Benzobis(imidazolium)-cucurbit[8]uril complexes for binding and sensing aromatic compounds in aqueous solution

Cucurbit[8]uril Binds Nonterminal Dipeptide Sites with High Affinity and Induces a Type II β-Turn

In an effort to target polypeptides at nonterminal sites, we screened the binding of the synthetic receptor cucurbit[8]uril (Q8) to a small library of tetrapeptides, each containing a nonterminal dipeptide binding site. The resulting leads were characterized in detail using a combination of isothermal titration calorimetry, 1H NMR spectroscopy, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and X-ray crystallography. The equilibrium dissociation constant values determined for the binding of Q8 to nonterminal dipeptide sites Lys-Phe (KF) and Phe-Lys (FK) were 60 and 86 nm, respectively. These are to the best of our knowledge the highest affinities reported to date for any synthetic receptor targeting a nonterminal site on an unmodified peptide. A 0.79 Å resolution crystal structure was obtained for the complex of Q8 with the peptide Gly-Gly-Leu-Tyr-Gly-Gly-Gly (GGLYGGG) and reveals structural details of the pair-inclusion motif. The molecular basis for recognition is established to be the inclusion of the side chains of Leu and Tyr residues, as well as an extensive network of hydrogen bonds between the peptide backbone, the carbonyl oxygens of Q8, and proximal water molecules. In addition, the crystal structure reveals that Q8 induces a type II β-turn. The sequence-selectivity, high affinity, reversibility, and detailed structural characterization of this system should facilitate the development of applications involving ligand-induced polypeptide folding.

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.

Molecular Recognition in Water Using Macrocyclic Synthetic Receptors

Molecular recognition in water using macrocyclic synthetic receptors constitutes a vibrant and timely research area of supramolecular chemistry. Pioneering examples on the topic date back to the 1980s. The investigated model systems and the results derived from them are key for furthering our understanding of the remarkable properties exhibited by proteins: high binding affinity, superior binding selectivity, and extreme catalytic performance. Dissecting the different effects contributing to the proteins' properties is severely limited owing to its complex nature. Molecular recognition in water is also involved in other appreciated areas such as self-assembly, drug discovery, and supramolecular catalysis. The development of all these research areas entails a deep understanding of the molecular recognition events occurring in aqueous media. In this review, we cover the past three decades of molecular recognition studies of neutral and charged, polar and nonpolar organic substrates and ions using selected artificial receptors soluble in water. We briefly discuss the intermolecular forces involved in the reversible binding of the substrates, as well as the hydrophobic and Hofmeister effects operating in aqueous solution. We examine, from an interdisciplinary perspective, the design and development of effective water-soluble synthetic receptors based on cyclic, oligo-cyclic, and concave-shaped architectures. We also include selected examples of self-assembled water-soluble synthetic receptors. The catalytic performance of some of the presented receptors is also described. The latter process also deals with molecular recognition and energetic stabilization, but instead of binding ground-state species, the targets become elusive counterparts: transition states and other high-energy intermediates.

SAMPL7 Host-Guest Challenge Overview: assessing the reliability of polarizable and non-polarizable methods for binding free energy calculations

The SAMPL challenges focus on testing and driving progress of computational methods to help guide pharmaceutical drug discovery. However, assessment of methods for predicting binding affinities is often hampered by computational challenges such as conformational sampling, protonation state uncertainties, variation in test sets selected, and even lack of high quality experimental data. SAMPL blind challenges have thus frequently included a component focusing on host-guest binding, which removes some of these challenges while still focusing on molecular recognition. Here, we report on the results of the SAMPL7 blind prediction challenge for host-guest affinity prediction. In this study, we focused on three different host-guest categories-a familiar deep cavity cavitand series which has been featured in several prior challenges (where we examine binding of a series of guests to two hosts), a new series of cyclodextrin derivatives which are monofunctionalized around the rim to add amino acid-like functionality (where we examine binding of two guests to a series of hosts), and binding of a series of guests to a new acyclic TrimerTrip host which is related to previous cucurbituril hosts. Many predictions used methods based on molecular simulations, and overall success was mixed, though several methods stood out. As in SAMPL6, we find that one strategy for achieving reasonable accuracy here was to make empirical corrections to binding predictions based on previous data for host categories which have been studied well before, though this can be of limited value when new systems are included. Additionally, we found that alchemical free energy methods using the AMOEBA polarizable force field had considerable success for the two host categories in which they participated. The new TrimerTrip system was also found to introduce some sampling problems, because multiple conformations may be relevant to binding and interconvert only slowly. Overall, results in this challenge tentatively suggest that further investigation of polarizable force fields for these challenges may be warranted.

