and the positively charged AcH(+) at pH of approximately 4.3. Whereas, the neutral Ac form of the dye undergoes weak complexation with SCX6 (Keq =0.9×10(3) M(-1) ) and the binding constant is lowered by one order of magnitude compared with that of the SCX6-AcH(+) system. The strong affinity of SCX6 to the protonated form leads to a large upward pKa shift (≈2 units) in the dye. In contrast, strong emission quenching upon SCX6 interaction and the regeneration of fluorescence intensity of the dye in the presence of Gd(3+) through competitive binding have also been demonstrated.

Stimuli-responsive supramolecular micellar assemblies of cetylpyridinium chloride with cucurbit[5/7]urils.

This article demonstrates, for the first time, construction of novel cucurbituril (CB)-adorned supramolecular micellar assemblies of a cationic surfactant, cetylpyridinium chloride (CPC), through noncovalent host-guest interactions. The distinct cation receptor features and cavity dimensions of the CB5 and CB7 homologues assert that the macrocyclic hosts remain complexed with the CPC monomers and take part in the micelle formation, a unique observation in contrast to that of the classical host, ß-cyclodextrin. The cooperative contributions of the CB macrocycles in the micelle formation have been documented by the photochemical, surface tension, conductivity, DOSY NMR, and SANS measurements. The contrasting downward and upward shifts in the cmc of the CPC surfactant, respectively, with CB5 and CB7 hosts provide a unique opportunity for the controlled tuning of the micellization region for CPC from 0.57 to 1.6 mM, by using a combination of the macrocyclic hosts. The article also establishes the reversible response of these soft supramolecular micellar structures to thermal-stimuli, which projects their utility for on-demand smart drug-delivery vehicles.

Enhanced Antibacterial Activity of Levofloxacin with Cucurbit[7]uril-Functionalized Gold Nanoparticles.

Bacterial infection is one of the major concerns of the growing society, and over the years, different permutations and combinations of various drugs and adjuvants have been attempted, which led to considerable improvements in the efficacy of the antibacterial drugs. In this regard, macrocyclic receptors such as cyclodextrin, cucurbiturils, calixarene, etc., have played a major role by modulating the drug properties that supplement the antibacterial efficacy. In this study, we have developed cucurbit[7]uril (CB7)-functionalized Au nanoparticles (CB7AuNPs) to modulate the activity of an antibiotic, levofloxacin (LOFL). From the spectroscopic and thermodynamic changes in the LOFL, it has been established that two of the prototropic forms, LOFLH and LOFLH2+, form strong 1:1 host/guest complexes with CB7/CB7AuNP. Both these interactions led to significant upward shifts in the pKa values as well as photostability of LOFL, thereby enhancing the availability of the active form for the antibacterial activity, at the physiological pH. Further, the LOFL uptake has also been established on CB7AuNP, which retained the CB7-LOFL activity at very low concentration of the CB7 host, functionalized on AuNP. Detailed antibacterial studies of LOFL, both as complexed with CB7 and CB7AuNP, were carried out using four food-borne pathogens (Escherichia coli, S. Typhimurium, Bacillus cereus, and Staphylococcus aureus), which revealed a creditable enhancement in the antibacterial property, irrespective of the bacterium strain. These results are quite promising at this stage for the development of drugs customized for multidrug-resistant bacteria.

p-Sulfonatocalix[6]arene-Functionalized Gold Nanoparticles: Applications in Drug Delivery and Bioimaging.

Surface-functionalized noble metal nanoparticles with macrocyclic hosts have attracted enormous research interest owing to their applications in drug delivery, catalysis, bioimaging, etc. Stable p-sulfonatocalix[6]arene-functionalized gold nanoparticles (SCx6AuNPs) of the sizes ∼7.5 nm have been synthesized and characterized by using UV-vis absorption, transmission electron microscopy, and surface-enhanced Raman spectroscopy measurements. The efficient uptake and stimuli-responsive release of doxorubicin (Dox), an anticancer drug, by the SCx6AuNPs have been established for targeted drug delivery application. The decreased cytotoxicity of Dox loaded on SCx6AuNPs, especially toward normal cell lines, and its multistimuli responsive release validated in both cancerous (A549) and normal (W126) cell lines find promising for selectively targeted drug delivery applications toward cancer cells. At the cellular level, this study also establishes the efficient uptake of the SCx6AuNP nanoconjugates, and its validation has been done by bioimaging measurement by using thioflavin T (ThT) dye loaded on to SCx6AuNPs instead of Dox as the fluorescent tracking probe. The bright fluorescence microscopic image of ThT-SCx6AuNP-stained cancerous cell lines corroborates the uptake of SCx6AuNPs by the cell lines and its projected utility for drug delivery and bioimaging applications.

