Host-Guest Stimuli-Responsive Click Chemistry

Click chemistry has reached its maturity as the weapon of choice for the irreversible ligation of molecular fragments, with over 20 years of research resulting in the development or improvement of highly efficient kinetically controlled conjugation reactions. Nevertheless, traditional click reactions can be disadvantageous not only in terms of efficiency (side products, slow kinetics, air/water tolerance, etc.), but also because they completely avoid the possibility to reversibly produce and control bound/unbound states. Recently, non-covalent click chemistry has appeared as a more efficient alternative, in particular by using host-guest self-assembled systems of high thermodynamic stability and kinetic lability. This review discusses the implementation of molecular switches in the development of such non-covalent ligation processes, resulting in what we have termed stimuli-responsive click chemistry, in which the bound/unbound constitutional states of the system can be favored by external stimulation, in particular using host-guest complexes. As we exemplify with handpicked selected examples, these supramolecular systems are well suited for the development of human-controlled molecular conjugation, by coupling thermodynamically regulated processes with appropriate temporally resolved extrinsic control mechanisms, thus mimicking nature and advancing our efforts to develop a more function-oriented chemical synthesis.

Cucurbit[8]uril-based supramolecular theranostics

Different from most of the conventional platforms with dissatisfactory theranostic capabilities, supramolecular nanotheranostic systems have unparalleled advantages via the artful combination of supramolecular chemistry and nanotechnology. Benefiting from the tunable stimuli-responsiveness and compatible hierarchical organization, host-guest interactions have developed into the most popular mainstay for constructing supramolecular nanoplatforms. Characterized by the strong and diverse complexation property, cucurbit[8]uril (CB[8]) shows great potential as important building blocks for supramolecular theranostic systems. In this review, we summarize the recent progress of CB[8]-based supramolecular theranostics regarding the design, manufacture and theranostic mechanism. Meanwhile, the current limitations and corresponding reasonable solutions as well as the potential future development are also discussed.

Overcoming barriers with non-covalent interactions: supramolecular recognition of adamantyl cucurbit[n]uril assemblies for medical applications

Adamantane, a staple in medicinal chemistry, recently became a cornerstone of a supramolecular host-guest drug delivery system, ADA/CB[n]. Owing to a good fit between the adamantane cage and the host cavity of the cucurbit[n]uril macrocycle, formed strong inclusion complexes find applications in drug delivery and controlled drug release. Note that the cucurbit[n]uril host is not solely a delivery vehicle of the ADA/CB[n] system but rather influences the bioactivity and bioavailability of drug molecules and can tune drug properties. Namely, as host-guest interactions are capable of changing the intrinsic properties of the guest molecule, inclusion complexes can become more soluble, bioavailable and more resistant to metabolic conditions compared to individual non-complexed molecules. Such synergistic effects have implications for practical bioapplicability of this complex system and provide a new viewpoint to therapy, beyond the traditional single drug molecule approach. By achieving a balance between guest encapsulation and release, the ADA/CB[n] system has also found use beyond just drug delivery, in fields like bioanalytics, sensing assays, bioimaging, etc. Thus, chemosensing in physiological conditions, indicator displacement assays, in vivo diagnostics and hybrid nanostructures are just some recent examples of the ADA/CB[n] applicability, be it for displacements purposes or as cargo vehicles.

Dual-Signal Chemical Exchange Saturation Transfer (Dusi-CEST): An Efficient Strategy for Visualizing Drug Delivery Monitoring in Living Cells

We report a dual-signal chemical exchange saturation transfer (Dusi-CEST) strategy for drug delivery and detection in living cells. The two signals can be detected by operators in complex environments. This strategy is demonstrated on a cucurbit[6]uril (CB[6]) nanoparticle probe, as an example. The CB[6] probe is equipped with two kinds of hydrophobic cavities: one is found inside CB[6] itself, whereas the other exists inside the nanoparticle. When the probe is dispersed in aqueous solution as part of a hyperpolarized 129Xe NMR experiment, two signals appear at two different chemical shifts (100 and 200 ppm). These two resonances correspond to the NMR signals of 129Xe in the two different cavities. Upon loading with hydrophobic drugs, such as paclitaxel, for intracellular drug delivery, the two resonances undergo significant changes upon drug loading and cargo release, giving rise to a metric enabling the assessment of drug delivery success. The simultaneous change of Dusi-CEST likes a mobile phone that can receive both LTE and Wi-Fi signals, which can help reduce the occurrence of false positives and false negatives in complex biological environments and help improve the accuracy and sensitivity of single-shot detection.

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.

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.

