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.

Selective Homocysteine Assay with Cucurbit[7]uril by pH Regulation

We report the effect of pH on the supramolecular complexation of two biothiols, viz., homocysteine (Hcy) and cysteine (Cys), with cucurbit[7]uril (CB[7]). Under basic pH conditions, Cys did not complex with CB[7], whereas Hcy efficiently complexed with CB[7], as confirmed by ¹H NMR spectroscopy and Ellman's reagent (5,5'-dithio-bis(2-nitrobenzoic acid), DTNB) assay. ¹H NMR and Raman spectroscopic studies revealed that, in the absence of CB[7], Hcy auto-oxidized slowly (~36 h) to homocystine (HSSH) under basic pH conditions. However, the rate of Hcy oxidation increased by up to 150 fold in the presence of CB[7], as suggested by the DTNB assay. Thus, supramolecular complexation under basic pH conditions led to the formation of a HSSH-CB[7] complex, and not Hcy-CB[7]. The results indicate that Hcy is rapidly oxidized to HSSH under the catalysis of CB[7], which acts as a reaction chamber, in basic pH conditions. Our studies suggest that Hcy concentration, a risk factor for cardiovascular disease, can be selectively and more easily quantified by supramolecular complexation with CB [7].

Back electron transfer rates determine the photoreactivity of donor-acceptor stilbene complexes in a macrocyclic host

Host-guest 2 : 1 complexation of photoreactive alkene guests improves the selectivity of [2 + 2] photodimerizations by templating alkene orientation prior to irradiation. Host-guest chemistry can also provide 1 : 1 : 1 complexes through the inclusion of electronically complementary donor and acceptor guests, but the photoreactivity of such complexes has not been investigated. We imagined that such complexes could enable selective cross-[2 + 2] photocycloadditions between donor and acceptor stilbenes. In pursuit of this strategy, we investigated a series of stilbenes and found 1 : 1 : 1 complexes with cucurbit[8]uril that exhibited charge-transfer (CT) absorption bands in the visible and near-IR regions. Irradiation of the CT band of an azastilbene, 4,4'-stilbenedicarboxylate, and cucurbit[8]uril ternary complex led to a selective cross-[2 + 2] photocycloaddition, while other substrate pairs exhibited no productive chemistry upon CT excitation. Using transient absorption spectroscopy, we were able to understand the variable photoreactivity of different stilbene donor-acceptor complexes. We found that back electron transfer following CT excitation of the photoreactive complex is positioned deep in the Marcus inverted region due to electrostatic stabilization of the ground state, allowing [2 + 2] to effectively compete with this relaxation pathway. Control reactions revealed that the cucurbit[8]uril host not only serves to template the reaction from the ground state, but also protects the long-lived radical ions formed by CT from side reactions. This protective role of the host suggests that donor-acceptor host-guest ternary complexes could be used to improve existing CT-initiated photochemistry or access new reactivity.

Cucurbiturils mimicked by low polarizability solvents with pre-formed cavities: an empirical model to predict hydrocarbon selectivity

Relative binding affinities of a series of nine rigid hydrocarbons towards the cavity formed by a portion of the inner wall of cucurbit[8]uril (CB[8]) and a positive auxiliary guest were determined by competitive ¹⁹F NMR titrations in deuterium oxide. The corresponding free binding energies were corrected by the hydrocarbon computed solvation energies to obtain their free energies of transfer from the gas phase to the CB[8]/auxiliary guest cavity. These energies correlate linearly with the hydrocarbon static polarizabilities, thereby suggesting that the selectivity is driven, perhaps exclusively, by dispersive interactions between the hydrocarbons and the tailor-made cavity, regardless of the degree of unsaturation of the guests. The free energies of transfer also correlate linearly with the energy released upon introduction of the hydrocarbon into a pre-formed cavity extruded from a solvent (benzene) selected to mimic the polarity and polarizability of the CB[8]/auxiliary probe cavity – and this, with a unity slope. Among other features, this empirical model also accurately predicts the relative binding affinities of various rigid hydrocarbons to CB[6] and CB[7], as well as noble gases to CB[5], when the macrocycles are mimicked with pre-formed cavities in perfluorohexane or perfluorohexane/benzene mixtures, both being notoriously non-polar and non-polarizable environments.

Mimicking cucurbiturils with low polarizability solvents and pre-formed cavities allows the in silico prediction of their selectivities towards hydrocarbons and noble gases in aqueous solution.

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.