Dual catalytic activity of a cucurbit[7]uril-functionalized metal alloy nanocomposite for sustained hydrogen generation: hydrolysis of ammonia borane and electrocatalysts for the hydrogen evolution reaction.

H2 is one of the most attractive fuel alternatives to the existing fossil fuels that cause detrimental environmental issues. Thus, there has been an upsurge in the research on the production of green hydrogen. In this view, cucurbit[7]uril (CB7)-functionalized Co:Ni alloy nanocomposites with different compositions, reported here for the first time, were synthesized to synergise the catalytic activities of a nanoalloy and CB7 and screened for hydrogen generation via hydrolysis of ammonia borane (AB). The (Co85:Ni15)50:(CB7)50 nanocomposite exhibited enhanced catalytic performance for AB hydrolysis even at room temperature as compared to the nanoalloy without CB7. Efficient release of ammonia-free green H2 is ensured by the retention of NH3 by the surface functionalized CB7 macrocycles. For sustained release, a novel and cost-effective procedure was used to regenerate AB from the by-product, and the H2 release activity was verified to be on par with commercial AB. The used nanocomposite magnetically separated from the by-product solution was shown to be an efficient electrochemical catalyst for the hydrogen evolution reaction (HER). The cucurbit[7]uril-functionalized Co:Ni nanocomposite demonstrates remarkable dual catalytic performance to generate clean hydrogen from both the hydrolysis of AB at room temperature and the electrochemical HER, thus opening new avenues in supramolecular chemistry for developing noble metal-free catalysts with high activity and long-term stability.

Thioflavin T as an efficient inducer and selective fluorescent sensor for the human telomeric G-quadruplex DNA.

The quest for a G-quadruplex specific fluorescent sensor among other DNA forms under physiological salt conditions has been addressed in this article. We demonstrate for the first time the application of a water-soluble fluorogenic dye, Thioflavin T (ThT), in a dual role of exclusively inducing quadruplex folding in the 22AG human telomeric DNA, both in the presence and absence of Tris buffer/salt, and sensing the same through its fluorescence light-up having emission enhancement of the order of 2100-fold in the visible region. Appropriate conditions allow an apparent switch over of the parallel quadruplex structure in 22AG-ThT (50 mM Tris, pH 7.2) solution to the antiparallel form just by the addition of K(+) ions in the range 10-50 mM. Moreover, addition of ThT cooperatively stabilizes the K(+) induced antiparallel quadruplexes by a ΔT(m) ∼11 °C. The distinction of ThT as a quadruplex inducer has been contrasted with the erstwhile used structurally related dye, Thiazole Orange (TO), which did not induce any quadruplex folding in the 22AG strand in the absence of salt. The striking fluorescence light-up in ThT on binding to the human telomeric G-quadruplex is shown to be highly specific compared to the less than 250-fold enhancement observed with other single/double strand DNA forms. This work has implication in designing new generation dyes based on the ThT scaffold, which are highly selective for telomeric DNA, for potential diagnostic, therapeutic, and ion-sensing applications.

Spectroscopic features of a perylenediimide probe for sensing amyloid fibrils: in vivo imaging of Aß-aggregates in a Drosophila model organism.

Customised perylenediimide (PDI) chromophores find diverse applications not only as chemosensors, inorganic-organic semiconductors, photovoltaics, photocatalysts, etc., but also in protein surface engineering, bio-sensors and drug delivery systems. This study focuses on the interaction of a custom synthesized phenylalanine derivatized perylenediimide (L-Phe-PDI) dye with a model protein, insulin, and its structurally distinct fibrils to develop fluorescence sensors for fibrillar aggregates and in vivo imaging applications. Detailed photophysical studies revealed that L-Phe-PDI gets aggregated in the presence of insulin and causes emission quenching at pH 7.4, which in the absence of insulin occurs only at pH ∼2. During in vitro incubation of insulin to its fibrils, the fluorescence intensity of the L-Phe-PDI probe is enhanced to ∼150 fold in a two-stage manner, manifesting the pathways of structural transformation to ß-sheet rich mature fibrils. The in vivo sensing has further been validated in living models of the Aß-mutant Drosophila fly, which is known to develop progressive neurodegeneration comparable to that of human brains with Alzheimer's disease (AD). Bioimaging of the L-Phe-PDI treated Aß-mutant Drosophila documented the blood-brain/blood-retina-barrier cross-over ability of L-Phe-PDI with no toxic effects. Comparison of the fibrillar images from the brain and eye region with the reference thioflavin T (ThT) probe established the uptake of L-Phe-PDI by the aggregate/fibrillar moieties. The samples from L-Phe-PDI-treated flies apparently displayed reduced fibrillar spots, a possible case of L-Phe-PDI-induced disintegration of fibrillar aggregates at large, an observation substantiated by the improved phenotype activities as compared to the untreated flies. The findings reported both in vitro and in vivo with the L-Phe-PDI material for the first time open up avenues to explore the therapeutic potential of custom-designed PDI derivatives for amyloid fibril sensors and bioimaging.