Simultaneous analyte indicator binding assay (SBA) for the monitoring of reversible host-guest complexation kinetics

Very little information is available on the kinetics of the self-assembly and dissociation of optically silent building blocks despite the importance of such data in the rational design of tailor-made host-guest systems. We introduce here a novel time-resolved method that enables the simultaneous determination of complex formation and complex dissociation rate constants for inclusion-type host-guest complexes. The simultaneous analyte indicator binding assay (SBA) gives also direct access to binding affinities, thus largely simplifying the experimental procedure for a full kinetic and thermodynamic characterisation of host-guest systems.

Self-sorted Compartmentalization by Simultaneous Use of Natural and Synthetic Amphiphiles

Exploiting the orthogonal molecular interactions of natural (phospholipids) and synthetic (mono-allyloxylated cucurbit[7]uril) amphiphiles to form their own vesicles, the formation of two different types of compartments in a self-sorted manner mimicking cellular compartments is demonstrated. Even after simultaneous extrusion of both vesicles through small pore membranes, which transformed them into smaller vesicles, both vesicles were not fused but still appeared as independent compartments in sucrose solution. The simultaneous use of natural and synthetic amphiphiles, forming independent compartments, holds great potential for in-depth investigation of self-sorted multi-compartments and their structures as prototype cells.

Glutathione sensitive vesicles prepared from supramolecular amphiphiles

Glutathione (GSH) sensitive vesicles were prepared by the self-assembly of amphiphilic inclusion complexes. These novel chemically sensitive supramolecular amphiphiles are anticipated to have applications in drug delivery; the nanocarriers can protect the encapsulated cargo and release it via triggered degradation in high concentrations of GSH. Additionally, the sensitivity of the vesicles to GSH indicates that the dynamic covalent disulfide bond at the vesicle surface can be used for post-modification of the nanocarrier via a thiol-disulfide exchange, a strategy that can be exploited to introduce targeting moieties to increase treatment specificity. Supramolecular amphiphiles containing a dynamic covalent disulfide bond were prepared via the host-guest inclusion complexes between alkylated β-cyclodextrin (β-CD) hosts and adamantane terminated polyethylene glycol derivatives. The significant difference between the critical micelle concentrations of the supramolecular amphiphiles and the individual host and guest components confirmed that a unique supramolecular amphiphile was formed. Fluorescence experiments and dynamic light scattering (DLS) revealed that the supramolecular amphiphiles self-assembled into vesicles of 130 nm diameter which were stable for 8 months. Degradation of the vesicles after incubation with GSH was monitored using DLS and by the release of encapsulated 5,6-carboxyfluorescein (CF), observed by an increase in fluorescence intensity. Degradation of the nanocarrier was faster at intracellular GSH concentrations than at extracellular GSH concentrations.

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.

A medium-firm drug-candidate library of cryptand-like structures on T7 phage: design and selection of a strong binder for Hsp90

We designed and synthesized a medium-firm drug-candidate library of cryptand-like structures possessing a randomized peptide linker on the bacteriophage T7. From the macrocyclic library with a 109 diversity, we obtained a binder toward a cancer-related protein (Hsp90) with an antibody-like strong affinity (KD = 62 nM) and the binding was driven by the enthalpy. The selected supramolecular ligand inhibited Hsp90 activity by site-specific binding outside of the well-known ATP-binding pocket on the N-terminal domain (NTD).

Potential Applications of Cucurbit[n]urils and Their Derivatives in the Capture of Hazardous Chemicals

Cucurbit[n]urils (Q[n]s) are a relatively young family of macrocycles, consisting of glycoluril units bridged by methylene groups, and their unique structures have attracted extensive attention from chemists in recent decades. Due to the presence of a rigid hydrophobic inner cavity and two polar outer portals lined with carbonyl groups, Q[n]s not only encapsulate guest species into the cavity, but also coordinate with metal ions/clusters. Considerable achievements have been obtained in the fields of Q[n]s-based host-guest chemistry, coordination chemistry, as well as the combination of host-guest and coordination chemistry. Furthermore, the outer surface of Q[n]s has been demonstrated to be capable of interacting with definite species to generate supramolecular architectures in recent years. With more in-depth research into Q[n]s, their application studies have also emerged as a hot topic. This Minireview focuses on recent advances in the potential applications of solid-state materials based on Q[n]s and their derivatives for the capture and adsorption of hazardous chemicals from a solution or a gas mixture.

Highly Selective Binding and Inhibition of Pyr-His-Pro-NH2 (TRH) Function using a Polypyridinyl Macrocyclic Receptor with an Amphiphilic Cavity

Macrocycle, cyclo[4] [(1,3-(4,6)-dimethylbezene)[4](2,6-(3,5)-dimethylpyridine (B4P4), shows highly selective binding affinity with protirelin (Pyr-His-Pro-NH₂ ; TRH) among the tested 26 drug or drug adductive substrates. The stable complexation in a 1:1 manner was fully characterized in solution, gas phase, and solid state study. Furthermore, B4P4 acts as an efficient TRH inhibitor even at [macrocycle]:[drug] <1:300, both in membrane transport and cellar incubation. The current work provides an unprecedented strategy for macrocycles to be efficiently used in drug target therapy.