Selective Recognition of Amino Acids and Peptides by Small Supramolecular Receptors

To this day, the recognition and high affinity binding of biomolecules in water by synthetic receptors remains challenging, while the necessity for systems for their sensing, transport and modulation persists. This problematic is prevalent for the recognition of peptides, which not only have key roles in many biochemical pathways, as well as having pharmacological and biotechnological applications, but also frequently serve as models for the study of proteins. Taking inspiration in nature and on the interactions that occur between several receptors and peptide sequences, many researchers have developed and applied a variety of different synthetic receptors, as is the case of macrocyclic compounds, molecular imprinted polymers, organometallic cages, among others, to bind amino acids, small peptides and proteins. In this critical review, we present and discuss selected examples of synthetic receptors for amino acids and peptides, with a greater focus on supramolecular receptors, which show great promise for the selective recognition of these biomolecules in physiological conditions. We decided to focus preferentially on small synthetic receptors (leaving out of this review high molecular weight polymeric systems) for which more detailed and accurate molecular level information regarding the main structural and thermodynamic features of the receptor biomolecule assemblies is available.

Ring-in-Ring(s) Complexes Exhibiting Tunable Multicolor Photoluminescence

One ring threaded by two other rings to form a non-intertwined ternary ring-in-rings motif is a challenging task in noncovalent synthesis. Constructing multicolor photoluminescence systems with tunable properties is also a fundamental research goal, which can lead to applications in multidimensional biological imaging, visual displays, and encryption materials. Herein, we describe the design and synthesis of binary and ternary ring-in-ring(s) complexes, based on an extended tetracationic cyclophane and cucurbit[8]uril. The formation of these complexes is accompanied by tunable multicolor fluorescence outputs. On mixing equimolar amounts of the cyclophane and cucurbit[8]uril, a 1:1 ring-in-ring complex is formed as a result of hydrophobic interactions associated with a favorable change in entropy. With the addition of another equivalent of cucurbit[8]uril, a 1:2 ring-in-rings complex is formed, facilitated by additional ion-dipole interactions involving the pyridinium units in the cyclophane and the carbonyl groups in cucurbit[8]uril. Because of the narrowing in the energy gaps of the cyclophane within the rigid hydrophobic cavities of cucurbit[8]urils, the binary and ternary ring-in-ring(s) complexes emit green and bright yellow fluorescence, respectively. A series of color-tunable emissions, such as sky blue, cyan, green, and yellow with increased fluorescence lifetimes, can be achieved by simply adding cucurbit[8]uril to an aqueous solution of the cyclophane. Notably, the smaller cyclobis(paraquat-p-phenylene), which contains the same p-xylylene linkers as the extended tetracationic cyclophane, does not form ring-in-ring(s) complexes with cucurbit[8]uril. The encapsulation of this extended tetracationic cyclophane by both one and two cucurbit[8]urils provides an incentive to design and synthesize more advanced supramolecular systems, as well as opening up a feasible approach toward achieving tunable multicolor photoluminescence with single chromophores.

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.

