A High-Affinity "Synthavidin" Receptor for Squaraine Dyes.

Strong-binding host-guest pairings in aqueous media have potential as "supramolecular glues" in biomedical techniques, complementing the widely-used (strept)avidin-biotin combination. We have previously found that squaraine dyes are bound very strongly by tetralactam macrocycles possessing anthracenyl units as cavity walls. Here we show that replacing the anthracenes with pentacyclic 5,7,12,14-tetrahydro-5,7,12,14-tetraoxapentacene (TOP) units generates receptors which bind squaraines with increased affinities (around Ka =1010  m-1 ) and improved selectivities. Binding can be followed through changes to squaraine fluorescence and absorbance. The TOP units are easy to prepare and potentially variable, while the TOP-based receptor shows improved photostability, both in itself and in complex with squaraines. The results suggest that this system could prove valuable in the further development of practical "synthavidin" chemistry.

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.

Hybrid Molecular Container Based on Glycoluril and Triptycene: Synthesis, Binding Properties, and Triggered Release.

We designed and synthesized a "hybrid" molecular container 1, which is structurally related to both cucurbit[n]uril (CB[n]) and pillar[n]arene type receptors. Receptor 1 was fully characterized by 1 H NMR, 13 C NMR, IR, MS and X-ray single crystal diffraction. The self-association behavior, host-guest recognition properties of 1, and the [salt] dependence of Ka were investigated in detail by 1 H NMR and isothermal titration calorimetry (ITC). Optical transmittance and TEM measurements provide strong evidence that receptor 1 undergoes co-assemble with amphiphilic guest C10 in water to form supramolecular bilayer vesicles (diameter 25.6±2.7 nm, wall thickness ≈3.5 nm) that can encapsulate the hydrophilic anticancer drug doxorubicin (DOX) and the hydrophobic dye Nile red (NR). The release of encapsulated DOX or NR from the vesicles can be triggered by hexamethonium (8 c) or spermine (10) which leads to the disruption of the supramolecular vesicles.

Synthetic mimics of biotin/(strept)avidin.

Biotin/(strept)avidin self-assembly is a powerful platform for nanoscale fabrication and capture with many different applications in science, medicine, and nanotechnology. However, biotin/(strept)avidin self-assembly has several well-recognized drawbacks that limit performance in certain technical areas and there is a need for synthetic mimics that can either become superior replacements or operational partners with bio-orthogonal recognition properties. The goal of this tutorial review is to describe the recent progress in making high affinity synthetic association partners that operate in water or biological media. The review starts with a background summary of biotin/(strept)avidin self-assembly and the current design rules for creating synthetic mimics. A series of case studies are presented that describe recent success using synthetic derivatives of cyclodextrins, cucurbiturils, and various organic cyclophanes such as calixarenes, deep cavitands, pillararenes, and tetralactams. In some cases, two complementary partners associate to produce a nanoscale complex and in other cases a ditopic host molecule is used to link two partners. The article concludes with a short discussion of future directions and likely challenges.

Blurring the Lines between Host and Guest: A Chimeric Receptor Derived from Cucurbituril and Triptycene.

We report the synthesis and X-ray crystal structure of a cucurbituril-triptycene chimeric receptor (1). Host 1 binds to guests typical of CB[6]-CB[8], but also binds to larger guests such as blue box (20) and the Fujita square (22). Intriguingly, the geometries of the 1⋅20 and 1⋅22 complexes blur the lines between host and guest in that both components fulfill both roles within each complex. The fluorescence output of 1 is fully quenched by the formation of complexes with pyridinium-derived guests.

Cucurbit[7]uril Enables Multi-Stimuli-Responsive Release from the Self-Assembled Hydrophobic Phase of a Metal Organic Polyhedron.