Supramolecular assembly of coumarin 7 with sulfobutylether-ß-cyclodextrin for biomolecular applications.

Coumarins, in general, exhibit a wide range of photophysical characteristics and are highly sensitive to their microenvironment, and, therefore, their fluorescence characteristics have attracted immense attention as sensors in chemical and biological systems. In the present study, the supramolecular interaction of a bichromophoric coumarin dye, namely, Coumarin 7 (C7) with sulfobutylether-ß-cyclodextrin (SBE7ßCD) macrocyclic host at different pH conditions has been investigated by using optical spectroscopic techniques such as absorption, steady-state and time-resolved emissions, and circular dichroism measurements and compared with that of ßCD. Considerable enhancement in the fluorescence intensity and lifetime of C7 on complexation with SBE7ßCD proposes that non-radiative processes like TICT behavior are strictly hindered due to the confinement in the host cavity experienced by the C7 dye. The increase in the rotational correlation time evaluated from the fluorescence anisotropy decay kinetics further confirms the formation of tightly bound inclusion complexes. The binding constant values reveal that the monocationic form of dye at pH 3 shows ∼3 times stronger interaction with SBE7ßCD than the neutral form of dye at pH 7 due to strong electrostatic cation-anion interaction. SBE7ßCD:C7 exhibits an improved photostability and an upward pK a shift of 0.4 unit compared to the contrasting downward pK a shift of 0.5 with the ßCD. The enhanced fluorescence yield and increased photostability have been exploited for bioimaging applications, and better images were captured by staining the Drosophila fly gut with the SBE7ßCD:C7 complex. The enhancement in the binding interaction and the emission intensity were found to be responsive to external stimuli such as small competitive binders or metal ions and nearly quantitative dissociation of the complex was demonstrated to release the dye and would find stimuli-responsive applications.

Non-covalent interactions of coumarin dyes with cucurbit[7]uril macrocycle: modulation of ICT to TICT state conversion.

Non-covalent interaction of coumarin laser dyes, namely coumarin-1 (C1), coumarin-481 (C481) and coumarin-6H (C6H), with a versatile macrocyclic host molecule cucurbit[7]uril (CB7), has been investigated in aqueous solution using photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the fluorescence yields, lifetimes and spectral features of C1, C481 and C6H on interaction with CB7, and are assigned to 1 : 1 complex formation between the dyes and the CB7 host. The complex formation is mainly driven by charge-dipole interaction, as evident from the binding constant values (K ~ 10(4)-10(5) M(-1)). The large changes in the excited state behaviour of C1 and C481 as compared to C6H in the presence of CB7 indicate that CB7 binds C1 and C481 through the encapsulation of the 7-N,N'-diethylamino group of the dyes and the structural rigidity imposed by this interaction dramatically alters the excited state properties of the dyes by preventing the conversion of their emissive intramolecular charge transfer (ICT) state to the non-radiative twisted intramolecular charge transfer (TICT) state. The present results direct towards the probable supramolecular approach using water soluble macrocyclic CB7, in the development of aqueous dye laser systems in the blue-green region.

Interaction of esculetin with aluminium ion by spectroscopic studies and isothermal titration calorimetry: a probable molecule for chelation therapy.