A Selective Cucurbit[8]uril-Peptide Beacon Ensemble for the Ratiometric Fluorescence Detection of Peptides

A convenient supramolecular strategy for constructing a ratiometric fluorescent chemosensing ensemble, consisting of a macrocyclic host (cucurbit[8]uril CB[8]), and a pyrene‐tagged amphiphilic peptide beacon (AP 1), is reported. AP 1 unfolds upon encapsulation of the pyrene termini into the hydrophobic CB[8] cavity. This changes pyrene excimer to monomer emission. Substrates with higher affinity for the CB[8] cavity can displace AP 1 from the ensemble. The released AP 1 folds again to form a pyrene excimer, which allows for the ratiometric fluorescence monitoring of the substrate. In this report, the ensemble capacity for ratiometric fluorescence monitoring of biological substrates, such as amino acid derivatives, specific peptides, and proteins, in aqueous media is demonstrated.

Fluorescent chemosensing for aromatic compounds by a supramolecular complex composed of tin(iv) porphyrin, viologen, and cucurbit[8]uril

We demonstrate fluorescent chemosensing for biologically important aromatic compounds by a supramolecular system consisting of tin(iv) porphyrin, viologen, and cucurbit[8]uril. The detection is successfully achieved by the inclusion of an aromatic analyte through the charge-transfer interaction with the viologen unit in the cavity of cucurbit[8]uril, in which the strong charge-transfer interaction interrupts the photo-induced electron transfer from the tin(iv) porphyrin core to the viologen ligands to lead to the efficient emission of the fluorescence from the porphyrin ring.

Molecular Recognition of Methionine-Terminated Peptides by Cucurbit[8]uril

This Article describes the molecular recognition of peptides containing an N-terminal methionine (Met) by the synthetic receptor cucurbit[8]uril (Q8) in aqueous solution and with submicromolar affinity. Prior work established that Q8 binds with high affinity to peptides containing aromatic amino acids, either by simultaneous binding of two aromatic residues, one from each of two different peptides, or by simultaneous binding of an aromatic residue and its immediate neighbor on the same peptide. The additional binding interface of two neighboring residues suggested the possibility of targeting nonaromatic peptides, which have thus far bound only weakly to synthetic receptors. A peptide library designed to test this hypothesis was synthesized and screened qualitatively for Q8 binding using a fluorescent indicator displacement assay. The large fluorescence response observed for several Met-terminated peptides suggested strong binding, which was confirmed quantitatively by the determination of submicromolar equilibrium dissociation constant values for Q8 binding to MLA, MYA, and MFA using isothermal titration calorimetry (ITC). This discovery of high affinity binding to Met-terminated peptides and, more generally, to nonaromatic peptides prompted a detailed investigation of the determinants of binding in this system using ITC, electrospray ionization mass spectrometry, and 1H NMR spectroscopy for 25 purified peptides. The studies establish the sequence determinants required for high-affinity binding of Met-terminated peptides and demonstrate that cucurbit[ n]uril-mediated peptide recognition does not require an aromatic residue for high affinity. These results, combined with the known ability of cucurbit[ n]urils to target N-termini and disordered loops in folded proteins, suggest that Q8 could be used to target unmodified, Met-terminated proteins.

Decoupled Associative and Dissociative Processes in Strong yet Highly Dynamic Host-Guest Complexes

Kinetics and thermodynamics in supramolecular systems are intimately linked, yet both are independently important for application in sensing assays and stimuli-responsive switching/self-healing of materials. Host-guest interactions are of particular interest in many water-based materials, sensing, and drug delivery applications. Herein we investigate the binding dynamics of a variety of electron-rich aromatic moieties forming hetero-ternary complexes with the macrocycle cucurbit[8]uril (CB[8]) and an auxiliary guest, dimethyl viologen, with high selectivity and equilibrium binding constants (K_eq up to 10¹⁴ M^-2). Using stopped-flow spectrofluorimetry, association rate constants were observed to approach the diffusion limit and were found to be insensitive to the structure of the guest. Conversely, the dissociation rate constants of the ternary complexes varied dramatically with the guest structure and were correlated with the thermodynamic binding selectivity. Hence differing molecular features were found to contribute to the associative and dissociative processes, mimicking naturally occurring reactions and giving rise to a decoupling of these kinetic parameters. Moreover, we demonstrate the ability to exploit these phenomena and selectively perturb the associative process with external stimuli (e.g., viscosity and pressure). Significantly, these complexes exhibit increased binding equilibria with increasing pressure, with important implications for the application of the CB[8] ternary complex for the formation of hydrogels, as these gels exhibit unprecedented pressure-insensitive rheological properties. A high degree of flexibility therefore exists in the design of host-guest systems with tunable kinetic and thermodynamic parameters for tailor-made applications across a broad range of fields.

