Acyclic Cucurbit[n]uril Receptors Function as Solid State Sequestrants for Organic Micropollutants.

The accumulation of organic micropollutants (OMP) in aquatic systems is a major societal problem that can be addressed by approaches including nanofiltration, flocculation, reverse osmosis and adsorptive methods using insoluble materials (e.g. activated carbon, MOFs, nanocomposites). More recently, polymeric versions of supramolecular hosts (e.g. cyclodextrins, calixarenes, pillararenes) have been investigated as OMP sequestrants. Herein, we report our study of the use of water insoluble dimethylcatechol walled acyclic cucurbit[n]uril (CB[n]) hosts as solid state sequestrants for a panel of five OMPs. A series of hosts (H1-H4) were synthesized by reaction of glycoluril oligomer (monomer-tetramer) with 3,6-dimethylcatechol and fully characterized by spectroscopic means and x-ray crystallography. The solid hosts sequester OMPs from water with removal efficiencies exceeding 90 % in some cases. The removal efficiencies of the new hosts parallel the known molecular recognition properties of analogous water soluble acyclic CB[n]. OMP uptake by solid host occurs rapidly (≈120 seconds). Head-to-head comparison with CB[6] in batch-mode separation and DARCO activated carbon in flow-through separation mode show that tetramer derived host (H4) performs very well under identical conditions. The work establishes insoluble acyclic CB[n]-type receptors as a promising new platform for OMP sequestration.

Voltage-Gated Membranes Incorporating Cucurbit[n]uril Molecular Containers for Molecular Nanofiltration.

Smart voltage-gated nanofiltration membranes have enormous potential for on-demand and precise separation of similar molecules, which is an essential element of sustainable water purification and resource recovery. However, the existing voltage-gated membranes are hampered by limited selectivity, stability, and scalability due to electroactive monomer dimerization. Here, for the first time, the host-guest recognition properties of cucurbit[7]uril (CB[7]) are used to protect the viologen derivatives and promote their assembly into the membrane by interfacial polymerization. Viologen functions as a voltage switch, whereas CB[7] complexation prevents its dimerization and improves its redox stability. The inhibited diffusion of the CB[7]-viologen complex enables the precise patterning of the surface structure. The resultant voltage-gated membrane displays 80% improved rejection performance, excellent recovery accuracy for similar molecules, and anti-fouling properties. This work not only provides an innovative strategy for the preparation of voltage-gated smart nanofiltration membranes but also opens up new avenues for ion-selective transmission in water treatment, bionic ion channels, and energy conversion.

DNA Strand Displacement Driven by Host-Guest Interactions.

Base-pair-driven toehold-mediated strand displacement (BP-TMSD) is a fundamental concept employed for constructing DNA machines and networks with a gamut of applications─from theranostics to computational devices. To broaden the toolbox of dynamic DNA chemistry, herein, we introduce a synthetic surrogate termed host-guest-driven toehold-mediated strand displacement (HG-TMSD) that utilizes bioorthogonal, cucurbit[7]uril (CB[7]) interactions with guest-linked input sequences. Since control of the strand-displacement process is salient, we demonstrate how HG-TMSD can be finely modulated via changes to the structure of the input sequence (including synthetic guest head-group and/or linker length). Further, for a given input sequence, competing small-molecule guests can serve as effective regulators (with fine and coarse control) of HG-TMSD. To show integration into functional devices, we have incorporated HG-TMSD into machines that control enzyme activity and layered reactions that detect specific microRNA.

Binding Methylarginines and Methyllysines as Free Amino Acids: A Comparative Study of Multiple Host Classes.

Methylated free amino acids are an important class of targets for host-guest chemistry that have recognition properties distinct from those of methylated peptides and proteins. We present comparative binding studies for three different host classes that are each studied with multiple methylated arginines and lysines to determine fundamental structure-function relationships. The hosts studied are all anionic and include three calixarenes, two acyclic cucurbiturils, and two other cleft-like hosts, a clip and a tweezer. We determined the binding association constants for a panel of methylated amino acids using indicator displacement assays. The acyclic cucurbiturils display stronger binding to the methylated amino acids, and some unique patterns of selectivity. The two other cleft-like hosts follow two different trends, shallow host (clip) following similar trends to the calixarenes, and the other more closed host (tweezer) binding certain less-methylated amino acids stronger than their methylated counterparts. Molecular modelling sheds some light on the different preferences of the various hosts. The results identify hosts with new selectivities and with affinities in a range that could be useful for biomedical applications. The overall selectivity patterns are explained by a common framework that considers the geometry, depth of binding pockets, and functional group participation across all host classes.