Complexation of trivalent metal cations (Al3+, Ga3+, In3+, La3+, Lu3+) to cucurbiturils: a DFT/SMD evaluation of the key factors governing the host-guest recognition

Cucurbiturils (CBs), the pumpkin-shaped macrocycles, are suitable hosts for an array of neutral and cationic species. A plethora of host-guest complexes between CBs and a variety of guest molecules has been studied. However, much remains unknown, even in the complexation of very simple guests such as metal cations. In the computational study herein, DFT molecular modeling has been employed to investigate the interactions of a series of trivalent metal cations (Al³⁺, Ga³⁺, In³⁺, La³⁺, Lu³⁺) to cucurbit[n]urils and to evaluate the main factors controlling the host-guest complexation. The thermodynamic descriptors (Gibbs energies in the gas phase and in a water environment) of the corresponding complexation reactions have been estimated. This research is a logical continuation of an earlier study on the interaction between CB[n]s and a series of biologically essential mono- and divalent metal cations (Na⁺/K⁺ and Mg²⁺/Ca²⁺, respectively).

Anthracene Walled Acyclic CB[n] Receptors: In Vitro and In Vivo Binding Properties Toward Drugs of Abuse

We report studies of the interaction of six acyclic CB[n]-type receptors toward a panel of drugs of abuse by a combination of isothermal titration calorimetry and 1H NMR spectroscopy.  Anthracene walled acyclic CB[n] host (M3) displays highest binding affinity toward methamphetamine (Kd = 15 nM) and fentanyl (Kd = 4 nM).  Host M3 is well tolerated by Hep G2 and HEK 293 cells up to 100 mM according to MTS metabolic and adenylate kinase release assays.  An in vivomaximum tolerated dose study with Swiss Webster mice showed no adverse effects at the highest dose studied (44.7 mg kg-1). Host M3 is not mutagenic based on the Ames fluctuation test and does not inhibit the hERG ion channel.  In vivoefficacy studies showed that pretreatment of mice with M3 significantly reduces the hyperlocomotion after treatment with methamphetamine, but M3 does not function similarly when administered 30 seconds after methamphetamine.

A light-responsive molecular switch based on cucurbit[7]uril and 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium dibromide displaying white light emission

By using ¹H NMR, ESI-MS and UV spectra, a novel light-responsive molecular switch constructed using 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium (1²⁺) and cucurbit[7]uril (Q[7]) is demonstrated. The E- to Z-isomerization of the double bond in 1²⁺ results in the transition of the switching states from the 1 : 2 complex E-1²⁺@Q[7]₂ to the stable 1 : 1 complex Z-1²⁺@Q[7]. In particular, both the 1 : 2 complex and the 1 : 1 complex can emit cold white fluorescence under UV light.

Molecular Approaches to Protein Dimerization: Opportunities for Supramolecular Chemistry

Protein dimerization plays a key role in many biological processes. Most cellular events such as enzyme activation, transcriptional cofactor recruitment, signal transduction, and even pathogenic pathways are significantly regulated via protein-protein interactions. Understanding and controlling the molecular mechanisms that regulate protein dimerization is crucial for biomedical applications. The limitations of engineered protein dimerization provide an opportunity for molecular chemistry to induce dimerization of protein in biological events. In this review, molecular control over dimerization of protein and activation in this respect are discussed. The well known molecule glue-based approaches to induced protein dimerization provide powerful tools to modulate the functionality of dimerized proteins and are shortly highlighted. Subsequently metal ion, nucleic acid and host-guest chemistry are brought forward as novel approaches for orthogonal control over dimerization of protein. The specific focus of the review will be on host-guest systems as novel, robust and versatile supramolecular approaches to modulate the dimerization of proteins, using functional proteins as model systems.

Switching plasmonic nanogaps between classical and quantum regimes with supramolecular interactions

In the realm of extreme nanophotonics, nanogap plasmons support reliable field enhancements up to 1000, which provide unique opportunities to access a single molecule for strong coupling and a single atom for quantum catalysis. The quantum plasmonics are intriguing but difficult to modulate largely because of the lack of proper spacers that can reversibly actuate the sub-1-nm gaps. Here, we demonstrate that supramolecular systems made of oligoamide sequences can reversibly switch the gap plasmons of Au nanoparticles on mirror between classical and quantum tunneling regimes via supramolecular interactions. The results reveal detailed plasmon shift near the quantum tunneling limit, which fits well with both classical- and quantum-corrected models. In the quantum tunneling regime, we demonstrate that plasmonic hot electron tunneling can further blue shift the quantum plasmons because of the increased conductance in the nanogaps, making it a promising prototype of optical tunable quantum plasmonic devices.

