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.

Influence of halogen-halogen interactions in the self-assembly of pillar[5]arene-based supramolecular polymers

Halogen-halogen interactions play a pivotal role in the formation and stability of supramolecular assemblies. Herein, we investigate the assembly dynamics and dissociation pathways of linear supramolecular polymers based on pillar[5]arene-mediated by guest halogen-halogen interactions (C-X × X-C) in both the solution and solid states. The structure of the solid-state supramolecular assembly was determined by single-crystal X-ray diffraction analysis. The binding affinities of four different 1,4-dihalobutane guests with pillar[5]arene were investigated by 1H NMR spectroscopic titration and isothermal titration calorimetry (ITC). The formation of the halogen-bonded linear supramolecular polymer in solution was demonstrated using diffusion-ordered spectroscopy (DOSY) and ITC. Our findings highlight the dependence of the dissociation process on halogen nature within the encapsulated guest, revealing that the process is entropically driven (TΔS = 27.12 kJ mol-1) and enthalpically disfavored (ΔH = 9.99 kJ mol-1). Moreover, the disassembly of supramolecular polymers promoted by N-containing compounds was investigated using 1H NMR spectroscopy and ITC, revealing that the process is driven both enthalpically (ΔH = -2.64 kJ mol-1) and entropically (TΔS = 15.70 kJ mol-1). Notably, the data suggest the formation of N⋯I bonding interactions at both ends of the inclusion guest, elucidating the intricate interplay of halogen interactions and host-guest chemistry in supramolecular polymer systems.

Supramolecular assembly of coumarin 7 with sulfobutylether-β-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-β-cyclodextrin (SBE7βCD) 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 βCD. Considerable enhancement in the fluorescence intensity and lifetime of C7 on complexation with SBE7βCD 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 SBE7βCD than the neutral form of dye at pH 7 due to strong electrostatic cation-anion interaction. SBE7βCD: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 βCD. 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 SBE7βCD: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.

Host Dynamics under General-Purpose Force Fields

Macrocyclic hosts as prototypical receptors to gaseous and drug-like guests are crucial components in pharmaceutical research. The external guests are often coordinated at the center of these macromolecular containers. The formation of host-guest coordination is accompanied by the broken of host-water and host-ion interactions and sometimes also involves some conformational rearrangements of the host. A balanced description of various components of interacting terms is indispensable. However, up to now, the modeling community still lacks a general yet detailed understanding of commonly employed general-purpose force fields and the host dynamics produced by these popular selections. To fill this critical gap, in this paper, we profile the energetics and dynamics of four types of popular macrocycles, including cucurbiturils, pillararenes, cyclodextrins, and octa acids. The presented investigations of force field definitions, refitting, and evaluations are unprecedently detailed. Based on the valuable observations and insightful explanations, we finally summarize some general guidelines on force field parametrization and selection in host-guest modeling.

Supramolecular Complexation of Azobenzene Dyes by Cucurbit[7]uril

This report describes cucurbit[7]uril (CB7) complexation of azobenzene dyes that have a 4-(N,N'-dimethylamino) or 4-amino substituent. Absorption and NMR data show that CB7 encapsulates the protonated form of the azobenzene and that the complexed dye exists as its azonium tautomer with a trans azo conformation and substantial quinoid resonance character. Because CB7 complexation stabilizes the dye conjugate acid, there is an upward shift in its pKa, and in one specific case, the pKa of the protonated azobenzene is increased from 3.09 to 4.47. Molecular modeling indicates that the CB7/azobenzene complex is stabilized by three major noncovalent factors: (i) ion-dipole interactions between the partially cationic 4-(N,N'-dimethylamino) or 4-amino group on the encapsulated protonated azobenzene and the electronegative carbonyl oxygens on CB7, (ii) inclusion of the upper aryl ring of the azobenzene within the hydrophobic CB7 cavity, and (iii) a hydrogen bond between the proton on the azo nitrogen and CB7 carbonyls. CB7 complexation enhances azobenzene stability and increases azobenzene hydrophilicity; thus, it is a promising way to improve azobenzene performance as a pigment or prodrug. In addition, the striking yellow/pink color change that accompanies CB7 complexation can be exploited to create azobenzene dye displacement assays with naked eye detection.