Helianthus-like cucurbit[4]uril and cucurbit[5]uril analogues

Two cucurbituril-like macrocycles were synthesized by condensing cyclopentano-substituted propanediurea with formaldehyde in the presence and absence of Ca²⁺.

Selective, non-covalent conjugation of synthetic peptides with recombinant proteins mediated by host-guest chemistry

The combination of potent chemical moieties with biologically active proteins is key to some of today's most innovative therapeutic drugs. In order to obviate any chemical modification of the proteins, we present a novel and powerful strategy for the selective conjugation of recombinant protein domains with synthetically derived peptides via a cucurbit[8]uril host-guest chemistry approach.

Host-Guest Complexes of Carboxylated Pillar[n]arenes With Drugs

Pillar[n]arenes are a new family of nanocapsules that have shown application in a number of areas, but because of their poor water solubility their biomedical applications are limited. Recently, a method of synthesizing water-soluble pillar[n]arenes was developed. In this study, carboxylated pillar[n]arenes (WP[n], n = 6 or 7) have been examined for their ability to form host-guest complexes with compounds relevant to drug delivery and biodiagnostic applications. Both pillar[n]arenes form host-guest complexes with memantine, chlorhexidine hydrochloride, and proflavine by ¹H nuclear magnetic resonance and modeling. Binding is stabilized by hydrophobic effects within the cavities, and hydrogen bonding and electrostatic interactions at the portals. Encapsulation within WP[6] results in the complete and efficient quenching of proflavine fluorescence, giving rise to "on" and "off" states that have potential in biodiagnostics. The toxicity of the pillar[n]arenes was examined using in vitro growth assays with the OVCAR-3 and HEK293 cell lines. The pillar[n]arenes are relatively nontoxic to cells except at high doses and after prolonged continuous exposure. Overall, the results show that there could be a potentially large range of medical applications for carboxylated pillar[n]arene nanocapsules.

Absolute binding free energy calculations of CBClip host-guest systems in the SAMPL5 blind challenge

Herein, we report the absolute binding free energy calculations of CBClip complexes in the SAMPL5 blind challenge. Initial conformations of CBClip complexes were obtained using docking and molecular dynamics simulations. Free energy calculations were performed using thermodynamic integration (TI) with soft-core potentials and Bennett's acceptance ratio (BAR) method based on a serial insertion scheme. We compared the results obtained with TI simulations with soft-core potentials and Hamiltonian replica exchange simulations with the serial insertion method combined with the BAR method. The results show that the difference between the two methods can be mainly attributed to the van der Waals free energies, suggesting that either the simulations used for TI or the simulations used for BAR, or both are not fully converged and the two sets of simulations may have sampled difference phase space regions. The penalty scores of force field parameters of the 10 guest molecules provided by CHARMM Generalized Force Field can be an indicator of the accuracy of binding free energy calculations. Among our submissions, the combination of docking and TI performed best, which yielded the root mean square deviation of 2.94 kcal/mol and an average unsigned error of 3.41 kcal/mol for the ten guest molecules. These values were best overall among all participants. However, our submissions had little correlation with experiments.

