Cucurbit[7]uril–Tetramethylrhodamine Conjugate for Direct Sensing and Cellular Imaging

Single Molecular Nanomedicines Based on Macrocyclic Carrier-Drug Conjugates for Concentration-Independent Encapsulation and Precise Activation of Drugs

Nanomedicines often rely on noncovalent self-assembly and encapsulation for drug loading and delivery. However, challenges such as reproducibility issues due to the multicomponent nature, off-target activation caused by premature drug release, and complex pharmacokinetics arising from assembly dissociation have hindered their clinical translation. In this study, we introduce an innovative design concept termed single molecular nanomedicine (SMNM) based on macrocyclic carrier-drug conjugates. Through the covalent linkage of two chemotherapy drugs to a hypoxia-cleavable macrocyclic carrier, azocalix[4]arene, we obtained two self-included complexes to serve as SMNMs. The intramolecular inclusion feature of the SMNMs has not only demonstrated comprehensive shielding and protection for the drugs but also effectively prevented off-target drug leakage, thereby significantly reducing their side effects and enhancing their antitumor therapeutic efficacy. Additionally, the attributes of being a single component and molecularly dispersed confer advantages such as ease of preparation and good reproducibility for SMNMs, which is desirable for clinical applications.

Adamantylglycine as a high-affinity peptide label for membrane transport monitoring and regulation

The non-canonical amino acid adamantylglycine (Ada) is introduced into peptides to allow high-affinity binding to cucurbit[7]uril (CB7). Introduction of Ada into a cell-penetrating peptide (CPP) sequence had minimal influence on the membrane transport, yet enabled up- and down-regulation of the membrane transport activity.

Phosphorescent acyclic cucurbituril solid supramolecular multicolour delayed fluorescence behaviour

Organic photoluminescent macrocyclic hosts have been widely advanced in many fields. Phosphorescent hosts with the ability to bind organic guests have rarely been reported. Herein, acyclic cucurbituril modified with four carboxylic acids (ACB-COOH) is mined to present uncommon purely organic room-temperature phosphorescence (RTP) at 510 nm with a lifetime of 1.86 μs. Its RTP properties are significantly promoted with an extended lifetime up to 2.12 s and considerable quantum yield of 6.29% after assembly with a polyvinyl alcohol (PVA) matrix. By virtue of the intrinsic self-crimping configuration of ACB-COOH, organic guests, including fluorescence dyes (Rhodamine B (RhB) and Pyronin Y (PyY)) and a drug molecule (morphine (Mor)), could be fully encapsulated by ACB-COOH to attain energy transfer involving phosphorescent acyclic cucurbituril. Ultimately, as-prepared systems are successfully exploited to establish multicolor afterglow materials and visible sensing of morphine. As an expansion of phosphorescent acyclic cucurbituril, the host afterglow color can be readily regulated by attaching different aromatic sidewalls. This study develops the fabrication strategies and application scope of a supramolecular phosphorescent host and opens up a new direction for the manufacture of intelligent long-lived luminescent materials.

Water-soluble pillar[6]arene bearing pyrene on alternating methylene bridges for direct spermine sensing

This paper describes the design and synthesis of a conjugate, which is composed of a percarboxylated water-soluble pillar[6]arene and three fluorescent pyrene chromophores on alternating methylene bridges. The optical characteristics are investigated. This conjugate is capable of encapsulating polycationic guest spermine, which results in an enhancement in the fluorescence intensity of pyrene. This host-pyrene conjugate is used for direct sensing of spermine, which shows selectivity towards a variety of biological analytes. The detection of spermine is demonstrated in live cells.