Supramolecular Room-Temperature Phosphorescent Hydrogel Based on Hexamethyl Cucurbit[5]uril for Cell Imaging

The host-guest interaction between hexamethyl cucurbit[5]uril (HmeQ[5]) and 1,4-diaminobenzene (DB) was investigated, and a new low-molecular-weight supramolecular gel was prepared by a simple heating/mixing cooling method. The structure and properties of the supramolecular gel were characterized. Results revealed that DB molecules did not enter the cavity of HmeQ[5] and that hydrogen bonding between the carbonyl group at the HmeQ[5] port and the DB amino groups, together with dipole-dipole interactions and outer wall interactions, were the main driving forces for the formation of the supramolecular gel. The HmeQ[5]/DB gel system exhibits temperature sensitivity. The phosphor 6-bromo-2-naphthol (BrNp) was embedded in the gel to give the gel fluorescent phosphorescence double emission. The double emission ability at room temperature can be attributed to the ordered microstructure of the supramolecular gel, which effectively avoids the nonradiative transition of BrNp. Meanwhile, HmeQ[5]/DB-BrNp has good biocompatibility and low biotoxicity, which is compatible with HeLa cells to achieve cytoplasmic staining of HeLa in the red channel. The supramolecular gels constructed by this supramolecular assembly strategy not only have good temperature sensitivity but also extend the application of Q[n]s in biomedical fields.

Synthetic star shaped tetra-tailed biocompatible supramolecular amphiphile as an efficient nanocarrier for Amphotericin B

Macrocycle-based amphiphiles are capable of self-assembling into multidimensional nano-architecture with defined dimensions for various applications. Herein we report the synthesis, physio-chemical characterizations and oral drug delivery profiling of resorcinarene-based amphiphilic supramolecular macrocycle. The macrocycle was synthesized in two-step reaction and characterized using ¹H NMR, Mass spectrometry and IR spectroscopic techniques. The synthesized macrocycle was assessed for vesicles formation, checked for biocompatibility and then Amphotericin B (Amp-B) was entrapped in macrocycle-based vesicles. The drug loaded vesicles were characterized for shape, size, homogeneity, drug entrapment, surface charge, in-vitro release profile and stability. Amp-B loaded macrocycle based vesicles were examined in rabbits for in-vivo bioavailability and compared with plan drug suspension. The synthesized macrocycle was non-toxic in normal mouse fibroblast cells, compatible with blood and safe in mice. The drug loaded macrocycle based vesicles appeared spherical with 279.4 nm size and - 12.2 mV zeta potential loading 85.45 % drug. The drug loaded vesicles storage stability for 30 days and gastric fluid stability for 1 h were it retained nearly 90 % drug at 30th day and 83.79 % drug at 1 h in gastric fluid. Oral bioavailability of Amp-B in rabbits was markedly enhanced when delivered in synthesized macrocycle based vesicles in comparison with plan drug suspension. Results of this study indicate that the synthesized star shaped tetra-tailed supramolecular amphiphile could be used as an efficient nanocarrier for enhancing oral bioavailability of drugs with solubility and bioavailability issues like Amp-B.

Chemiluminescent Cucurbit[n]uril-Based Chemosensor for the Detection of Drugs in Biofluids

Chemiluminescence-based detection methods offer a superior signal-to-noise ratio and are commonly adopted for biosensors. This work presents the design and implementation of a supramolecular assay based on a chemiluminescent chemosensor. Specifically, an indicator displacement assay (IDA) with the supramolecular host-guest complex of chemiluminescent phenoxy 1,2-dioxetane and cucurbit[8]uril enables the low-micromolar detection of drugs in human urine and human serum samples. Cucurbit[8]uril thereby acts as a non-surfactant chemiluminescence enhancer and a synthetic receptor. Additionally, we show that adding an equimolar amount of cucurbit[8]uril to a commercially available dioxetane used in standard enzymatic chemiluminescence immunoassays enhances the chemiluminescence by more than 15 times. Finally, we demonstrate that a chemiluminescence resonance energy transfer between a unimolecular macrocyclic cucurbit[7]uril-dye conjugate and a phenoxy 1,2-dioxetane can be utilized to detect the herbicide paraquat at a micromolar concentration in aqueous media.