New Synthetic Route to Water-Soluble Prism[5]arene Hosts and Their Molecular Recognition Properties.

We report that the direct macrocyclization of naphthalene monomers bearing ethyl ester functional groups delivers prism[5]arene derivatives, which can be deprotected to yield water-soluble prism[5]arenes (H1 and H3). 1 H NMR spectroscopy showed that dicationic guests bind with the hydrophobic cores buried inside the anisotropic magnetically shielding cavity. Isothermal titration calorimetry measurements showed that H1 and H3 are high-affinity hosts in PBS-buffered water with Ka values exceeding 109  M-1 for a select guest. The complexation events are driven by the non-classical hydrophobic effect, CH⋅⋅⋅π interactions, and electrostatic interactions. Host H1 displays somewhat higher affinity toward a common guest than pillar[6]arene bearing carboxylic acid functional groups but is significantly less potent than pillar[6]arene bearing sulfate groups. H1 and H3 should be considered alongside other high affinity hosts for a variety of chemical and biological applications.

Molecular recognition of methylated amino acids and peptides by Pillar[6]MaxQ.

We report the molecular recognition properties of Pillar[n]MaxQ (P[n]MQ) toward a series of (methylated) amino acids, amino acid amides, and post-translationally modified peptides by a combination of 1H NMR, isothermal titration calorimetry, indicator displacement assays, and molecular dynamics simulations. We find that P6MQ is a potent receptor for N-methylated amino acid side chains. P6MQ recognized the H3K4Me3 peptide with Kd = 16 nM in phosphate buffered saline.

Double-Cavity Nor-Seco-Cucurbit[10]uril Enables Efficient and Rapid Separation of Pyridine from Mixtures of Toluene, Benzene, and Pyridine.

Benzene, toluene and pyridine are widely used as essential raw materials in industry and laboratory research, with stringent purity requirements. Herein, we show that solid double-cavity host nor-seco-cucurbit[10]uril (ns-Q[10]) functions as an efficient adsorption separator for pyridine; benzene and toluene can be obtained in >99.9 % purity by this method. Thermal removal of pyridine from ns-Q[10]⋅Py2 allows 10 separation cycles without erosion of performance. The geometry of the ns-Q[10]⋅Py2 ternary complex was established by single-crystal X-ray diffraction methods. The ns-Q[10] cage facilitates the removal of pyridine from toluene and benzene, with >99.9 % purity for the separated compounds.

In Vitro and In Vivo Sequestration of Phencyclidine by Me4 Cucurbit[8]uril*.

We report investigations of the use of cucurbit[8]uril (CB[8]) macrocycles as an antidote to counteract the in vivo biological effects of phencyclidine. We investigate the binding of CB[8] and its derivative Me4 CB[8] toward ten drugs of abuse (3-9, 12-14) by a combination of 1 H NMR spectroscopy and isothermal titration calorimetry in phosphate buffered water. We find that the cavity of CB[8] and Me4 CB[8] are able to encapsulate the 1-amino-1-aryl-cyclohexane ring system of phencyclidine (PCP) and ketamine as well as the morphinan skeleton of morphine and hydromorphone with Kd values ≤50 nm. In vitro cytotoxicity (MTS metabolic and adenylate kinase cell death assays in HEK293 and HEPG2 cells) and in vivo maximum tolerated dose studies (Swiss Webster mice) which were performed for Me4 CB[8] indicated good tolerability. The tightest host⋅guest pair (Me4 CB[8]⋅PCP; Kd =2 nm) was advanced to in vivo efficacy studies. The results of open field tests demonstrate that pretreatment of mice with Me4 CB[8] prevents subsequent hyperlocomotion induction by PCP and also that treatment of animals previously dosed with PCP with Me4 CB[8] significantly reduces the locomotion levels.

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

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

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.

Self-assembled, optically-active {naphthalene diimide}U{cucurbit[8]uril} ensembles in an aqueous environment.

Naphthalene diimides (NDIs) are shown to arrange spontaneously co-facially with cucurbit[8]uril (CB[8]) in an aqueous environment through purely non-covalent interactions. The resultant 2 : 2 supramolecular complex of NDI and CB[8] is highly fluorescent (>30 times more than the constituent NDIs) due to the formation of NDI-NDI excimers within the supramolecular complex.

Acyclic Cucurbituril Featuring Pendant Cyclodextrins.