Mixed self-assembly of ligands 1, 2, 1,6-hexanediamine (HDA), and Pd(NO3)2 afforded Fujita-type metal organic polyhedron MOP1 (diameter ≈ 8.2 nm), which is covalently functionalized with an average of 18 cucurbit[7]uril (CB[7]) units, as evidenced by 1H NMR, diffusion-ordered spectroscopy NMR, and transmission electron microscopy measurements. By virtue of the host-guest properties of CB[7], the inner cavity of MOP can be rendered hydrophobic by using octadecyl HDA (3) as guest during the self-assembly process. The hydrophobic cavity was successfully utilized to trap the hydrophobic dye Nile Red (NR) and the anticancer drug doxorubicin (DOX). The stimuli-responsive release of encapsulated NR or DOX occurs (1) upon addition of a competitive binder (e.g., adamantane ammonium (ADA)) for CB[7], (2) by a dual pH-chemical stimulus involving the protonation state change of adamantane carboxylate at pH 5.8, and (3) by a dual pH-photochemical stimulus involving photoisomerization of trans-6 to cis-6 at pH 5.8. NR is released from NR@MOP2 within HeLa cancer cells. This body of work suggests that the covalent attachment of cucurbit[n]uril to metal organic polyhedra constitutes a promising vehicle for the development of both diagnostic and therapeutic nanoparticles.

Metal-Organic Polyhedron Capped with Cucurbit[8]uril Delivers Doxorubicin to Cancer Cells.

Self-assembly of ligand 1 and Pd(NO3)2 delivers Fujita-type metal-organic polyhedron (MOP) 3 which bears 24 covalently attached methyl viologen units on its external surface, as evidenced by 1H NMR, diffusion-ordered spectroscopy NMR, electrospray mass spectrometry, transmission electron microscopy, and atomic force microscopy measurements. MOP 3 undergoes noncovalent complexation with cucurbit[n]urils to yield MOPs 4-6 with diameter ≈5-6 nm. MOP 5 can be fully loaded with doxorubicin (DOX) prodrug 2 via hetero-ternary complex formation to yield 7. The MOPs exhibit excellent stability toward neutral to slightly acidic pH in 10 mM sodium phosphate buffer, mitigating the concern of disassembly during circulation. The results of MTS assays show that MOP 7 is 10-fold more cytotoxic toward HeLa cells than equimolar quantities of DOX prodrug 2. The enhanced cytotoxicity can be traced to a combination of enhanced cellular uptake of 7 and DOX release as demonstrated by flow cytometry and confocal fluorescence microscopy. The confluence of properties imparted by the polycationic MOP architecture and plug-and-play CB[n] complexation provides a potent new platform for drug delivery application.

Acyclic Cucurbit[n]uril-Type Molecular Container Enables Systemic Delivery of Effective Doses of Albendazole for Treatment of SK-OV-3 Xenograft Tumors.

Approximately, 40-70% of active pharmaceutical ingredients (API) are severely limited by their extremely poor aqueous solubility, and consequently, there is a high demand for excipients that can be used to formulate clinically relevant doses of these drug candidates. Here, proof-of-concept studies demonstrate the potential of our recently discovered acyclic cucurbit[n]uril-type molecular container Motor1 (M1) as a solubilizing agent for insoluble drugs. M1 did not induce significant rates of mutations in various Salmonella typhimurium test strains during the Ames test, suggesting low genotoxicity. M1 also has low risk of causing cardiac toxicity in humans since it did not inhibit the human Ether-à-go-go-Related Gene channel as tested on transfected CHO cell lines via patch clamp analysis. Albendazole (ABZ) is a widely used antihelminthic agent but that has also shown promising efficacy against cancerous cells in vitro. However, due to its low aqueous solubility (2.7 µM) and poor pharmacokinetics, ABZ is clinically limited as an anticancer agent. Here we investigated the potential of M1 as a solubilizing excipient for ABZ formulation. A pharmacokinetic study indicated that ABZ escapes the peritoneal cavity resulting in 78% absolute bioavailability, while its active intermediate metabolite, albendazole sulfoxide, achieved 43% absolute bioavailability. The daily dosing of 681 mg/kg M1 complexed with 3.2 mg/kg of ABZ for 14 days did not result in significant weight loss or pathology in Swiss Webster mice. In vivo efficacy studies using this M1·ABZ inclusion complex showed significant decreases in tumor growth rates and increases in survival of mice bearing SK-OV-3 xenograft tumors. In conclusion, we provide substantial new evidence demonstrating that M1 is a safe and efficient excipient that enables in vivo parenteral delivery of poorly water-soluble APIs.

Conformational Slippage Determines Rotational Frequency in Five-Component Nanorotors.