Synthetic Receptors for Early Detection and Treatment of Cancer

Over recent decades, synthetic macrocyclic compounds have attracted interest from the scientific community due to their ability to selectively and reversibly form complexes with a huge variety of guest moieties. These molecules have been studied within a wide range of sensing and other fields. Within this review, we will give an overview of the most common synthetic macrocyclic compounds including cyclodextrins, calixarenes, calixresorcinarenes, pillarenes and cucurbiturils. These species all display the ability to form a wide range of complexes. This makes these compounds suitable in the field of cancer detection since they can bind to either cancer cell surfaces or indeed to marker compounds for a wide variety of cancers. The formation of such complexes allows sensitive and selective detection and quantification of such guests. Many of these compounds also show potential for the detection and encapsulation of environmental carcinogens. Furthermore, many anti-cancer drugs, although effective in in vitro tests, are not suitable for use directly for cancer treatment due to low solubility, inherent instability in in vivo environments or an inability to be adsorbed by or transported to the required sites for treatment. The reversible encapsulation of these species in a macrocyclic compound can greatly improve their solubility, stability and transport to required sites where they can be released for maximum therapeutic effect. Within this review, we intend to present the use of these species both in cancer sensing and treatment. The various macrocyclic compound families will be described, along with brief descriptions of their synthesis and properties, with an outline of their use in cancer detection and usage as therapeutic agents. Their use in the sensing of environmental carcinogens as well as their potential utilisation in the clean-up of some of these species will also be discussed.

Binding affinity-based intracellular drug detection enabled by a unimolecular cucurbit[7]uril-dye conjugate

Label-free fluorescence-based chemosensing has been increasingly brought into focus due to its simplicity and high sensitivity for intracellular monitoring of molecules. Currently used methods, such as conventional indicator displacement assays (IDAs), pose limitations related to dissociation upon dilution, random diffusion of the released indicators, and high sensitivity to interference by agents from the ambient cellular environment (e.g., salts, enzymes, and proteins). Herein we report a potentially widely applicable strategy to overcome the limitations of conventional IDAs by employing a macrocyclic cucurbit[7]uril (CB7) host covalently coupled to a nitrobenzoxadiazole (NBD) fluorescent dye (CB7-NBD conjugate). As a proof of concept, we demonstrated that the CB7-NBD unimolecular conjugate responded to various target analytes even in the complex live cell system. Moreover, the sensing system was compatible with fluorescence imaging, fluorescence-assisted cell sorting (FACS), and fluorescence spectrometry with a microplate reader. These experiments demonstrated an application of covalently bound unimolecular CB7-NBD conjugate as a sensor for detecting diverse analytes in the intracellular compartment of live cells.

Challenges and Opportunities of Functionalized Cucurbiturils for Biomedical Applications

Cucurbit[n]uril (CB[n]) macrocycles (especially CB[5] to CB[8]) have shown exceptional attributes since their discovery in 2000. Their stability, water solubility, responsiveness to several stimuli, and remarkable binding properties have enabled a growing number of biological applications. Yet, soon after their discovery, the challenge of their functionalization was set. Nevertheless, after more than two decades, a myriad of CB[n] derivatives has been described, many of them used in cells or in vivo for advanced applications. This perspective summarizes key advances of this burgeoning field and points to the next opportunities and remaining challenges to fully express the potential of these fascinating macrocycles in biology and biomedical sciences.

Cucurbit[8]uril-based water-dispersible assemblies with enhanced optoacoustic performance for multispectral optoacoustic imaging

Organic small-molecule contrast agents have attracted considerable attention in the field of multispectral optoacoustic imaging, but their weak optoacoustic performance resulted from relatively low extinction coefficient and poor water solubility restrains their widespread applications. Herein, we address these limitations by constructing supramolecular assemblies based on cucurbit[8]uril (CB[8]). Two dixanthene-based chromophores (DXP and DXBTZ) are synthesized as the model guest compounds, and then included in CB[8] to prepare host-guest complexes. The obtained DXP-CB[8] and DXBTZ-CB[8] display red-shifted and increased absorption as well as decreased fluorescence, thereby leading to a substantial enhancement in optoacoustic performance. Biological application potential of DXBTZ-CB[8] is investigated after co-assembly with chondroitin sulfate A (CSA). Benefiting from the excellent optoacoustic property of DXBTZ-CB[8] and the CD44-targeting feature of CSA, the formulated DXBTZ-CB[8]/CSA can effectively detect and diagnose subcutaneous tumors, orthotopic bladder tumors, lymphatic metastasis of tumors and ischemia/reperfusion-induced acute kidney injury in mouse models with multispectral optoacoustic imaging.