Preparation, characterization, and biological activity study of thymoquinone-cucurbit[7]uril inclusion complex

In this study, the formation of a host–guest inclusion complex between cucurbit[7]uril (CB[7]) and thymoquinone (TQ) was investigated in aqueous solution. The formation of a stable inclusion complex, CB[7]–TQ, was confirmed by using different techniques, such as ¹H NMR and UV-visible spectroscopy. The aqueous solubility of TQ was clearly enhanced upon the addition of CB[7], which provided an initial indication for supramolecular complexation. The complexation stoichiometry and the binding constant of the inclusion complex were determined through a combination of two sets of titration methods, including UV-visible and fluorescence displacement titrations. Both methods suggested the formation of a 1 : 1 stoichiometry between CB[7] and TQ with moderate binding affinity of 3 × 10³ M⁻¹. Density functional theory (DFT) calculations were also performed to verify the structure of the resulted host–guest complex and to support the complexation stoichiometry. The theoretical calculations were in agreement with experimental results obtained by ¹H NMR spectroscopy. Most importantly, the cytotoxic effect of the CB[7]–TQ complex was investigated against cancer and normal cell lines. The results showed that the anticancer activity of TQ against MDA-MB-231 cells was enhanced by the complexation with CB[7], while no significant effect was observed in MCF-7 cells. The results also confirmed the low toxicity of the CB[7] host molecule that supports the use of CB[7] as a drug carrier.

The host–guest inclusion complexation of thymoquinone by cucurbit[7]uril in aqueous solution is established, which results in an enhanced biological activity.

Photomodulation of ultrastable host–guest complexes in water and their application in light-controlled steroid release

Host–guest complexation of dithienylethene photoswitches with cucurbit[8]uril leads to photoresponsive binding pairs with picomolar affinity in water.

Theoretical Study of Macrocyclic Host Molecules: From Supramolecular Recognition to Self-Assembly

Supramolecular chemistry focuses on molecular recognition and self-assembly of various building blocks through weak non-covalent interactions, including anion-π, hydrogen bond (HB), hydrophobic interactions, van der Waals (vdW) interactions, etc, which...

Insights into β-amyloid transition prevention by cucurbit[7]uril from molecular modeling

In this study, comparable molecular dynamic (MD) simulations of 1.2 microseconds were performed to clarify the prevention of the β-amyloid peptide (Aβ_1-42) aggregation by cucurbit[7]uril (CB[7]). The accumulation of this peptide in the brain is one of the most harmful in Alzheimer's disease. The inhibition mechanism of Aβ_1-42 aggregation by different molecules is attributed to preventing of Aβ_1-42 conformational transition from α-helix to the β-sheet structure. However, our structural analysis shows that the pure water and aqueous solution of the CB[7] denature the native Aβ_1-42 α-helix structure forming different compactness and unfolded conformations, not in β-sheet form. On the other hand, in the three CB[7]@Aβ_1-42 complexes, it was observed the encapsulation of N-terminal (Asp1), Lys16, and Val36 by CB[7] along the MD trajectory, and not with aromatic residues as suggested by the literature. Only in one CB[7]@Aβ_1-42 complex was observed stable Asp23-Lys28 salt bridge with an average distance of 0.36 nm. All CB[7]@Aβ_1-42 complexes are very stable with binding free energy lowest than ∼-50 kcal/mol between the CB[7] and Aβ_1-42 monomer from MM/PBSA calculation. Therefore, herein we show that the mechanism of the prevention of elongation protofibril by CB[7] is due to the disruption of the Asp23-Lys28 salt bridge and steric effects of CB[7]@Aβ_1-42 complex with the fibril lattice, and not due to the transition from α-helix to β-sheet following the dock-lock mechanism.Communicated by Ramaswamy H. Sarma.

Encapsulation of l-valine, d-leucine and d-methionine by cucurbit[8]uril

The binding interactions of cucurbit[8]uril (Q[8]) with l-valine, d-leucine, and d-methionine, both in aqueous solution and solid state, have been studied by 1H NMR spectroscopy and X-ray crystallography.

Porous materials formed by four self-construction processes

Multiple self-construction behavior of cyclic oligoesters is described.

Unravelling binding effects in cyclodextrin inclusion complexes with diamondoid ammonium salt guests

A hydrophobic tornado – complexation of diamondoid ammonium salts with cyclodextrins in water.