Several five-component nanorotors ROT-3 that rotate at different rates were prepared by adding phenanthrolines of distinct lateral size as brake blocks to the four-component nanorotor ROT-2. The brake blocks interfere with the 180° rotor causing the rotational frequency to drop from 97 kHz to 5 kHz. The effect of the rotating brake blocks on the rotational frequency in ROT-3 is accurately predicted by a nanomechanical model called "conformational slippage". For quantification, the interaction of the brake blocks with the trajectory of the main rotator is gauged based on the number of interfering vs. non-interfering conformations as computed by PM6.

Four-component supramolecular nanorotors.

A family of supramolecular four-component nanorotors was quantitatively self-assembled from two different zinc(II) porphyrins: one representing the stator and the other the rotator with DABCO as an interconnecting axle and copper(I) ions. Rotational spinning in ROT-1' occurs at 97,000 s(-1) at 25 °C but is virtually stopped at -75 °C. The activation vs binding data suggest that spinning is an intrasupramolecular process occurring to >99.9% without dissociation. Addition and removal of two further equiv of Cu(+) reversibly switches the mode of the stochastic rotation between pure 180° and mixed 90°/180° steps and reversibly regulates the speed between 97,000 and ~80,000 s(-1).

Guest encapsulation and coronene-C60 exchange in supramolecular zinc porphyrin tweezers, grids and prisms.

Using a variant of the HETPHEN concept, heteroleptic 2D and 3D metallosupramolecular structures, such as tweezer T, grid G and tetragonal prism P, were fabricated quantitatively and characterised by (1)H NMR, (13)C NMR, (1)H-(1)H COSY, DOSY as well as ESI-MS. All structures encapsulate C60, with P showing the highest binding affinity (Kassoc = 3.3 × 10(6) M(-1)). The association constant increases along the series T < G < P, most likely due to the enhanced structural rigidity and better coplanarity of the two zinc porphyrin units. In contrast to T and G, the tetragonal prism P shows notable encapsulation of coronene (Kassoc = 1.1 × 10(4) M(-1)). In T and G, on the other hand, complexation of coronene is kinetically inhibited by the bulky mesityl rings at the porphyrin periphery. As illustrated in the facile displacement of coronene by C60 in coronene@P to furnish C60@P, P behaves as a flexible and guest-adaptive host.

DABCO as a dynamic hinge between cofacial porphyrin panels and its tumbling inside a supramolecular cavity.

The heteroleptic supramolecular double-decker porphyrin 1 was synthesized with DABCO as a guest between two cofacial porphyrin units as characterized by (1)H NMR and ESI-MS. While DABCO is not seen to tumble inside the cavity, even at higher temperatures (80 °C), such motion was triggered upon addition of various coordinating ligands (quinuclidine, 4-bromopyridine, or excess of DABCO). Different stoichiometric amounts were needed depending on the n donor quality of the added ligands to initiate tumbling of the "inside" DABCO. As demonstrated in an example with excess DABCO, the tumbling was stopped by lowering the temperature to -50 °C.

Orthogonal actuation of a supramolecular double-porphyrin tweezer.

A supramolecular three-component double-porphyrin tweezer (PT) is prepared quantitatively using heteroleptic complex formation along the HETTAP methodology. Insertion of guest molecules, such as DABCO or pyrazine, into the coupled porphyrin cavities of PT leads to an actuation of the double-porphyrin tweezer function. Evidence from (1)H NMR, VT NMR, and UV-vis titration suggests a rapid association/dissociation of the DABCO molecules at the central porphyrin. Upon addition of an equimolar mixture of DABCO and pyrazine to PT, a dynamic five-component self-assembled structure was furnished exclusively. (1)H NMR and K(assoc) values validate the greater stability of the heteroloaded PT-(DABCO)(py) system in comparison to the two homoloaded systems, PT-(DABCO)(2) and PT-(py)(2). The higher stability of PT-(DABCO)(py) seems to be the result of charge transfer from DABCO to the vacant pi* orbital of pyrazine across the porphyrin plane.

Aqueous recognition of purine and pyrimidine bases by an anthracene-based macrocyclic receptor.

A water-soluble bis-anthracenyl tetralactam binds biogenic heterocycles with high affinities in aqueous solution, rising to 107 M-1 for the purine hypoxanthine. Recognition occurs through a combination of hydrogen bonding and hydrophobic interactions, and results in fluorescence changes which suggest applications in sensing.

Selective glucose sensing in complex media using a biomimetic receptor.