Preparation and In Vitro Validation of a Cucurbit[7]uril-Peptide Conjugate Targeting the LDL Receptor

Here we report the coupling of a cyclic peptide (VH4127) targeting the low density lipoprotein (LDL) receptor (LDLR) noncompetitively to cucurbit[7]uril (CB[7]) to develop a new kind of drug delivery system (DDS), namely, CB[7]-VH4127, with maintained binding affinity to the LDLR. To evaluate the uptake potential of this bismacrocyclic compound, another conjugate was prepared comprising a high-affinity group for CB[7] (adamantyl(Ada)-amine) coupled to the fluorescent tracker Alexa680 (A680). The resulting A680-Ada·CB[7]-VH4127 supramolecular complex demonstrated conserved LDLR-binding potential and improved LDLR-mediated endocytosis and intracellular accumulation potential in LDLR-expressing cells. The combination of two technologies, namely, monofunctionalized CB[7] and the VH4127 LDLR-targeting peptide, opens new avenues in terms of targeting and intracellular delivery to LDLR-expressing tissues or tumors. The versatile transport capacity of CB[7], known to bind a large spectrum of bioactive or functional compounds, makes this new DDS suitable for a wide range of therapeutic or imaging applications.

Two-photon fluorescence imaging and specifically biosensing of norepinephrine on a 100-ms timescale

Norepinephrine (NE) is a key neurotransmitter in the central nervous system of organisms; however, specifically tracking the transient NE dynamics with high spatiotemporal resolution in living systems remains a great challenge. Herein, we develop a small molecular fluorescent probe that can precisely anchor on neuronal cytomembranes and specifically respond to NE on a 100-ms timescale. A unique dual acceleration mechanism of molecular-folding and water-bridging is disclosed, which boosts the reaction kinetics by ˃10⁵ and ˃10³ times, respectively. Benefiting from its excellent spatiotemporal resolution, the probe is applied to monitor NE dynamics at the single-neuron level, thereby, successfully snapshotting the fast fluctuation of NE levels at neuronal cytomembranes within 2 s. Moreover, two-photon fluorescence imaging of acute brain tissue slices reveals a close correlation between downregulated NE levels and Alzheimer's disease pathology as well as antioxidant therapy.

A supramolecular cucurbit[8]uril-based rotaxane chemosensor for the optical tryptophan detection in human serum and urine

Sensing small biomolecules in biofluids remains challenging for many optical chemosensors based on supramolecular host-guest interactions due to adverse interplays with salts, proteins, and other biofluid components. Instead of following the established strategy of developing alternative synthetic binders with improved affinities and selectivity, we report a molecular engineering approach that addresses this biofluid challenge. Here we introduce a cucurbit[8]uril-based rotaxane chemosensor feasible for sensing the health-relevant biomarker tryptophan at physiologically relevant concentrations, even in protein- and lipid-containing human blood serum and urine. Moreover, this chemosensor enables emission-based high-throughput screening in a microwell plate format and can be used for label-free enzymatic reaction monitoring and chirality sensing. Printed sensor chips with surface-immobilized rotaxane-microarrays are used for fluorescence microscopy imaging of tryptophan. Our system overcomes the limitations of current supramolecular host-guest chemosensors and will foster future applications of supramolecular sensors for molecular diagnostics.

Cucurbituril Enhanced Electrochemiluminescence of Gold Nanoclusters via Host-Guest Recognition for Sensitive D-Dimer Sensing

Gold nanoclusters (AuNCs) are promising electrochemiluminescence (ECL) signal probes for their outstanding biocompatibility, unusual molecule-like structures, and versatile optical and electrochemical properties. Nevertheless, their relatively low ECL efficiency and poor stability in aqueous solutions hindered their application in the ECL sensing field. Herein, a facile host-guest recognition strategy was proposed to enhance the ECL efficiency and stability of Au NCs by rigidifying the surface of ligand-stabilized AuNCs via supramolecular self-assembly between cucurbiturils[7] (CB[7]) and l-phenylalanine (l-Phe). Meanwhile, mercaptopropionic acid (MPA) was introduced as a ligand in order to cooperatively enhance the performance of the AuNCs and facilitate the link between AuNCs and bioactive substances. The prepared CB[7]/l-Phe/MPA-AuNCs had a higher ECL emission efficiency, achieving about 2-fold stronger ECL intensity than that of l-Phe/MPA-AuNCs. In addition, after non-covalent modification with CB[7], the finite stability of the papered AuNCs was significantly improved. The prepared CB[7]/l-Phe/MPA-AuNCs showed excellent D-dimer sensing results, exhibiting a linear range from 50.00 fg/mL to 100.0 ng/mL and a detection limit of 29.20 fg/mL (S/N = 3). Our work demonstrated that the host-guest self-assembly strategy provided a universal approach for strengthening the ECL efficiency and stability of nanostructures on an ultra-small scale.