An inclusion complex of cucurbit[7]uril with benzimidazolyl benzyl viologen exhibits fluorescence and photochromic properties

A self-assembled supramolecular inclusion complex of Q[7] with benzimidazolyl benzyl viologen exhibits interesting fluorescence emission and reversible photochromism.

Engineering living cells with cucurbit[7]uril-based supramolecular polymer chemistry: from cell surface engineering to manipulation of subcellular organelles

Supramolecular polymer chemistry, which closely integrates noncovalent interactions with polymeric structures, is a promising toolbox for living cell engineering. Here, we report our recent progress in exploring the applications of cucurbit[7]uril (CB[7])-based supramolecular polymer chemistry for engineering living cells. First, a modular polymer-analogous approach was established to prepare multifunctional polymers that contain CB[7]-based supramolecular recognition motifs. The supramolecular polymeric systems were successfully applied to cell surface engineering and subcellular organelle manipulation. By anchoring polymers on the cell membranes, cell–cell interactions were established by CB[7]-based host–guest recognition, which further facilitated heterogeneous cell fusion. In addition to cell surface engineering, placing the multifunctional polymers on specific subcellular organelles, including the mitochondria and endoplasmic reticulum, has led to enhanced physical contact between subcellular organelles. It is highly anticipated that the CB[7]-based supramolecular polymer chemistry will provide a new strategy for living cell engineering to advance the development of cell-based therapeutic materials.

Cucurbit[7]uril-based supramolecular polymer chemistry, which closely integrates host–guest recognition with multifunctional polymeric structures, is a promising toolbox for living cell engineering.

Synthesis of a novel aminobenzene-containing hemicucurbituril and its fluorescence spectral properties with ions

A novel hemicucurbituril-based macrocycle, alternately consisting of amidobenzene and 2-imidazolidione moieties was designed and synthesized. Based on the fragment coupling strategy, nitrobenzene-containing hemicucurbituril was firstly prepared facilely under alkaline environment, and reduction of the nitro groups produced the desired amidobenzene-containing hemicucurbituril. As an original fluorescent chemosensor, it exhibited strong interactions with Fe³⁺ over other metal cations. The experimental evidence of fluorescence spectra suggested that a 1:1 complex was formed between this macrocycle and Fe³⁺ with an association constant up to (2.1 ± 0.3) × 10⁴ M⁻¹. Meanwhile, this macrocycle showed no obvious or only slight enhancement of the fluorescence intensity with selected anions.

A General Picture of Cucurbit[8]uril Host-Guest Binding

Describing, understanding, and designing complex interaction networks within macromolecular systems remain challenging in modern chemical research. Host-guest systems, despite their relative simplicity in both the structural feature and interaction patterns, still pose problems in theoretical modeling. The barrel-shaped supramolecular container cucurbit[8]uril (CB8) shows promising functionalities in various areas, e.g., catalysis and molecular recognition. It can stably coordinate a series of structurally diverse guests with high affinities. In this work, we examine the binding of seven commonly abused drugs to the CB8 host, aiming at providing a general picture of CB8-guest binding. Extensive sampling of the configurational space of these host-guest systems is performed, and the binding pathway and interaction patterns of CB8-guest complexes are investigated. A thorough comparison of widely used fixed-charge models for drug-like molecules is presented. Iterative refitting of the atomic charges suggests significant conformation dependence of charge generation. The initial model generated at the original conformation could be inaccurate for new conformations explored during conformational search, and the newly fitted charge set improves the prediction-experiment correlation significantly. Our investigations of the configurational space of CB8-drug complexes suggest that the host-guest interactions are more complex than expected. Despite the structural simplicities of these molecules, the conformational fluctuations of the host and the guest molecules and orientations of functional groups lead to the existence of an ensemble of binding modes. The insights of the binding thermodynamics, performance of fixed-charge models, and binding patterns of the CB8-guest systems are useful for studying and elucidating the binding mechanism of other host-guest complexes.

Organic Host Encapsulation Effects on Nitrosobenzene Monomer-Dimer Distribution and C-NO Bond Rotation in an Aqueous Solution

The intermolecular (monomer-dimer equilibrium) and intramolecular (C-NO and C-NMe₂ rotations) dynamics of 4-nitrosocumene (1a) and 4-(N,N-dimethylamino)nitrosobenzene (1b), respectively, were found to be controlled by the medium (water) and the host environment (organic capsules and cavitands). The ability of water to shift the equilibrium toward the dimer appears to result from dipolar stabilization of the polar dimer structure and has a resemblance to water's known ability to favor organic cycloaddition reactions. In an aqueous medium, a range of organic hosts selectively include only the nitrosocumene monomer 1a. Encapsulation in the octa acid duplex (OA₂) selects two 1a monomers rather than a dimer structure. Octa acid encapsulation also results in more restricted intramolecular C-N rotations of the guest 1b.