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.

Assembly and Applications of Macrocyclic-Confinement-Derived Supramolecular Organic Luminescent Emissions from Cucurbiturils

Cucurbit[n]urils (Q[n]s or CB[n]s), as a classical of artificial organic macrocyclic hosts, were found to have excellent advantages in the fabricating of tunable and smart organic luminescent materials in aqueous media and the solid state with high emitting efficiency under the rigid pumpkin-shaped structure-derived macrocyclic-confinement effect in recent years. This review aims to give a systematically up-to-date overview of the Q[n]-based supramolecular organic luminescent emissions from the confined spaces triggered host-guest complexes, including the assembly fashions and the mechanisms of the macrocycle-based luminescent complexes, as well as their applications. Finally, challenges and outlook are provided. Since this class of Q[n]-based supramolecular organic luminescent emissions, which have essentially derived from the cavity-dependent confinement effect and the resulting assembly fashions, emerged only a few years ago, we hope this review will provide valuable information for the further development of macrocycle-based light-emitting materials and other related research fields.

Supramolecular Regulation of Catalytic Activity for an Amphiphilic Pyrene-Ruthenium Complex in Water

A switchable catalytic system has been designed and constructed with a host-guest interaction between cucurbituril (CB) and an amphiphilic metal complex pyrene-ruthenium (Py-Ru). Py-Ru can self-assemble into positively charged nanoparticles in water, and exhibits an enhanced catalytic efficiency in the transfer hydrogenation of NAD⁺ to NADH. After forming an inclusion complex with CB, Py-Ru aggregates are broken, leading to a decrease in catalytic efficiency, which can be recovered by competitive replacement with amantadine. This supramolecular strategy provides an efficient and flexible method for constructing reversible catalytic system, which also extends the application scope of the host-guest interaction.

Photophysical and NMR studies of encapsulation of 2-cyano-6-hydroxy benzothiazole in p-sulfonatocalix[6]arene and its biological applications

This work deals with the study of the interaction between 2-cyano-6-hydroxy benzothiazole (CHBT) and p-sulfonatocalix[6]arene (SCX6) at different pH values in aqueous medium by UV-visible absorption spectroscopy and steady-state fluorescence spectroscopy. The results demonstrate the strong influence of SCX6 on the fluorescence properties of CHBT. The steady-state emission of CHBT shows strong sensitivity to its environment. The mode of inclusion complexation of CHBT and SCX6 has also been investigated using HR-MS, FT-IR, NMR, 2D NMR, and FESEM analysis. With the increase in SCX6 concentration, absorbance decreased with an isosbestic point at 305 nm. The binding constant is calculated by a spectrofluorimetric method and stoichiometry by Job's method. The formation of an inclusion complex has been confirmed by 2D NMR NOESY, COSY, ROESY, HMBC, and HSQC spectroscopic methods. The complex is seen to be stabilized by electrostatic interactions between CHBT and the nanocavity of SCX6. Studies with cellular systems support that the CHBT-SCX6 complex is more effective in killing cancerous cells and hence, SCX6 may prove to be an effective carrier for drug molecules like CHBT.