Supramolecular Activation of S8 by Cucurbiturils in Water and Mechanism of Reduction to H2S by Thiols: Insights into Biological Sulfane Sulfur Trafficking

Reactive sulfur species (RSS) play critical roles in diverse chemical environments. Molecules containing sulfane sulfur (S⁰) have emerged as key species involved in cellular redox buffering as well as RSS generation, translocation, and action. Using cucurbit[7]uril (CB[7]) as a model hydrophobic host, we demonstrate here that S₈ can be encapsulated to form a 1:1 host guest complex, which was confirmed by solution state experiments, mass spectrometry, and X-ray crystallography. The solid state structure of CB[7]/S₈ shows that the encapsulated S₈ is available to nucleophiles through the carbonyl portals of the host. Treatment of CB[7]/S₈ with thiols results in efficient reduction of S₈ to H₂S in water at physiological pH. We establish that encapsulated S₈ is attacked by a thiol within the CB[7] host and that the resultant soluble hydropolysulfide is ejected into solution, where it reacts further with thiols to generate soluble sulfane sulfur carriers and ultimately H₂S. The formation of these intermediate is supported by observed kinetic saturation behavior, competitive inhibition experiments, and alkylative trapping experiments. We also demonstrate that CB[7]/S₈ can be used to increase sulfane sulfur levels in live cells using fluorescence microscopy. More broadly, this work suggests a general activation mechanism of S₈ by hydrophobic motifs, which may be applicable to proteins, membranes, or other bimolecular compartments that could transiently bind and solubilize S₈ to promote reaction with thiols to solubilize and shuttle S₈ back into the redox labile sulfane sulfur pool. Such a mechanism would provide an attractive manifold in which to understand the RSS translocation and trafficking.

Cucurbit[7]uril Macrocyclic Sensors for Optical Fingerprinting: Predicting Protein Structural Changes to Identifying Disease-Specific Amyloid Assemblies

In a three-dimensional (3D) representation, each protein molecule displays a specific pattern of chemical and topological features, which are altered during its misfolding and aggregation pathway. Generating a recognizable fingerprint from such features could provide an enticing approach not only to identify these biomolecules but also to gain clues regarding their folding state and the occurrence of pathologically lethal misfolded aggregates. We report here a universal strategy to generate a fluorescent fingerprint from biomolecules by employing the pan-selective molecular recognition feature of a cucurbit[7]uril (CB[7]) macrocyclic receptor. We implemented a direct sensing strategy by covalently tethering CB[7] with a library of fluorescent reporters. When CB[7] recognizes the chemical and geometrical features of a biomolecule, it brings the tethered fluorophore into the vicinity, concomitantly reporting the nature of its binding microenvironment through a change in their optical signature. The photophysical properties of the fluorophores allow a multitude of probing modes, while their structural features provide additional binding diversity, generating a distinct fluorescence fingerprint from the biomolecule. We first used this strategy to rapidly discriminate a diverse range of protein analytes. The macrocyclic sensor was then applied to probe conformational changes in the protein structure and identify the formation of oligomeric and fibrillar species from misfolded proteins. Notably, the sensor system allowed us to differentiate between different self-assembled forms of the disease-specific amyloid-β (Aβ) aggregates and segregated them from other generic amyloid structures with a 100% identification accuracy. Ultimately, this sensor system predicted clinically relevant changes by fingerprinting serum samples from a cohort of pregnant women.