Glucose is a key biomedical analyte, especially relevant to the management of diabetes. Current methods for glucose determination rely on the enzyme glucose oxidase, requiring specialist instrumentation and suffering from redox-active interferents. In a new approach, a powerful and highly selective achiral glucose receptor is mixed with a sample, l-glucose is added, and the induced CD spectrum is measured. The CD signal results from competition between the enantiomers, and is used to determine the d-glucose content. The involvement of l-glucose doubles the signal range from the CD spectrometer and allows sensitivity to be adjusted over a wide dynamic range. It also negates medium effects, which must be equal for both enantiomers. The method has been demonstrated with human serum, pre-filtered to remove proteins, giving results which closely match the standard biochemical procedures, as well as a cell culture medium and a beer sample containing high (70 mM) and low (0.4 mM) glucose concentrations respectively.

A synthetic transcription factor pair mimic for precise recruitment of an epigenetic modifier to the targeted DNA locus.

We developed an epigenetically active, cooperative DNA binding transcription factor platform assisted by cucurbit[7]uril (CB7) host-guest modules. This new type of molecule termed ePIP-HoGu not only mimics the operation of transcription factors as a pair but also recruits the epigenetic modifier to a particular DNA locus.

Metal Organic Polyhedra: A Click-and-Clack Approach Toward Targeted Delivery.

Mixed self-assembly of ligands 1 and 2, PXDA (3), and Pd(NO3)2 afforded metal organic polyhedra (MOP 1 - MOP 3) which bear 24 covalently attached CB[7] and cyclooctyne moieties. Post assembly modification (PAM) of MOP 3 by covalent strain promoted alkyne azide click reaction provided MOP 4 R bearing covalently attached functionality (PEG, sulfonate, biotin, c-RGD, fluorescein and cyanine). Orthogonal CB[7] guest mediated non-covalent PAM of MOP 4 R with Ad-FITC afforded MOP 5 RGD Ad-FITC and MOP 5 biotin 0020Ad-FITC. Flow cytometry analysis of the uptake of MOP 5 RGD Ad-FITC toward U87 cells demonstrated improved uptake relative to control MOP lacking c-RGD ligands. These results suggest a broad applicability of orthogonally functionalizable (covalent and non-covalent) MOPs in targeted drug delivery and imaging applications.

Self-assembly of cucurbit[7]uril based triangular [4]molecular necklaces and their fluorescence properties.

Self-assembly of rigid-rod dipyridine ligand 1 with M(en)(NO3)2 (M = Pd, Pt) affords triangular (3, 5) and square (4, 6) supramolecular coordination complexes (SCCs). The binding affinity of 1 toward CB[n]-type containers results in the formation of triangular [4]molecular necklaces ([4]MNs, 7-10) by either one-pot or post complexation approaches as evidenced by 1H NMR, diffusion ordered spectroscopy, and ESI-MS.

A five-component nanorotor with speed regulation.

A five-component supramolecular nanorotor with reversibly acting brakes has been prepared from a four-component nanorotor by adding the photo- and heat-responsive 2,2'-diazastilbene as a signal transducer. The rotational speed was reversibly switched between 86 and 38 kHz.

Reversible cargo shipping between orthogonal stations of a nanoscaffold upon redox input.

The sterically shielded terpyridine was prepared, both as a new ligand and as part of the four-station nanoscaffold . Mixing of terpyridine , the parent phenanthroline and the shielded phenanthroline in the presence of Zn(2+) (1 : 1 : 1 : 1) furnished quantitatively the inverse HETTAP complex [Zn()()](2+) by self-sorting, while in the presence of Cu(+) the HETPHEN complex [Cu()()](+) was preferred (89%). Due to the akin coordination preferences of Cu(2+) and Zn(2+), the above self-sorting was implemented for Cu(+)/Cu(2+) on nanoscaffold , the latter equipped with the binding motifs of (PhenAr2) and (TerpyAr2). When was reacted with Cu(+) and phenanthroline () in a 1 : 2 : 2 ratio, only the PhenAr2 stations became involved in complex formation (= (Cu)phen). In contrast, upon oxidative formation of Cu(2+), ligand was exclusively moved to the TerpyAr2 stations ((Cu)terpy). Electrochemical oxidation-reduction prompted the cargo to be shipped reversibly on a subsecond time scale between the two different stations of .