The use of aluminium has made significant impact in our life by virtue of its attractive properties. The lack of essentiality of aluminium in biosphere indicated that its accumulation above certain level is undesirous. Esculetin (6,7-dihydroxy coumarin) is an excellent aluminium ion chelator and the chelation interaction was studied by exploiting the absorption and fluorescence behavior of esculetin. In presence of aluminium ion, the absorption band of esculetin was shifted from 350 to 380 nm suggesting the possibility of complex formation. The fluorescence intensity of esculetin at 466 nm was significantly quenched in presence of aluminium ion. The fluorescence quenching was interpreted in terms of chelation-quenched fluorescence (CHQF) mechanism where the strong Lewis acid character of aluminium ion accepts electrons from the chelating catechol moiety of the excited esculetin. From the absorption and fluorescence changes the association constant was estimated in the order of 105 M-1. The association constant was further evaluated by isothermal titration calorimetry (ITC) and there was close agreement to that of obtained from spectroscopic studies. Form ITC studies, the binding enthalpy and binding entropy were estimated as -20.6 kcal/mol and -46.7 cal/mol/K respectively. The complex was less toxic compared to the individual complexing agents when studied in Chinese hamster ovary cells. Considering the present investigation, esculetin can be a probable molecule for chelation therapy where rapid complex formation ability of esculetin will help to reduce the aluminium accumulation through chelation and water soluble nature of the complex will help for faster elimination from the system.Communicated by Ramaswamy H. Sarma.

A combination of a graphene quantum dots-cationic red dye donor-acceptor pair and cucurbit[7]uril as a supramolecular sensor for ultrasensitive detection of cancer biomarkers spermine and spermidine.

In a unique approach, the combination of a donor-acceptor pair of hydroxy graphene quantum dots (GQDs-OH) and a red-emissive donor-two-acceptor (D-2-A) type dye with pyridinium units (BPBP) and the well-known host cucurbit[7]uril (CB[7]) has been exploited as a supramolecular sensing assembly for the detection of cancer biomarkers spermine and spermidine in aqueous media at the sub-ppb level based on the affinity-driven exchange of guests from the CB[7] portal. In the binary conjugate, green fluorescent GQDs-OH transfers energy to trigger the emission of the dye BPBP and itself remains in the turn-off state. CB[7] withdraws the dye from the surface of GQDs-OH by strong host-guest interactions with its portal, making GQDs-OH fluoresce again to produce a ratiometric response. In the presence of spermine (SP) or spermidine (SPD), their strong affinity with CB[7] forces the ejection of the fluorophore to settle on the GQDs-OH surface, and the strong green emission of GQDs-OH turns off to device a supramolecular sensor for the detection of SP/SPD. The DFT studies revealed interesting excited-state charge-transfer conjugate formation between BPBP and GQDs leading to turn-on emission of the dye, and further supported the stronger binding modes of BPBP-CB[7], indicating the retrieval of the emission of GQDs. The assembly-disassembly based sensing mechanism was also established by Job's plot analysis, particle size analysis, zeta potential, time-resolved spectroscopy, ITC studies, microscopic studies, etc. The supramolecular sensing assembly is highly selective to SP and SPD, and showed nominal interference from other biogenic amines, amino acids, various metal ions, and anions. The limits of detection (LODs) were 0.1 ppb and 0.9 ppb for spermine and spermidine, respectively. The potential for the real-world application of this sensing assembly was demonstrated by spiking SP and SPD in human urine and blood serum with a high %recovery.

pH and temperature dependent relaxation dynamics of Hoechst-33258: a time resolved fluorescence study.

The photophysical behavior of Hoechst 33258 (H33258) in aqueous solution has been studied by steady-state and time-resolved fluorescence measurements. The intriguing intramolecular geometrical orientations of the dye bring out major modulation on its photophysical behavior, especially in the fluorescence emission characteristics with pH. It has been seen that a change in the solution pH from 7 to 4.5 enhances the emission yield by ~20 fold and this change is ~80-fold on changing the pH from 1.5 to 4.5. While a fast flipping motion among the two benzimidazole rings is considered to be one of the most probable mechanisms for the fast fluorescence decay, a more planar structure of the dicationic form at pH 4.5 having a double bond character between the two benzimidazolium groups is suggested to be the most likely fluorescent species. A similar planar structure is in fact considered to be the fluorescent emitting species of H33258 on minor groove binding to DNA. On the basis of temperature dependent fluorescence decay dynamics explored for the dye in solutions at pH 7 and 4.5, it is understood that a nearly isoenergetic double-well excited state potential is possibly involved in the excited state relaxation dynamics of the dye at pH 7. On increasing the temperature, the conversion to the planar structure is facilitated from the non-planar LE state, enhancing the emission probability of the dye.

Supramolecular assembly of Hoechst-33258 with cucurbit[7]uril macrocycle.