Further Dimensions for Sensing in Biofluids: Distinguishing Bioorganic Analytes by the Salt-Induced Adaptation of a Cucurbit[7]uril-Based Chemosensor

Insufficient binding selectivity of chemosensors often renders biorelevant metabolites indistinguishable by the widely used indicator displacement assay. Array-based chemosensing methods are a common workaround but require additional effort for synthesizing a chemosensor library and setting up a sensing array. Moreover, it can be very challenging to tune the inherent binding preference of macrocyclic systems such as cucurbit[n]urils (CBn) by synthetic means. Using a novel cucurbit[7]uril-dye conjugate that undergoes salt-induced adaptation, we now succeeded in distinguishing 14 bioorganic analytes from each other through the facile stepwise addition of salts. The salt-specific concentration-resolved emission provides additional information about the system at a low synthetic effort. We present a data-driven approach to translate the human-visible curve differences into intuitive pairwise difference measures. Ion mobility experiments combined with density functional theory calculations gave further insights into the binding mechanism and uncovered an unprecedented ternary complex geometry for CB7. TThis work introduces the non-selectively binding, salt-adaptive cucurbit[n]uril system for sensing applications in biofluids such as urine, saliva, and blood serum.

Clustering emission of cucurbit[n]urils in the solid- and solution-state induced by the outer surface interactions of cucurbit[n]urils

Atypical luminescent compounds that do not contain conventional chromophores emit light due to clustering and have important basic research value and a broad range of potential applications. To date, most atypical luminescent compounds are small molecules or polymers containing groups such as cyano, carbonyl and hydroxyl. In this work, driven by some sporadic and accidental luminescence phenomena observed for cucurbit[n]urils (Q[n]s), the luminescent properties and mechanism of Q[n]s in the solid- and solution-state were systematically studied and the clustering emission of Q[n]s confirmed. Our experiments have revealed that the self-induced outer-surface interactions of Q[n]s (OSIQ) are the most important driving force resulting in the clustering emission of Q[n]s. Substances that can weaken the effect of self-induced OSIQ, such as the presence of various aromatic compounds and anions, may weaken or quench the clustering emission of Q[n]s. This not only reveals the new characteristics and mechanism of the clustering emission of Q[n]s, but also provides new insights on how to utilize the clustering emission of Q[n]s and construct new types of macrocyclic luminescence systems.

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 CRISPR-OFF switches with host-guest chemistry

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeat) technology is a powerful tool in biology and medicine. However, the safety and application of this technology is hampered by excessive activity of CRISPR machinery. It is particularly important to develop methods for switching off CRISPR activity in human cells. The current study demonstrates the concept of supramolecular CRISPR-OFF switches by employing host-guest chemistry. We demonstrate that the CRISPR systems show considerable tolerance to adamantoylation on guide RNAs (gRNAs), whereas supramolecular complexation tremendously affects the function of adamantoyl gRNAs. Host-guest chemistry is demonstrated to be novel and effective tools to reduce unwanted excessive activities of CRISPR complexes in human cells. This work indicates considerable potential of supramolecular strategy for controlling and enhancing CRISPR systems.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

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.

Cell-surface Labeling via Bioorthogonal Host-Guest Chemistry

The widespread adoption of the bioorthogonal chemical reporter strategy revolutionized chemical biology. However, its translation to living mammals has been challenging, due to the size/stability properties of the chemical reporter group and/or the reaction kinetics of the labeling step. While developing new bioorthogonal reactions has been the traditional approach to optimizing the bioorthogonal chemical reporter strategy, here we present a different avenue, leveraging intermolecular interactions, to create bioorthogonal host-guest pairs. This approach, deemed "bioorthogonal complexation, does not rely on activated functional groups or second-order rate constants. We utilize the cucurbit[7]uril (CB[7]) scaffold to showcase bioorthogonal complexation and determine that medium-affinity (K_a ≈ 10⁸-10⁹ M^-1) guests efficiently label cell surfaces and outperform the strain-promoted azide-alkyne cycloaddition. Finally, we implement bioorthogonal complexation in the chemical reporter strategy through the metabolic incorporation of ortho-carborane into cell-surface glycans and detection with a CB[7]-fluorescein conjugate.

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.

