Voltage-Driven Reversible Insertion into and Leaving from a Lipid Bilayer: Tuning Transmembrane Transport of Artificial Channels

A redox-responsive selenium-containing pillar[5]arene-based macrocyclic amphiphile: synthesis, controllable self-assembly in water, and application in controlled release

A selenium-containing pillar[5]arene-based macrocyclic amphiphile was prepared. This amphiphile self-assembled in water to form vesicles with redox responsiveness. Then, these pillar[5]arene-based vesicles were used in the controlled release of DOX.

Pillar[5]arene-based amphiphilic supramolecular brush copolymer: fabrication, controllable self-assembly and application in self-imaging targeted drug delivery

Supramolecular brush copolymers have attracted continuing interest due to their unusual architectures, fascinating properties, and potential applications in many fields involving smart stimuli-responsive drug delivery systems. Herein, the first pillararene-based amphiphilic supramolecular brush copolymer (P5-PEG-Biotin⊃PTPE) was constructed on the basis of the host-guest molecular recognition between a water-soluble pillar[5]arene (P5) and a viologen salt (M). P5-PEG-Biotin⊃PTPE self-assembled into supramolecular nanoparticles (SNPs), which were utilized as a self-imaging drug delivery vehicle by taking advantage of the aggregation-induced emission (AIE) effect. Encapsulation of anticancer drug doxorubicin (DOX) caused deactivation of the fluorescences of both the tetraphenylethene (TPE) and DOX chromophores due to the energy transfer relay (ETR) effect, mediated by Förster resonance energy transfer (FRET) and aggregation-caused quenching (ACQ). The release of loaded DOX molecules can be triggered by low pH and reductase, recovering the "silenced" fluorescence caused by the interruption of the ETR effect, achieving in situ visualization of the drug release process by observing the location and magnitude of the energy transfer-dependent fluorescence variation. The biotin ligands on the surfaces of the DOX-loaded SNPs act as targeting agents to deliver DOX preferentially to cancer cells over-expressing biotin receptor. In vitro studies demonstrated that the loading of DOX by this supramolecular nanomaterial exhibited selective cytotoxicity towards cancer cells over normal cells. The potency of this sophisticated supramolecular drug delivery system in cancer therapy was further evaluated in HeLa tumor-bearing mice. In vivo experiments confirmed that the DOX-loaded SNPs possess excellent antitumor efficacy with negligible systemic toxicity.

Cationic pillar[6]arene/ATP host-guest recognition: selectivity, inhibition of ATP hydrolysis, and application in multidrug resistance treatment

Due to the differences in the cavity size of the hosts and the charge and length of the guests, a cationic water-soluble pillar[6]arene (WP6) selectively complexes with ATP to form a stable 1 : 1 inclusion complex WP6⊃ATP. This host-guest complexation was utilized to efficiently inhibit the hydrolysis of ATP, arising from the existence of the hydrophobic cavity of WP6. A folic acid functionalized diblock copolymer (FA-PEG-b-PAA) was employed to PEGylate WP6 to endow the polyion complex (PIC) micelles with specific targeting ability, preferentially delivering WP6 to folate receptor over-expressing KB cell. This host-guest complexation was further used to block the efflux pump to transport anticancer drugs out of cells by cutting off the energy source, which enhanced the efficacy of the cancer chemotherapy of DOX·HCl towards drug resistant MCF-7/ADR cell. This supramolecular method provides an extremely distinct strategy to potentially overcome multidrug resistance (MDR).

Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

In 2008, we reported a new class of pillar-shaped macrocyclic hosts, known as "pillar[n]arenes". Today, pillar[n]arenes are recognized as key players in supramolecular chemistry because of their facile synthesis, unique pillar shape, versatile functionality, interesting host-guest properties, and original supramolecular assembly characteristics, which have resulted in numerous electrochemical and biomedical material applications. In this Review, we have provided historical background to macrocyclic chemistry, followed by a detailed discussion of the fundamental properties of pillar[n]arenes, including their synthesis, structure, and host-guest properties. Furthermore, we have discussed the applications of pillar[n]arenes to materials science, as well as their applications in supramolecular chemistry, in terms of their fundamental properties. Finally, we have described the future perspectives of pillar[n]arene chemistry. We hope that this Review will provide a useful reference for researchers working in the field and inspire discoveries concerning pillar[n]arene chemistry.