Site-selective modification of proteins using cucurbit[7]uril as supramolecular protection for N-terminal aromatic amino acids

Cucurbit[7,8]urils are known to form inclusion complexes with aromatic amino acids, hosting the hydrohobic side chains within the cavity and adjacent cations within the portal of the macrocyclic host. Here we show that cucurbit[7]uril binding with N-terminal phenylalanine significantly reduces the nucleophilicity of the amine, likely due to an increase in stability of the ammonium ion, rendering it unreactive at neutral pH. Using insulin as a model protein, we show that this supramolecular protection strategy can drive selectivity of N-terminal amine conjugation away from the preferred B chain N-terminal phenylalanine towards the A chain N-terminal glycine. Cucurbit[7]uril can therefore be used as a supramolecular protecting group for site-selective protein modification.

pH/ROS Dual-Responsive Supramolecular Vesicles Fabricated by Carboxylated Pillar[6]arene-Based Host-Guest Recognition and Phenylboronic Acid Pinacol Ester Derivative

The pH and reactive oxygen species (ROS) dual-responsive supramolecular vesicle utilizing a novel host-guest molecular recognition between a phenylboronic acid pinacol ester derivative carrying long alkyl chain (PBEC₁₂A) and carboxylated pillar[6]arene (CP[6]) is developed. The host-guest complexation between CP[6] and PBEC₁₂A was first studied in aqueous solution. PBEC₁₂A was encapsulated within CP[6] forming a stable host-guest complex with a binding constant as high as 10⁶ M^-1 order of magnitude. The driving force behind such a host-guest recognition was the combination of electrostatic interaction and hydrophobic effect. Then, the self-assembly of the supra-amphiphiles of PBEC₁₂A-CP[6] host-guest complexes was investigated in aqueous solution through high-resolution transmission electron microscope and dynamic light scattering. It was found that the supra-amphiphiles self-assembled into supramolecular vesicles and the size of the self-assembled supramolecular vesicles could be tuned from 25 to 200 nm by varying the ratio of CP[6] to PBEC₁₂A. To demonstrate the pH- and ROS-responsive properties of the self-assembled vesicles, the supramolecular vesicles self-assembled from PBEC₁₂A/CP[6] (5:1) were utilized. The Nile Red loading and release studies demonstrated that the supramolecular vesicles possessed good pH/ROS dual-responsive properties. This study enriches the field of supra-amphiphile based on noncovalent interactions. It is anticipated that the pH/ROS dual-responsive supramolecular vesicles have potential applications in drug-delivery systems because both the stimuli are in close relation with specific microenvironments of tumors and relevant diseases of the human body.

Alkyl-Substituted Cucurbit[6]uril Bridged β-Cyclodextrin Dimer Mediated Intramolecular FRET Behavior

A novel triazolyl bridged cucurbituril (CB)-cyclodextrin (CD) dimer was synthesized via click reaction of monopropargyl modified octamethylcucurbit[6]uril and mono-6-azido-β-cyclodextrin. Moreover, it could form stable supramolecular inclusion complexes possessing efficient fluorescence resonance energy transfer, which benefited from the fact that CD and CB can bind amantadine- and pyridinium-containing fluorophores simultaneously. The supramolecular inclusion complex behaviors were investigated by NMR spectroscopy, UV-vis absorption, and fluorescence spectroscopy.

Morphology transformation of pillararene-based supramolecular nanostructures

In this feature article, the construction methods and the factors that influence the morphological transformation of pillararene-based supramolecular nanostructures are reviewed.

Cucurbit[8]uril-based supramolecular nanocapsules with a multienzyme-cascade antioxidative effect

A supramolecular nanocapsule was constructed by the ternary host-guest complexation of azobenzene (Azo) and methylviologen (MV) to cucurbit[8]uril (CB[8]) and the subsequent self-assembly. The supramolecular nanocapsule with both glutathione peroxidase (GPx) and superoxide dismutase (SOD) activities can mimic the intracellular enzymatic reactive oxygen species (ROS) defense system.

Fabrication of nor-seco-cucurbit[10]uril based supramolecular polymers via self-sorting

A nor-seco-CB[10] (ns-CB[10]) based linear supramolecular polymer is firstly fabricated via self-sorting strategy. Through self-sorting of the monomer, ns-CB[10] and CB[7], the unfavorable factors for supramolecular polymerization are avoided. Therefore, supramolecular polymer with high molecular weight is successfully fabricated, and the molecular weight can be controllably regulated.