Chiroptical sensing of amino acids, amines, amino alcohols, alcohols and terpenes with π-extended acyclic cucurbiturils

The efficiency and scope of two acyclic π-wall extended cucurbiturils, M2 and M3, exhibiting rapidly interconverting helical conformers for chiroptical sensing of amines, amino acids, alcohols, and terpenes at micromolar concentrations in water is evaluated. The formation of 1 : 1 host-guest complexes results in spontaneous induction of circular dichroism signals that can be used for accurate determination of the absolute configuration and enantiomeric composition of the analyte based on a simple mix-and-measure protocol.

Synthesis of an AIEgen functionalized cucurbit[7]uril for subcellular bioimaging and synergistic photodynamic therapy and supramolecular chemotherapy

Aggregation-induced emission (AIE) based fluorophores (AIEgens) have attracted increasing attention for biomedical applications due to their unique optical properties. Here we report an AIE photosensitizer functionalized CB[7], namely AIECB[7], which could spontaneously self-assemble into nanoaggregates in aqueous solutions. Interestingly, the carbonyl-lace of CB[7] may potentially act as a proton acceptor in an acidic environment to fine-tune the fluorescence and singlet oxygen generation of AIECB[7] nanoaggregates by regulating the inner stacking of AIEgens. Additionally, benefiting from the guest-binding properties of CB[7], oxaliplatin was included into AIECB[7] nanoaggregates for combined photodynamic therapy and supramolecular chemotherapy. To show the modular versatility of this supramolecular system, a hypoxia-activatable prodrug banoxantrone (AQ4N) was loaded into AIECB[7] nanoaggregates, which exhibited synergistic antitumor effects on a multicellular tumor spheroid model (MCTS). This work not only provides AIECB[7] for versatile theranostic applications, but also offers important new insights into the design and development of macrocycle-conjugated AIE materials for diverse biomedical applications.

Mimicking Biological Recognition: Lessons in Binding Hydrophilic Guests in Water

Selective molecular recognition of hydrophilic guests in water plays a fundamental role in a vast number of biological processes, but synthetic mimicry of biomolecular recognition in water still proves challenging both in terms of achieving comparable affinities and selectivities. This Review highlights strategies that have been developed in the field of supramolecular chemistry to selectively and non-covalently bind three classes of biologically relevant molecules: nucleotides, carbohydrates, and amino acids. As several groups have systematically modified receptors for a specific guest, an evolutionary perspective is also provided in some cases. Trends in the most effective binding forces for each class are described, providing insight into selectivity and potential directions for future work.

Carborane Guests for Cucurbit[7]uril Facilitate Strong Binding and On-Demand Removal

High-affinity guests have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but hindering CB[7] materials from being returned to their guest-free state for reuse. Here, we present polyhedral boron clusters (carboranes) as strongly binding, yet easily removable, guests for CB[7]. Aided by a Pd-catalyzed coupling of an azide anion, we prepared boron-functionalized 9-amino-ortho-carborane that binds to CB[7] with a K_a ≈ 10¹⁰ M^-1. Upon basic treatment, ortho-carborane readily undergoes deboronation to yield anionic nido-carborane, a poor guest for CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified ortho-carborane guest by recycling a CB[7]-functionalized resin. With this report, we introduce stimuli-responsive decomplexation as an additional consideration in the design of high-affinity host-guest complexes.

Covalent cucurbit[7]uril-dye conjugates for sensing in aqueous saline media and biofluids

Non-covalent chemosensing ensembles of cucurbit[n]urils (CBn) have been widely used in proof-of-concept sensing applications, but they are prone to disintegrate in saline media, e.g. biological fluids. We show here that covalent cucurbit[7]uril-indicator dye conjugates are buffer- (10× PBS buffer) and saline-stable (up to 1.4 M NaCl) and allow for selective sensing of Parkinson's drug amantadine in human urine and saliva, where the analogous non-covalent CB7⊃dye complex is dysfunctional. The in-depth analysis of the covalent host-dye conjugates in the gas-phase, and deionized versus saline aqueous media revealed interesting structural, thermodynamic and kinetic effects that are of general interest for the design of CBn-based supramolecular chemosensors and systems. This work also introduces a novel high-affinity indicator dye for CB7 through which fundamental limitations of indicator displacement assays (IDA) were exposed, namely an impractical slow equilibration time. Unlike non-covalent CBn⊃dye reporter pairs, the conjugate chemosensors can also operate through a SN2-type guest-dye exchange mechanism, which shortens assay times and opens new avenues for tailoring analyte-selectivity.