DFT-D4 Insight into the Inclusion of Amphetamine and Methamphetamine in Cucurbit[7]uril: Energetic, Structural and Biosensing Properties

The host-guest interactions of cucurbit[7]uril (CB[7]) as host and amphetamine (AMP), methamphetamine (MET) and their enantiomeric forms (S-form and R-form) as guests were computationally investigated using density functional theory calculations with the recent D4 atomic-charge dependent dispersion corrections. The analysis of energetic, structural and electronic properties with the aid of frontier molecular orbital analysis, charge decomposition analysis (CDA), extended charge decomposition analysis (ECDA) and independent gradient model (IGM) approach allowed to characterize the host-guest interactions in the studied systems. Energetic results indicate the formation of stable non-covalent complexes where R-AMP@CB[7] and S-AMP@CB[7] are more stable thermodynamically than R-MET@CB[7] and S-MET@CB[7] in gas phase while the reverse is true in water solvent. Based on structural analysis, a recognition mechanism is proposed, which suggests that the synergistic effect of van der Waals forces, ion-dipole interactions, intermolecular charge transfer interactions and intermolecular hydrogen bonding is responsible for the stabilization of the complexes. The geometries of the complexes obtained theoretically are in good agreement with the X-ray experimental structures and indicate that the phenyl ring of amphetamine and methamphetamine is deeply buried into the cavity of CB[7] through hydrophobic interactions while the ammonium group remains outside the cavity to establish hydrogen bonds with the portal oxygen atoms of CB[7].

In Vitro and In Vivo Sequestration of Methamphetamine by a Sulfated Acyclic CB[n]-Type Receptor

We report the synthesis of two new acyclic sulfated acyclic CB[n]-type receptors (TriM0 and Me4 TetM0) and investigations of their binding properties toward a panel of drugs of abuse (1-13) by a combination of 1 H NMR spectroscopy and isothermal titration calorimetry. TetM0 is the most potent receptor with Ka ≥106  M-1 toward methamphetamine, fentanyl, MDMA and mephedrone. TetM0 is not cytotoxic toward HepG2 and HEK 293 cells below 100 μM according to MTS metabolic and adenylate kinase release assays and is well tolerated in vivo when dosed at 46 mg kg-1 . TetM0 does not inhibit the hERG ion channel and is not mutagenic based on the Ames fluctuation test. Finally, in vivo efficacy studies show that the hyperlocomotion of mice treated with methamphetamine can be greatly reduced by treatment with TetM0 up to 5 minutes later. TetM0 has potential as a broad spectrum in vivo sequestrant for drugs of abuse.

An overview from simple host-guest systems to progressively complex supramolecular assemblies

Supramolecular chemistry involving macrocyclic hosts is a highly interdisciplinary and fast-growing research field in chemistry, biochemistry, and materials science. Host-guest based supramolecular assemblies, as constructed through non-covalent interactions, are highly dynamic in nature, and can be tuned easily using their responses to various external stimuli, providing a convenient approach to achieve excellent functional materials. Macrocyclic hosts, particularly cyclodextrins, cucurbit[n]urils, and calix[n]arenes, which have unique features like possessing hydrophobic cavities of different sizes, along with hydrophilic external surfaces, which are also amenable towards easy derivatizations, are versatile cavitands or host molecules to encapsulate diverse guest molecules to form stable host-guest complexes with many unique structures and properties. Interestingly, host-guest complexes possessing amphiphilic properties can easily lead to the formation of various advanced supramolecular assemblies, like pseudorotaxanes, rotaxanes, polyrotaxanes, supramolecular polymers, micelles, vesicles, supramolecular nanostructures, and so on. Moreover, these supramolecular assemblies, with varied morphologies and responsiveness towards external stimuli, have immense potential for applications in nanotechnology, materials science, biosensors, drug delivery, analytical chemistry and biomedical sciences. In this perspective, we present a stimulating overview, discussing simple host-guest systems to complex supramolecular assemblies in a systematic manner, aiming to encourage future researchers in this fascinating area of supramolecular chemistry to develop advanced supramolecular materials with superior functionalities, for their deployment in diverse applied areas.