Fluorescence detected circular dichroism (FDCD) for supramolecular host-guest complexes

Fluorescence-detected circular dichroism (FDCD) spectroscopy is applied for the first time to supramolecular host-guest and host-protein systems and compared to the more known electronic circular dichroism (ECD). We find that FDCD can be an excellent choice for common supramolecular applications, e.g. for the detection and chirality sensing of chiral organic analytes, as well as for reaction monitoring. Our comprehensive investigations demonstrate that FDCD can be conducted in favorable circumstances at much lower concentrations than ECD measurements, even in chromophoric and auto-emissive biofluids such as blood serum, overcoming the sensitivity limitation of absorbance-based chiroptical spectroscopy. Besides, the combined use of FDCD and ECD can provide additional valuable information about the system, e.g. the chemical identity of an analyte or hidden aggregation phenomena. We believe that simultaneous FDCD- and ECD-based chiroptical characterization of emissive supramolecular systems will be of general benefit for characterizing fluorescent, chiral supramolecular systems due to the higher information content obtained by their combined use.

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.

Supramolecular and suprabiomolecular photochemistry: a perspective overview

In this perspective review article, we have attempted to bring out the important current trends of research in the areas of supramolecular and suprabiomolecular photochemistry. Since the spans of the subject areas are very vast, it is impossible to cover all the aspects within the limited space of this review article. Nevertheless, efforts have been made to assimilate the basic understanding of how supramolecular interactions can significantly change the photophysical and other related physiochemical properties of chromophoric dyes and drugs, which have enormous academic and practical implications. We have discussed with reference to relevant chemical systems where supramolecularly assisted modulations in the properties of chromophoric dyes and drugs can be used or have already been used in different areas like sensing, dye/drug stabilization, drug delivery, functional materials, and aqueous dye laser systems. In supramolecular assemblies, along with their conventional photophysical properties, the acid-base properties of prototropic dyes, as well as the excited state prototautomerization and related proton transfer behavior of proton donor/acceptor dye molecules, are also largely modulated due to supramolecular interactions, which are often reflected very explicitly through changes in their absorption and fluorescence characteristics, providing us many useful insights into these chemical systems and bringing out intriguing applications of such changes in different applied areas. Another interesting research area in supramolecular photochemistry is the excitation energy transfer from the donor to acceptor moieties in self-assembled systems which have immense importance in light harvesting applications, mimicking natural photosynthetic systems. In this review article, we have discussed varieties of these aspects, highlighting their academic and applied implications. We have tried to emphasize the progress made so far and thus to bring out future research perspectives in the subject areas concerned, which are anticipated to find many useful applications in areas like sensors, catalysis, electronic devices, pharmaceuticals, drug formulations, nanomedicine, light harvesting, and smart materials.

Interaction of Cucurbit[7]uril With Protease Substrates: Application to Nanosecond Time-Resolved Fluorescence Assays

We report the use of the macrocyclic host cucurbit[7]uril (CB7) as a supramolecular additive in nanosecond time-resolved fluorescence (Nano-TRF) assays for proteases to enhance the discrimination of substrates and products and, thereby, the sensitivity. A peptide substrate was labeled with 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as a long-lived (>300 ns) fluorescent probe and 3-nitrotyrosine was established as a non-fluorescent fluorescence resonance energy transfer (FRET) acceptor that acts as a “dark quencher.” The substrate was cleaved by the model proteases trypsin and chymotrypsin and the effects of the addition of CB7 to the enzyme assay mixture were investigated in detail using UV/VIS absorption as well as steady-state and time-resolved fluorescence spectroscopy. This also allowed us to identify the DBO and nitrotyrosine residues as preferential binding sites for CB7 and suggested a hairpin conformation of the peptide, in which the guanidinium side chain of an arginine residue is additionally bound to a vacant ureido rim of one of the CB7 hosts.

Supramolecular interaction of sanguinarine dye with sulfobutylether-β-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-β-cyclodextrin (SBE7βCD, 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 SBE7βCD. The changes in the pK a values, absorption and fluorescence spectra, and fluorescence lifetime values of these two forms of SG with SBE7βCD indicate complex formation between them. The cationic form shows 3 times higher interaction towards SEB7βCD (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 SBE7βCD results in increasing the antibacterial activity of the SBE7βCD: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 β-cyclodextrin.