Host-Guest Protein Assembly for Affinity Purification of Methyllysine Proteomes

Protein-protein interactions drive self-assembly of biomacromolecules and thus enable important physiological functions at a cellular level. Supramolecular chemists have developed artificial host-guest interactions that are similar with, yet distinct from and orthogonal to, the natural protein-protein interactions. For instance, cucurbit[n]urils are synthetic receptors that can specifically recognize proteins with N-terminal aromatic residues with high affinities, yet this interaction can be reversed by the competition of small molecules such as amantadine. Herein, we develop a site-specific, oriented protein-display method by combining the host-guest interaction based on cucurbit[7]uril and a covalent protein-peptide reaction. A methyllysine-binding protein HP1β chromodomain (CD) is immobilized via host-guest interactions and used as the "bait" to capture methyllysine proteomes from cancer cells. The captured "fish"-methyllysine-containing proteins-can be released via competitive displacement by amantadine in a nondenaturing and traceless manner. This affinity purification method found 73 novel methyllysine sites from 101 identified sites among 66 methylated proteins from 255 HP1β CD-binding proteins in cancer cells via subsequent mass spectrometric analysis. This work thereby presents a new strategy of artificial host-guest protein assembly in affinity purification of methyllysine proteins in coupling to mass spectrometry.

Cucurbit[7]uril-conjugated dyes as live cell imaging probes: investigation on their cellular uptake and excretion pathways

Here we report the endocytosis and excretion pathways of two different dye-conjugated cucurbit[7]urils, (cyanine 3-conjugated CB[7] and rhodamine X-conjugated CB[7]), which have great potential as molecular probes for live cell imaging. The dye-CB[7]s are translocated into live cells (human breast carcinoma cells, MCF-7) via multiple pathways, predominantly by clathrin-mediated endocytosis, and excreted from cells via lysosome-associated exocytosis. Interestingly, the CB[7] moiety has a substantial influence on the uptake and excretion pathways. These findings may widen the applications of the dyes conjugated to CB[7] and assist in the design of new molecular probes for live cell imaging.

Supramolecular chemistry of substituted cucurbit[n]urils

This review covers important advances in the field of substituted cucurbit[n]urils.

Dynamic host-guest interaction enables autonomous single molecule blinking and super-resolution imaging

Synthetic host-guest complexes are inherently dynamic as they employ weak and reversible noncovalent interactions for their recognition processes. We strategically exploited dynamic supramolecular recognition between fluorescently labeled guest molecules to complementary cucurbit[7]uril hosts to obtain stochastic switching between fluorescence ON- and OFF-states, enabling PAINT-based nanoscopic imaging in cells and tissues.

Rational design and implementation of a cucurbit[8]uril-based indicator-displacement assay for application in blood serum

In this study, we report the first supramolecular indicator displacement assay (IDA) based on cucurbit[n]uril (CBn) host and a [2.2]paracyclophane derivative as indicator that is operational in blood serum.

In this study, we report the first supramolecular indicator-displacement assay (IDA) based on cucurbit[n]uril (CBn) hosts that is operational in blood serum. Rational design principles for host–guest chemosensing in competitively binding media were derived through detailed mathematical simulations. It was shown that currently known CBn-based chemosensing ensembles are not suited for use in highly competitive matrices such as blood serum. Conversely, the simulations indicated that a combination of cucurbit[8]uril (CB8) and an ultra-high affinity dye would be a promising IDA reporter pair for the detection of Alzheimer's drug memantine in blood serum. Therefore, a novel class of [2.2]paracyclophane-derived indicator dyes for the host CB8 was developed that possesses one of the highest host–guest affinities (K_a > 10¹² M^–1 in water) known in supramolecular host–guest chemistry, and which provides a large Stokes shift (up to 200 nm). The novel IDA was then tested for the detection of memantine in blood serum in a physiologically relevant sub- to low micromolar concentration range.