In Vitro selectivity of an acyclic cucurbit[n]uril molecular container towards neuromuscular blocking agents relative to commonly used drugs

An acyclic cucurbit[n]uril (CB[n]) based molecular container (2, a.k.a. Calabadion 2) binds to both amino-steroidal and benzylisoquinolinium type neuromuscular blocking agents (NMBAs) in vitro, and reverses the effect of these drugs in vivo displaying faster recovery times than placebo and the γ-cyclodextrin (CD) based and clinically used reversal agent Sugammadex. In this study we have assessed the potential for other drugs commonly used during and after surgery (e.g. antibiotics, antihistamines, and antiarrhythmics) to interfere with the ability of 2 to bind NMBAs rocuronium and cisatracurium in vitro. We measured the binding affinities (Ka, M(-1)) of twenty seven commonly used drugs towards 2 and simulated the equilibrium between 2, NMBA, and drug based on their standard clinical dosages to calculate the equilibrium concentration of 2·NMBA in the presence of the various drugs. We found that none of the 27 drugs studied possess the combination of a high enough binding affinity with 2 and a high enough standard dosage to be able to promote the competitive dissociation (a.k.a. displacement interactions) of the 2·NMBA complex with the formation of the 2·drug complex. Finally, we used the simulations to explore how the potential for displacement interactions is affected by a number of factors including the Ka of the 2·NMBA complex, the Ka of the AChR·NMBA complex, the Ka of the 2·drug complex, and the dosage of the drug.

Highly fluorescent cucurbit[8]uril–perylenemonoimide host–guest complexes as efficient fluorescent probes for N-terminal phenylalanine

We have developed host–guest fluorescent complexes based on cucurbit[8]uril and a perylenemonoimide for selective and sensitive detection of N-terminal phenylalanine peptides.

Supramolecular protection from the enzymatic tyrosine phosphorylation in a polypeptide

Two pseudopeptidic cages bind the EYE peptide motif of poly(EY) in buffered water, as shown by NMR and fluorescence spectroscopy. This supramolecular interaction protects the Tyr residues from the enzymatic phosphorylation by PTK.

Stereoselective recognition of the Ac-Glu-Tyr-OH dipeptide by pseudopeptidic cages

Pseudopeptidic molecular cages are appealing receptors since they can display different polar and non-polar interaction sites in a modular framework and a controlled disposition. Inspired by previous host-guest knowledge, two pseudopeptidic molecular cages based on serine and threonine (CySer and CyThr, respectively) were designed and synthesized as hosts for the binding of the four possible stereoisomers of the Ac-Glu-Tyr-OH dipeptide, a target sequence of tyrosine kinases. The careful NMR titration experiments in aqueous acetonitrile allowed the determination of the binding constants and reflected a difference in the stability of the corresponding diastereomeric host-guest complexes. The CySer cage proved to be slightly more efficient than the CyThr counterpart, although both showed similar stereoselectivity trends: LL > DD ≥ LD > DL. This stereoselective binding was retained in the gas phase, as shown by ESI-MS competition experiments using the enantiomer-labelled method (EL), as well as CID experiments. Thus, the MS-determined discriminations follow the same trends observed by NMR, suggesting that the stereoselectivity observed for these systems must be mainly dictated by the polar host-guest interactions. Despite the stereoselective binding of short peptide sequences in competitive media being a challenging issue in supramolecular chemistry, our results demonstrate the power of pseudopeptidic cages in molecular recognition with foreseen implications in chemical biology.

Binding Studies of Cucurbit[7]uril with Gold Nanoparticles Bearing Different Surface Functionalities

Host-guest interactions between a synthetic receptor, cucurbit[7]uril (CB[7]), and gold nanoparticles (AuNPs) have been quantified using isothermal titration calorimetry. AuNPs were functionalized with ligands containing tertiary or quaternary benzylamine derivatives, with electron donating or withdrawing groups at the para position of the benzene ring. Analysis of binding interactions reveals that functional groups at the para position have no significant effect on binding constant. However, headgroups bearing a permanent positive charge increased the binding of AuNPs to CB[7] ten-fold compared to monomethyl counterparts.