Evaluation of the supramolecular interaction of Congo red with cucurbiturils using mass spectrometry and spectroscopic methods

Cucurbit[n]urils decolourise aqueous solutions of Congo red by forming outer-surface adducts, which are also detected in gas-phase ESI-MS studies.

Host–guest interaction based supramolecular photodynamic therapy systems: a promising candidate in the battle against cancer

This article summarizes recent advances in the development of supramolecular photodynamic therapy based on host–guest interactions.

Acidity constant of pH indicators in the supramolecular systems studied by two CE-based methods compared using the RGB additive color model

Acid-base properties of methyl orange, bromocresol green, bromophenol blue, and bromothymol blue were thoroughly investigated in the past due to their application as colorimetric pH indicators. However, it is still unknown how these properties change upon the supramolecular host-guest interactions. Owing to the growing interest in using supramolecular host-guest interactions to reach expected modification of various physicochemical properties of guests, we decided to address this question in the present article. We estimated the shifts of pK_a values induced by diverse hosts (cyclodextrins, cucurbiturils, calixarenes, micelles, and serum albumin) and performed a thermodynamic analysis of the selected systems. To make a deeper insight, we confronted the aforementioned dyes with the other kinds of molecules studied by us in the past. In overall, the results obtained demonstrate a large multiplicity of possible pK_a behaviors, their poor predictability, and the existence of subtle structure-acidity relationships. In addition, we observed three thermodynamically different mechanisms of pK_a alteration. Therefore, more studies are needed to bring closer the promising perspective of a programmable acidity's tuning. Our methodology was based on capillary electrophoresis (CE) applied in two parallel variants: a classical method based on the fitting of a nonlinear function, and an alternative two-value method (TVM), which requires over twice less measurements to estimate pK_a. To identify the optimal approach for further studies, both methods were comprehensively compared and discussed based on the RGB additive color model, a user-friendly scale that integrates three primary aspects of an analytical method: analytical performance, green chemistry, and practicality.

Applications of Cucurbiturils in Medicinal Chemistry and Chemical Biology

The supramolecular chemistry of cucurbit[n]urils (CBn) has been rapidly developing to encompass diverse medicinal applications, including drug formulation and delivery, controlled drug release, and sensing for bioanalytical purposes. This is made possible by their unique recognition properties and very low cytotoxicity. In this review, we summarize the host-guest complexation of biologically important molecules with CBn, and highlight their implementation in medicinal chemistry and chemical biology.

Host-guest interactions in nor-seco-cucurbit[10]uril: novel guest-dependent molecular recognition and stereoisomerism

The unique monomer and excimer fluorescence emissions of pyrene were first exploited as distinctly photophysical signals to identify the possible diastereomers of guests within nor-seco-cucurbit[10]uril (NS-CB[10]) cavities. Further experiments revealed that balancing the hydrophilic and hydrophobic effects of the guest in aqueous solution can improve the molecular recognition and binding ability of NS-CB[10].

Supramolecular Nanorods of (N-Methylpyridyl) Porphyrin With Captisol: Effective Photosensitizer for Anti-bacterial and Anti-tumor Activities

Porphyrins, especially the 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin (TMPyP), are well-accepted as photosensitizers due to strong absorption from visible to near-infrared region, good singlet oxygen quantum yields as well as chemical versatility, all of which can be further modulated through planned supramolecular strategies. In this study, we report the construction of supramolecular nanorods of TMPyP dye/drug with captisol [sulfobutylether-β-cyclodextrin (SBE₇βCD)] macrocycle through host-guest interaction. The availability of four cationic N-methylpyridyl groups favors multiple binding interaction with the captisol host, building an extended supramolecular assembly of captisol and TMPyP. In addition to the spectroscopic characterizations for the assembly formation, the same has been pictured in SEM and FM images as nanorods of ~10 μm in length or more. Complexation of TMPyP has brought out beneficial features over the uncomplexed TMPyP dye; enhanced singlet oxygen yield, improved photostability, and better photosensitizing effect, all supportive of efficient photodynamic therapy activity. The Captisol:TMPyP complex displayed enhanced antibacterial activity toward E. coli under white light irradiation as compared to TMPyP alone. Cell viability studies performed in lung carcinoma A549 cells with light irradiation documented increased cytotoxicity of the complex toward the cancer cells whereas reduced dark toxicity is observed toward normal CHO cells. All these synergistic effects of supramolecular nanorods of Captisol-TMPyP complex make the system an effective photosensitizer and a superior antibacterial and antitumor agent.