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

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

Protein Assembly by Design

Proteins are nature's primary building blocks for the construction of sophisticated molecular machines and dynamic materials, ranging from protein complexes such as photosystem II and nitrogenase that drive biogeochemical cycles to cytoskeletal assemblies and muscle fibers for motion. Such natural systems have inspired extensive efforts in the rational design of artificial protein assemblies in the last two decades. As molecular building blocks, proteins are highly complex, in terms of both their three-dimensional structures and chemical compositions. To enable control over the self-assembly of such complex molecules, scientists have devised many creative strategies by combining tools and principles of experimental and computational biophysics, supramolecular chemistry, inorganic chemistry, materials science, and polymer chemistry, among others. Owing to these innovative strategies, what started as a purely structure-building exercise two decades ago has, in short order, led to artificial protein assemblies with unprecedented structures and functions and protein-based materials with unusual properties. Our goal in this review is to give an overview of this exciting and highly interdisciplinary area of research, first outlining the design strategies and tools that have been devised for controlling protein self-assembly, then describing the diverse structures of artificial protein assemblies, and finally highlighting the emergent properties and functions of these assemblies.

Self-Assembly Hydrosoluble Coronenes: A Rich Source of Supramolecular Turn-On Fluorogenic Sensing Materials in Aqueous Media

Water-soluble coronenes, that form nanoparticles by self-association, work as new fluorescent materials by complexation with cucurbit[7]uril, as well as selective turn-on fluorogenic sensors for nitroaromatic explosives with remarkable selectivity, by using only water as solvent.

Cucurbiturils as Effectors on the Self-Assembly of Pd(II) and Pt(II) Metallacycles

Four bidentate, dicationic ligands (L1²⁺-L4²⁺) were prepared and investigated as guests for binding by the cucurbit[7]uril (CB[7]) host and structural components for metal (Pd and Pt)-coordinated self-assembly into metallacycles. In aqueous solutions, all the ligands were found to form stable complexes of variable stoichiometries with CB[7], and only one (L2²⁺) failed to self-assemble, induced by the presence of suitable Pd or Pt complexes, into metallacycles. Exposure of the Pd-based metallacycles to CB[7] led to their disassembly at room temperature, while the Pt-based metallacycles remained stable under these conditions. However, heating of the Pt metallacycles in the presence of CB[7] also led to their disassembly. This interplay between the interactions in aqueous media of the L1²⁺, L3²⁺, and L4²⁺ ligands with the CB[7] host and Pd (or Pt) complexes suggests the possibility of using these or related systems for controlled drug delivery applications.

Halide Size-Selective Binding by Cucurbit[5]uril-Alkali Cation Complexes in the Gas Phase

We report data that suggest complexes with alkali cations capping the portals of cucurbit[5]uril (CB[5]) bind halide anions size-selectively as observed in the gas phase: Cl^- binds inside the CB[5] cavity, Br^- is observed both inside and outside, and I^- binds weakly outside. This is reflected in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: all detected Cl^- complexes dissociate at higher energies, and Br^- complexes exhibit unusual bimodal dissociation behavior, with part of the ion population dissociating at very low energies and the remainder dissociating at significantly higher energies comparable to those observed for Cl^-. Decoherence cross sections measured in SF₆ using cross-sectional areas by Fourier transform ion cyclotron resonance techniques for [CB[5] + M₂X]⁺ (M = Na, X = Cl or Br) are comparable to or less than that of [CB[5] + Na]⁺ over a wide energy range, suggesting that Cl^- or Br^- in these complexes are bound inside the CB[5] cavity. In contrast, [CB[5] + K₂Br]⁺ has a cross section measured about 20% larger than that of [CB[5] + Na]⁺, suggesting external binding that may correspond with the weakly bound component seen in SORI. While I^- complexes with alkali cation caps were not observed, alkaline earth iodides with CB[5] yielded complexes with cross sections 5-10% larger than that of [CB[5] + Na]⁺, suggesting externally bound iodide. Geometry optimization at the M06-2X/6-31+G* level of ab initio theory suggests that internal anion binding is energetically favored by approximately 50-200 kJ mol^-1 over external binding; thus, the externally bound complexes observed experimentally must be due to large energetic barriers hindering the passing of large anions through the CB[5] portal, preventing access to the interior. Calculation of the barriers to anion egress using MMFF//M06-2X/6-31+G* theory supports this idea and suggests that the size-selective binding we observe is due to anion size-dependent differences in the barriers.