We report the design and synthesis of the acyclic cucurbit[n]uril-ß-cyclodextrin chimeric host H1. The goal of the study is to deepen the cavity of the receptor to allow ß-CD complexation of moieties on the guest (especially fentanyl) that protrude from the cavity of the primary acyclic CB[n] binding site to enhance binding affinity and deliver new supramolecular antidotes for fentanyl intoxication. 1H NMR spectroscopy was used to deduce the geometry of the complexes between H1 and H2 and the guest panel (G1 - G8 and fentanyl) whereas isothermal titration calorimetry was used to determine the thermodynamic parameters of complexation. Hosts H1 and H2 retain the essential molecular recognition features of CB[n] receptors, but chimeric host H1 binds slightly stronger toward the guest panel than H2 for reasons that remain unclear. Compared to tetraanionic hosts M1 and M2, the dianionic hosts H1 and H2 are less potent receptors which reflects the importance of electrostatic (ion-ion and ion-dipole) interactions in this series of hosts. The work highlights the challenges inherent in the optimization of binding affinity of hosts as potential supramolecular antidotes.

Self Assembled Cages with Mechanically Interlocked Cucurbiturils.

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

Supramolecular hosts as in vivo sequestration agents for pharmaceuticals and toxins.

Pharmaceutical agents, drugs of abuse, and toxic substances have a large impact, positive and negative, on modern society. Efforts to mitigate the side effects of pharmaceuticals and counteract the life threatening effects of drugs of abuse and toxins can occur either by pharmacodynamic (PD) approaches based on bioreceptor·drug antagonism or by pharmacokinetic (PK) approaches that seek to reduce the concentration of free drug. In this tutorial review, we present the use of supramolecular hosts (cyclodextrins, calixarenes, (acyclic) cucurbiturils, and pillararenes) as in vivo sequestration agents for neuromuscular blockers, drugs of abuse (methamphetamine and fentanyl), anesthetics, neurotoxins, the pesticide paraquat, and heparin anti-coagulants by the PK approach. The review presents the basic physical and molecular recognition features of the supramolecular hosts and some of the principles used in their selection and structural optimization for in vivo sequestration applications. The influence of host·guest complexation on other relevant in vivo properties of drugs (e.g. distribution, circulation time, excretion, redox properties) is also mentioned. The article concludes with a discussion of future directions.

Biomedical Applications of Metal Organic Polygons and Polyhedra (MOPs).

Since the discovery and structural characterization of metal organic polygons and polyhedra (MOPs), scientists have explored their potential in various applications like catalysis, separation, storage, and sensing. In recent years, scientists have explored the potential of supramolecular MOPs in biomedical application. Pioneering works by Ehrlich, Rosenberg, Lippard, Stang and others have demonstrated that MOPs have great potential as a novel class of metallo-therapeutics that can deliver cargoes (drugs and dyes) selectively. In this article, we document the progress made over the past two decades on the biomedical applications of MOPs and discuss the future prospects of this emerging field.

Acyclic Cucurbit[n]uril-Type Receptors: Aromatic Wall Extension Enhances Binding Affinity, Delivers Helical Chirality, and Enables Fluorescence Sensing.

We report the linear extension from M1 to M2 to anthracene walled M3 which adopts a helical conformation (X-ray) to avoid unfavorable interactions between sidewalls. M3 is water soluble (=30 mm) and displays enhanced optical properties (ϵ=1.28×105 m-1 cm-1 , λmax =370 nm) relative to M2. The binding properties of M3 toward guests 1-29 were examined by 1 H NMR and ITC. The M3⋅guest complexes are stronger than the analogous complexes of M2 and M1. The enhanced binding of M3 toward neuromuscular blockers 25, 27-29 suggests that M3 holds significant promise as an in vivo reversal agent. The changes in fluorescence observed for M3⋅guest complexes are a function of the relative orientation of the anthracene sidewalls, guest concentration, Ka , and guest electronics which rendered M3 a superb component of a fluorescence sensing array. The work establishes M3 as a next generation sequestering agent and a versatile component of fluorescence sensors.

Calabadion 1 selectively reverses respiratory and central nervous system effects of fentanyl in a rat model.