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

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

A Supramolecular Approach for Enhanced Antibacterial Activity and Extended Shelf-life of Fluoroquinolone Drugs with Cucurbit[7]uril

The host-guest interactions of a third-generation fluoroquinone, danofloxacin (DOFL), with the macrocyclic host cucurbit[7]uril (CB7) have been investigated at different pH values (~3.5, 7.5, and 10). The photophysical properties have been positively affected, that is, the fluorescence yield and lifetime increased, as well as the photostability of DOFL improved in the presence of CB7. The antibacterial activity of DOFL is enhanced in the presence of CB7, as tested against four pathogenic bacteria; highest activity has been found towards B. cereus and E. coli, and lower activity towards S. aureus and S. typhi. The antibacterial activity of two additional second-generation fluoroquinones, i.e., norfloxacin and ofloxacin, has also been investigated in the absence as well as the presence of CB7 and compared with that of DOFL. In case of all drugs, the minimum inhibitory concentration (MIC) was reduced 3–5 fold in the presence of CB7. The extended shelf-life (antibacterial activity over time) of the fluoroquinone drugs in the presence of CB7, irrespective of four types of bacteria, can be attributed to the enhanced photostability of their CB7 complexes, which can act as better antibiotics with a longer expiry date than uncomplexed DOFL.

Ultra-Bright Rhodamines with Sulfobutylether-β-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-β-cyclodextrin (SBE₇ βCD). Spectral changes in the absorption and fluorescence behavior of dyes in presence of the SBE₇ βCD 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 βCD host. Consequently, SBE₇ βCD 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 βCD : RhB system and ∼10 % higher for SBE₇ βCD : Rh6G system), better beam profile, and enhanced durability compared to the respective dyes in optically matched ethanol solutions.

2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril: Encapsulation-Promoted Dimerization of Electrostatically Repulsing Pyridiniums

Rigid linear compounds G1 and G2, which contained two 4-phenylpyridinium (PhPy⁺ ) units, have been prepared to investigate their binding with cucurbit[8]uril (CB[8]). X-ray crystallographic structures revealed that in the solid state both compounds were included by CB[8], through antiparallel stacking, to form 2:2 quaternary complexes (G1)₂ @(CB[8])₂ and (G2)₂ @(CB[8])₂ . For the former complex, CB[8] entrapped G1 by holding two heterodimers of its Py⁺ and benzyl units, which were at opposite ends of the backbone. In contrast, for the first time, the second complex disclosed parallel stacking of two cationic Py⁺ units of G2 in the cavity of CB[8] in the solid state, despite the generation of important electrostatic repulsion. Isothermal titrations in water afforded high apparent association constants of 4.36×10⁶ and 6.43×10⁶  m^-1 for 1:1 complexes G1@CB[8] and G2@CB[8], respectively, and ¹ H NMR spectroscopy experiments in D₂ O confirmed a similar stacking pattern to that observed in the solid state. A previous study and crystal structures of the 2:1 complexes formed between three new controls, G3-5, and CB[8] did not display such unusual stacking of the cationic Py⁺ unit; this may be attributed to the multivalency of the two CB[8] encapsulation interactions.

pH-responsive molecular assemblies of pyridylbutadiene derivative with cucurbit[7]uril

pH-responsive emission behavior of supramolecular complexes between pyridylbutadiene with CB7 and formation of molecular assemblies is described.

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

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