Cucurbituril-Ferrocene: Host-Guest Based Pretargeted Positron Emission Tomography in a Xenograft Model

Pretargeted positron emission tomography is a macromolecule-driven nuclear medicine technique that involves targeting a preadministered antigen target-bound macromolecule with a radioligand in vivo, aiming to minimize the overall radiation dose. This study investigates the use of antibody based host-guest chemistry methodology for pretargeted positron emission tomography. We hypothesize that the novel pretargeting approach reported here overcomes the challenges the current pretargeting platforms have with the in vivo stability and modularity of the pretargeting components. A cucurbit[7]uril host molecule modified, anti-carcinoembryonic antigen antibody (M5A; CB7-M5A) and a 68Ga-radiolabeled ferrocene guest radioligand ([68Ga]Ga-NOTA-PEG3-NMe2-Fc) were studied as potential host-guest chemistry pretargeting agents for positron emission tomography in BxPC3 xenografted nude mice. The viability of the platform was studied via in vivo biodistribution and positron emission tomography. Tumor uptake of [68Ga]Ga-NOTA-PEG3-NMe2-Fc was significantly higher in mice which received CB7-M5A prior to the radioligand injection (pretargeted) (3.3 ± 0.7%ID/g) compared to mice which only received the radioligand (nonpretargeted) (0.2 ± 0.1%ID/g).

Exploring the pH-dependent kinetics, thermodynamics and photochemistry of a flavylium-based pseudorotaxane

Flavylium-based molecular switches are attractive molecular components to devise stimuli-responsive host-guest systems such as rotaxanes and pseudorotaxanes. These compounds display a pH-dependent reaction network of several species that reversibly interconvert within different time scales. Therefore, to explore and take profit of exceptional stimuli-responsive properties of these systems, detailed kinetic and thermodynamic characterizations are often required. In this work, we present the results of such characterization for a new flavylium compound decorated with a trimethylalkylammonium substituent designed to form a pseudorotaxane with cucurbit[7]uril (CB7). The formation of the pseudorotaxane was characterized in detail, and the thermodynamic and kinetic aspects of the flavylium interconversion reactions in the assembly were investigated and compared with the free molecular switch.

Self Assembled Cages with Mechanically Interlocked Cucurbiturils

We report preparation of (bis)aniline ligand 4 which contains a central viologen binding domain and its subcomponent self-assembly with aldehyde 5 and Fe(OTf)₂ in CH₃CN to yield tetrahedral assembly 6. Complexation of ligand 4 with CB[7] in the form of CB[7]•4•2PF₆ allows the preparation of assembly 7 which contains an average of 1.95 (range 1-3) mechanically interlocked CB[7] units. Assemblies 6 and 7 are hydrolytically unstable in water due to their imine linkages. Redesign of our system with water stable 2,2'-bipyridine end groups was realized in the form of ligands 11 and 16 which also contain a central viologen binding domain. Self-assembly of 11 with Fe(NTf₂)₂ gave tetrahedral MOP 12 as evidenced by ¹H NMR, DOSY, and mass spectrometric analysis. In contrast, isomeric ligand 16 underwent self-assembly with Fe(OTf)₂ to give cubic assembly 17. Precomplexation of ligands 11 and 16 with CB[7] gave the acetonitrile soluble CB[7]•11•2PF₆ and CB[7]•16•2PF₆ complexes. Self-assembly of CB[7]•11•2PF₆ with Fe(OTf)₂ gave tetrahedron 13 which contains on average 1.8 mechanically interlocked CB[7] units as determined by ¹H NMR, DOSY, and ESI-MS analysis. Self-assembly of CB[7]•16•2PF₆ with Fe(OTf)₂ gave cube 13 which contains 6.59 mechanically interlocked CB[7] units as determined by ¹H NMR and DOSY measurements.

Stimuli Responsive Confinement of a Molecular Rotor Based BODIPY Dye inside a Cucurbit[7]uril Nanocavity