Ratiometric DNA sensing with a host-guest FRET pair

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

Cucurbit[n]uril-based amphiphiles that self-assemble into functional nanomaterials for therapeutics

In this feature article, the two types (molecular amphiphile and supramolecular amphiphile) of CB-based amphiphiles, their self-assemblies and their applications for useful nanotherapeutics and theranostics are presented with future perspectives.

Synthetic Host-Guest Assembly in Cells and Tissues: Fast, Stable, and Selective Bioorthogonal Imaging via Molecular Recognition

Bioorthogonal strategies are continuing to pave the way for new analytical tools in biology. Although a significant amount of progress has been made in developing covalent reaction based bioorthogonal strategies, balanced reactivity, and stability are often difficult to achieve from these systems. Alternatively, despite being kinetically beneficial, the development of noncovalent approaches that utilize fully synthetic and stable components remains challenging due to the lack of selectivity in conventional noncovalent interactions in the living cellular environment. Herein, we introduce a bioorthogonal assembly strategy based on a synthetic host–guest system featuring Cucurbit[7]uril (CB[7]) and adamantylamine (ADA). We demonstrate that highly selective and ultrastable host–guest interaction between CB[7] and ADA provides a noncovalent mechanism for assembling labeling agents, such as fluorophores and DNA, in cells and tissues for bioorthogonal imaging of molecular targets. Additionally, by combining with covalent reaction, we show that this CB[7]–ADA based noncovalent interaction enables simultaneous bioorthogonal labeling and multiplexed imaging in cells as well as tissue sections. Finally, we show that interaction between CB[7] and ADA fulfills the demands of specificity and stability that is required for assembling molecules in the complexities of a living cell. We demonstrate this by sensitive detection of metastatic cancer-associated cell surface protein marker as well as by showing the distribution and dynamics of F-actin in living cells.

Polymeric Nanomedicine with "Lego" Surface Allowing Modular Functionalization and Drug Encapsulation

Surface functionalization of nanoparticles (NPs) is of pivotal importance in nanomedicine. However, current strategies often require covalent conjugation that involves laborious design and synthesis. Herein, cucurbit[7]uril (CB[7])-decorated poly(lactic acid) (PLA)/poly(lactic-co-glycolic acid) (PLGA) NPs are developed and exploited for the first time as a novel, biocompatible, and versatile drug delivery platform with a noncovalently tailorable surface. CB[7] on the surface of NPs, acting as a "Lego" base block, allowed facile, modular surface modification with a variety of functional moieties or tags that are linked with amantadine (a complementary "Lego" piece to the base block), including amantadine-conjugated folate, polyethylene glycol, and fluorescein isothiocyanate. In addition, surface CB[7] also provided an opportunity for encapsulation of a secondary drug, such as oxaliplatin, into the cavity of the base block CB[7], in addition to a primary drug (e.g., paclitaxel) loaded into PLA/PLGA NPs, for a possible synergistic chemotherapy. This proof of concept not only provides the first versatile PLA/PLGA nanomedicine platform with "Lego" surface for modular functionalization and improved drug delivery but also offers new insights into the design and development of novel nanomedicine with a modular surface.

Rational design of boron-dipyrromethene (BODIPY) reporter dyes for cucurbit[7]uril

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

Supramolecular latching system based on ultrastable synthetic binding pairs as versatile tools for protein imaging

Here we report ultrastable synthetic binding pairs between cucurbit[7]uril (CB[7]) and adamantyl- (AdA) or ferrocenyl-ammonium (FcA) as a supramolecular latching system for protein imaging, overcoming the limitations of protein-based binding pairs. Cyanine 3-conjugated CB[7] (Cy3-CB[7]) can visualize AdA- or FcA-labeled proteins to provide clear fluorescence images for accurate and precise analysis of proteins. Furthermore, controllability of the system is demonstrated by treating with a stronger competitor guest. At low temperature, this allows us to selectively detach Cy3-CB[7] from guest-labeled proteins on the cell surface, while leaving Cy3-CB[7] latched to the cytosolic proteins for spatially conditional visualization of target proteins. This work represents a non-protein-based bioimaging tool which has inherent advantages over the widely used protein-based techniques, thereby demonstrating the great potential of this synthetic system.

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.