BACKGROUND: We hypothesised that Calabadion 1, an acyclic cucurbit[n]uril molecular container, reverses fentanyl-induced respiratory depression and dysfunction of the CNS. METHODS: Experiments were conducted in male Sprague-Dawley rats. A constant-rate i.v. infusion of fentanyl (12.5 or 25 µg kg-1 over 15 min) was administered followed by an i.v. bolus of Calabadion 1 (0.5-200 mg kg-1) or placebo. The primary outcome was reversal of ventilatory and respiratory depression, assessed by pneumotachography and arterial blood gas analysis, respectively. Key secondary outcomes were effects on fentanyl-induced central nervous dysfunction quantified by righting reflex, balance beam test, and electromyography (EMG). RESULTS: Calabadion 1 reversed fentanyl-induced respiratory depression across the endpoints minute ventilation, pH, and Paco2 (P=0.001). Compared with placebo, Calabadion 1 dose dependently (P for trend <0.001) reversed fentanyl-induced hypoventilation {81.9 [5.1] (mean [standard error of the mean]) vs 45.5 [12.4] ml min-1; P<0.001}, acidosis (pH 7.43 [0.01] vs 7.28 [0.04]; P=0.005), and hypercarbia (Paco2 43.4 [1.6] vs 63.4 [8.1] mm Hg; P=0.018). The effective Calabadion 1 doses required to reverse respiratory depression by 50% and 90% (ED50Res and ED90Res) were 1.7 and 15.6 mg kg-1, respectively. Higher effective doses were needed for recovery of righting reflex (ED50CNS: 9.6 mg kg-1; ED90CNS: 86.1 mg kg-1), which was accelerated by Calabadion 1 (4.6 [0.3] vs 9.0 [0.7] min; P<0.001). Calabadion 1 also significantly accelerated recovery of full functional mobility and reversal of muscle rigidity. CONCLUSIONS: Calabadion 1 selectively and dose dependently reversed the respiratory system and CNS side-effects of fentanyl.

Conformationally Mobile Acyclic Cucurbit[n]uril-Type Receptors Derived from an S-shaped Methylene Bridged Glycoluril Pentamer.

We report the synthesis of the conformationally mobile S-shaped glycoluril pentamer building block 3a and two new acyclic CB[n]-type receptors P1 and P2. P1 (9 mM) and P2 (11 mM) have moderate aqueous solubility but their host•guest complexes are poorly soluble. Host P1 does not undergo intermolecular self-association whereas P2 does (Ks = 189±27 M-1). 1H NMR titrations show that P1 and P2 are poor hosts toward hydrophobic (di)cations 6 - 11 (P1: Ka = 375-1400 M-1; P2: Ka = 1950-19800 M-1) compared to Tet1 and Tet2 (Tet1: Ka = 3.09 × 106 to 4.69 × 108 M-1; Tet2: Ka = 4.59 × 108 to 1.30 × 1010 M-1). Molecular modelling shows that P1 and P2 exist as a mixture of three different conformers due to the two S-shaped methylene bridged glycoluril dimer subunits that each possess two different conformations. The lowest energy conformers of P1 and P2 do not feature a well-defined central cavity. In the presence of guests, P2 adapts its conformation to form 1:1 P2•guest complexes; the binding free energy pays the energetic price of conformer selection. This energetically unfavorable conformer selection results in significantly decreased Ka values of P1 and P2 compared to Tet1 and Tet2.

Pillar[n]MaxQ: A New High Affinity Host Family for Sequestration in Water.

We report the synthesis, X-ray crystal structure, and molecular recognition properties of pillar[n]arene derivative P[6]AS, which we refer to as Pillar[6]MaxQ along with analogues P[5]AS and P[7]AS toward guests 1-18. The ultratight binding affinity of P[5]AS and P[6]AS toward quaternary (di)ammonium ions renders them prime candidates for in vitro and in vivo non-covalent bioconjugation, for imaging and delivery applications, and as in vivo sequestration agents.

Chaperone-Assisted Host-Guest Interactions Revealed by Single-Molecule Force Spectroscopy.

The recent discovery of ultra-high binding affinities in cucurbit[7]uril (CB7)-based host-guest pairs in an aqueous environment has rendered CB7 a rather attractive material in analytical and biomedical applications. Due to the lack of a molecular platform that can follow the same host-guest complex during repetitive mechanical measurements, however, mechanical stabilities of the CB7 system have not been revealed, hindering its potential to rival widely used conjugation pairs, such as streptavidin-biotin. Here, we assembled a DNA template in which a flexible DNA linker was exploited to keep the host (CB7) and guest (adamantane) in proximity. This platform not only increased the efficiency of the single-molecule characterization in optical tweezers but also clearly revealed mechanical features of the same host-guest complex. We found that positively charged adamantane guest demonstrated higher mechanical stability (49 pN) than neutral adamantane (44 pN), a trend consistent with the chemical affinity between guest molecules and the CB7 host. Surprisingly, we found that a hexyl group adjacent to the adamantane served as a chaperone to assist the formation of the adamantane-CB7 pairs. The discovery of an unprecedented chaperone-assisted interaction mechanism provides new approaches to efficiently assemble host-guest-based supramolecules with increased mechanical stabilities, which can be exploited in various biomedical, biosensing, and materials fields.