Present study reports the interaction of a molecular rotor based BODIPY dye, 8-anilino-BODIPY (ABP), with a versatile macrocyclic molecule, cucurbit[7]uril (CB7), investigated through various techniques such as ground-state absorption, steady-state fluorescence, time-resolve emission, proton NMR, and quantum chemical studies. Although BODIPY dyes have widespread applications due to their intriguing photochemical properties, studies on their noncovalent interactions with different macrocyclic hosts, especially regarding their supramolecularly induced modulations in photophysical properties are very limited. The investigated BODIPY dye, especially its protonated ABPH⁺ form (pH ∼ 1), shows a large fluorescence enhancement on its interaction with the CB7 host, due to large reduction in the structural flexibility for the bound dye, causing a suppression in its nonradiative de-excitation process in the excited state. Unlike ABPH⁺, the neutral ABP form (pH ∼ 7) shows considerably weaker interaction with CB7. For ABPH⁺-CB7 system, observed photophysical results indicate formation of both 1:1 and 1:2 dye-to-host complexes. Plausible geometries of these complexes are obtained from quantum chemical studies which are substantiated nicely from ¹H NMR results. The response of the ABPH⁺-CB7 system toward changing temperature of the solution have also been investigated elaborately to understand the potential of the system in different stimuli-responsive sensor applications.

Cucurbitimines - imine cages with concave walls

The variety of shape-persistent organic cages by imine bond formation has tremendously enlarged in recent years by using different building blocks (aldehydes and amines) in the condensation reactions. Here, we describe the use of a kinked tetraldehyde to generate pumpkin-shaped cages with concave walls, similar to cucurbiturils.

Kinked tetraaldehyde building blocks lead in condensation reactions with diamines to pumpkin shaped cages – the cucurbitimines.

Cucurbituril Based Luminescent Materials in Aqueous Media and Solid State

Cucurbit[n]urils, the pumpkin shaped macrocyclic host molecules possessing a hydrophobic cavity and two identical carbonyl portals, have drawn a lot of attention in recent years due to their high-affinity yet dynamic molecular recognition properties in water. The reversible and stimuli-responsive nature of their host-guest complexes imparts "smart" features leading to materials with intriguing optical, mechanical and morphological properties. In this review, we focus on the design of cucurbituril based luminescent materials in aqueous media as well in solid or film state. The design principles of fluorescent complexes, small assemblies as well as supramolecular polymers along with their stimuli-responsive properties and applications in diverse areas such as optoelectronic devices, light harvesting, anti-counterfeiting and information technology, cell imaging, etc are highlighted with selected examples from recent literature. We also discuss examples of room temperature phosphorescent materials derived from purely organic luminogens in the presence of cucurbiturils.

Toward Light-Controlled Supramolecular Peptide Dimerization

The selective photodeprotection of the NVoc-modified FGG tripeptide yields the transformation of its 1:1 receptor-ligand complex with cucurbit[8]uril into a homoternary FGG₂@CB8 assembly. The resulting light-induced dimerization of the model peptide provides a tool for the implementation of stimuli-responsive supramolecular chemistry in biologically relevant contexts.

Design and recognition of cucurbituril-secured platinum-bound oligopeptides

Platinum terpyridyl complexes, stacked on top of one another and secured as dimers with cucurbit[8]uril (CB[8]) in aqueous medium, were functionalized quantitatively and in situ with a pair of pentapeptides Phe-(Gly)₃-Cys by grafting their cysteine residues to the Pt centers. The resulting CB[8]·(Pt·peptide)₂ assemblies were used to target secondary hosts CB[7] and CB[8] via their pair of phenylalanine residues, again in situ. A series of well-defined architectures, including a supramolecular “pendant necklace” with hybrid head-to-head and head-to-tail arrangements inside CB[8], were obtained during the self-sorting process after combining only 3 or 4 simple building units.

A platinum terpyridyl complex, pentapeptide Phe-(Gly)₃-Cys and cucurbit[8]uril assemble into a “pendant necklace” with hybrid head-to-head and head-to-tail arrangements in aqueous medium.

Photoresponsive Binding Dynamics in High-Affinity Cucurbit[8]uril-Dithienylethene Host-Guest Complexes

The use of external stimuli to control the binding kinetics in supramolecular systems is of critical importance for the development of advanced molecular machines and devices. In this work, a study focused on the kinetics of a water-soluble host-guest system based on cucurbit[8]uril and two dithienylethene (DTE) photoswitches is reported. It is shown that for the DTE guest comprising two anionic sulfonate side arms appended to pyridinium moieties, the formation/dissociation of the pseudorotaxane structures is slowed down by more than 100000-fold with respect to its bipyridinium analogue. The decrease in ingression rate leads to the emergence of a competitive metastable product with the open DTE isomer that has an important influence in the overall binding kinetics. Moreover, the host-guest dissociation kinetics is demonstrated to be approximately 100-fold slower for the closed DTE isomer (t_1/2 =107 h vs. t_1/2 =1.2 h for the open isomer) allowing control over the dissociation rate with light.