Molecular assemblies of potential guest molecules through non-covalent host-guest interactions have found immense use in many applied areas. In this study supramolecular interaction of a biologically important dye Hoechst-33258 (H33258) has been investigated in aqueous solutions at different pHs, in the presence of a macrocyclic host, namely, cucurbit[7]uril (CB7). The pH dependent emission behaviour of H33258 is inherently connected with its protolytic equilibria which allow the dye in different geometrical conformations. This pH dependent structural orientation is greatly affected by the complexation with CB7. The significant structural changes in the monocationic H33258 brought out by CB7 at pH 7 have been documented in the fluorescence emission and lifetime data, which are comparatively less affected in case of the dicationic form, which is prominent in dye solutions at pH 4.5. The strong ion-dipole interactions provided by the carbonyl portals of the CB7 host adequately stabilize the CB7-H33258 complex, both in 1:1 and 2:1 stoichiometries at both the pH conditions. The Job's plot method, fluorescence anisotropy, NMR measurements and geometry optimization calculations confirm the stoichiometric arrangement and are found to be tunable with the addition of metal ions. The non-covalently stabilized assembly brings out large enhancement in the fluorescence emission due to the unique structural orientation attained by H33258, which reduces the non-radiative relaxation pathways. Comparison of the spectral data of the dye at different pH conditions in the absence and presence of CB7 proposes a large upward pK(a) shift due to CB7 encapsulation, thus providing a handy tool to modulate the photophysical characteristics of the guest molecules.

Metal-Free Supramolecular Catalytic Hydrolysis of Ammonia Borane through Cucurbituril Nanocavitands.

Ammonia borane (AB) is considered a potential "on-board" hydrogen storage material. However, its implementation as a hydrogen reservoir in fuel cells is lacking due to the extremely slow release of hydrogen at room-temperature hydrolysis. In this study, a metal-free supramolecular strategy is demonstrated at room temperature to increase the hydrolysis rate and yield of hydrogen along with significant reduction in ammonia release by using cucurbit[5/8]uril (CB5/CB8) nanocavitands as catalysts. The complex of AB with CB stabilizes the ammonium ion at the host portals, which reduces ammonia release and enhances hydrogen yield. The complexation brings down the activation energy of hydrolysis from 103.8 to ∼27.5 kJ mol-1 (for CB5), a value close to the Pt/Pd nanoparticle-based catalysts reported so far. The high catalytic performance and reusability of CB catalysts at very low concentration make AB a promising supramolecular alternative for a sustainable "on-board" energy source.

Fibril-induced neurodegenerative disorders in an Aß-mutant Drosophila model: therapeutic targeting using ammonium molybdate.

The ability of polyanionic molybdate to inhibit and degrade protein fibrils both in vitro (insulin protein) and in vivo (Drosophila fly model) has been demonstrated. We establish the disappearance of fibrillar structures and recovery from neurodegenerative disorders in molybdate-treated Aß42-mutant Drosophila flies as compared to the untreated ones, corroborating the therapeutic ability of ammonium molybdate towards the treatment of Alzheimer's disease.

Supramolecular interaction of sanguinarine dye with sulfobutylether-ß-cyclodextrin: modulation of the photophysical properties and antibacterial activity.

The noncovalent host-guest interaction of sanguinarine (SGR), a benzophenanthridine alkaloid, with a nontoxic, water soluble sulfobutylether-ß-cyclodextrin (SBE7ßCD, commercially available as Captisol) macrocyclic host has been investigated using ground-state optical absorption, and steady-state and time-resolved fluorescence measurements. The pH-dependent changes in the absorbance of the dye at 327 nm showed a pK a value of 7.5, which has been shifted to 8.1 in the presence of SBE7ßCD. The changes in the pK a values, absorption and fluorescence spectra, and fluorescence lifetime values of these two forms of SG with SBE7ßCD indicate complex formation between them. The cationic form shows 3 times higher interaction towards SEB7ßCD (K = 1.2 × 104 M-1) as compared to the neutral form (K = 3.9 × 103 M-1) which leads to a moderate upward pK a shift (pK a values of SGR shifted by more than 0.6 units). The subsequent fluorescence "turn off" was demonstrated to be responsive to chemical stimuli, such as metal ions (Ca2+ ions). Upon addition of Ca2+ ions, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence "turn on". Apart from improving the stability under ambient light conditions, the upward pK a shift of SGR in the presence of SBE7ßCD results in increasing the antibacterial activity of the SBE7ßCD:SGR complex compared to that of the free dye towards four pathogenic micro-organisms at the physiological pH range. This work further compares SGR interaction with parent ß-cyclodextrin.