Systematic strategy for designing immidazolium containing precursors to produce N-heterocyclic carbenes: a DFT study

A series of cationic N-heterocyclic carbene (NHC) precursors that can be utilized as fluorescent chemosensors for carbon dioxide capture were investigated by density functional theory (DFT) calculations. Activation energy barriers for the reactions of the cationic NHC precursors and hydrogen carbonate (HCO3(-)) based on intrinsic reaction coordinate (IRC) profiles as well as proton affinity of the precursors were compared. The calculated proton affinity of 1-ethyl-3-methylimidazol-2-yliene was in good agreement with experimental one within the margin of error. We clarified main factors to lower the activation energy barrier based on the correlation among the number of N-heterocyclic functional group, aromatic ring size, and structural characteristics for the candidate compounds. On the basis of the results, it was verified that some of our model systems spontaneously generate NHCs without any specific catalyst.

Host–guest interaction between fluoro-substituted azobenzene derivative and cyclodextrins

This system is completely opposite to the conventional azobenzene/β-CD system in that cis-F-azo-COOH fits β-CD more tightly than its trans form.

Associative chemosensing by fluorescent macrocycle–dye complexes – a versatile enzyme assay platform beyond indicator displacement

Enzymatic reactions of aromatic substrates can be monitored by fluorescence with μM sensitivity in real time by using self-assembled fluorescent receptors.

The hydrophobic effect revisited--studies with supramolecular complexes imply high-energy water as a noncovalent driving force

Traditional descriptions of the hydrophobic effect on the basis of entropic arguments or the calculation of solvent-occupied surfaces must be questioned in view of new results obtained with supramolecular complexes. In these studies, it was possible to separate hydrophobic from dispersive interactions, which are strongest in aqueous systems. Even very hydrophobic alkanes associate significantly only in cavities containing water molecules with an insufficient number of possible hydrogen bonds. The replacement of high-energy water in cavities by guest molecules is the essential enthalpic driving force for complexation, as borne out by data for complexes of cyclodextrins, cyclophanes, and cucurbiturils, for which complexation enthalpies of up to -100 kJ mol(-1) were reached for encapsulated alkyl residues. Water-box simulations were used to characterize the different contributions from high-energy water and enabled the calculation of the association free enthalpies for selected cucurbituril complexes to within a 10% deviation from experimental values. Cavities in artificial receptors are more apt to show the enthalpic effect of high-energy water than those in proteins or nucleic acids, because they bear fewer or no functional groups in the inner cavity to stabilize interior water molecules.

Topical cream-based dosage forms of the macrocyclic drug delivery vehicle cucurbit[6]uril

The macrocycle family of molecules called cucurbit[n]urils are potential drug delivery vehicles as they are able to form host-guest complexes with many different classes of drugs. This study aimed to examine the utility of Cucurbit[6]uril (CB[6]) in topical cream-based formulations for either localised treatment or for transdermal delivery. Cucurbit[6]uril was formulated into both buffered cream aqueous- and oily cream-based dosage forms. The solid state interaction of CB[6] with other excipients was studied by differential scanning calorimetry and the macrocycle's transdermal permeability was determined using rat skin. Significant solid state interactions were observed between CB[6] and the other dosage form excipients. At concentrations up to 32% w/w the buffered aqueous cream maintained its normal consistency and could be effectively applied to skin, but the oily cream was too stiff and is not suitable as a dosage form. Cucurbit[6]uril does not permeate through skin; as such, the results imply that cucurbituril-based topical creams may potentially only have applications for localised skin treatment and not for transdermal drug delivery.

Cyclodextrin-promoted energy transfer for broadly applicable small-molecule detection

Reported herein is the development of non-covalent, proximity-induced energy transfer from small-molecule toxicants to organic fluorophores bound in the cavity of γ-cyclodextrin. This energy transfer occurs with exceptional efficiency for a broad range of toxicants in complex biological media, and is largely independent of the spectral overlap between the donor and acceptor. This generally applicable phenomenon has significant potential in the development of new turn-on detection schemes.

Sequential inclusion of two berberine cations in cucurbit[8]uril cavity: kinetic and thermodynamic studies

The formation and dissociation kinetics of 1 : 1 and 2 : 1 inclusion of berberine in the cucurbit[8]uril cavity is revealed.