Self-Assembly of X-Shaped Bolapolyphiles in Lipid Membranes: Solid-State NMR Investigations

A novel class of rigid-rod bolapolyphilic molecules with three philicities (rigid aromatic core, mobile aliphatic side chains, polar end groups) has recently been demonstrated to incorporate into and span lipid membranes, and to exhibit a rich variety of self-organization modes, including macroscopically ordered snowflake structures with 6-fold symmetry. In order to support a structural model and to better understand the self-organization on a molecular scale, we here report on proton and carbon-13 high-resolution magic-angle spinning solid-state NMR investigations of two different bolapolyphiles (BPs) in model membranes of two different phospholipids (DPPC, DOPC). We elucidate the changes in molecular dynamics associated with three new phase transitions detected by calorimetry in composite membranes of different composition, namely, a change in π-π-packing, the melting of lipid tails associated with the superstructure, and the dissolution and onset of free rotation of the BPs. We derive dynamic order parameters associated with different H-H and C-H bond directions of the BPs, demonstrating that the aromatic cores are well packed below the final phase transition, showing only 180° flips of the phenyl ring, and that they perform free rotations with additional oscillations of the long axis when dissolved in the fluid membrane. Our data suggests that BPs not only form ordered superstructures, but also rather homogeneously dispersed π-packed filaments within the lipid gel phase, thus reducing the corrugation of large vesicles.

Artificial water channels – incipient innovative developments

This Feature Article discusses the incipient developments of the first artificial water channels, including only systems that integrate synthetic elements in their water selective translocation unit.

Photolysis of a bola-type supra-amphiphile promoted by water-soluble pillar[5]arene-induced assembly

A novel monolayer supramolecular vesicle assembled from a pillararene-based bola-type supra-amphiphile was successfully constructed, which showed excellent photodegradable properties and might have potential applications in phototherapy.

Pillar[7]arene-based host–guest complex in water: dual-responsiveness and application in controllable self-assembly

A dual-responsive supra-amphiphile was constructed based on a water-soluble pillar[7]arene and a pyrene-containing guest. The self-assembly morphology of this supra-amphiphile could be adjusted by changing pH or irradiation by UV light.

Stimuli-responsive supramolecular gel constructed by pillar[5]arene-based pseudo[2]rotaxanes via orthogonal metal–ligand coordination and hydrogen bonding interaction

A supramolecular polyrotaxane constructed by pillar[5]arene-based pseudo[2]rotaxanes via the incorporation of metal–ligand coordination could further self-assemble to form a stimuli-responsive supramolecular gel at high concentration.

Design of artificial membrane transporters from gold nanoparticles with controllable hydrophobicity

Gold nanoparticles with variable hydrophobicity have been prepared in three different size regimes following established methods. The control of hydrophobicity was achieved by complexation of the 18-crown-6-CH₂-thiolate ligand shell with potassium ions. Potassium dependent phase transfer of these particles from dispersion in water to chloroform was demonstrated, and the equilibrium partitioning of the particles in water–chloroform liquid/liquid systems was quantified by optical spectroscopy. The gradual complexation of the ligand shell with potassium ions was further monitored by zeta potential measurements. Potassium dependent insertion of nanoparticles into the phospholipid bilayer membrane of vesicles in aqueous dispersion has been demonstrated by cryogenic transmission electron microscopy (cryo-TEM). Nanoparticle-dependent potassium ion transport across the vesicle membrane has been established by monitoring the membrane potential with fluorescence spectroscopy using a potential sensitive dye.

Oxacalix[2]arene[2]triazine based ion-pair transporters

Heteracalixaromatics are a new generation of macrocyclic hosts showing a unique structure and versatile recognition properties towards various guests.

GSH- and pH-responsive drug delivery system constructed by water-soluble pillar[5]arene and lysine derivative for controllable drug release

Novel GSH- and pH-responsive supramolecular vesicles constructed by an amphiphilic inclusion complex formed from water-soluble pillar[5]arene and lysine derivative have been successfully developed, which can efficiently encapsulate anticancer drug MTZ and show rapid MTZ-release in a simulated acidic tumor environment with high GSH concentration, and exhibit potent antitumor activity.

A pillar[6]arene-based [2]pseudorotaxane in solution and in the solid state and its photo-responsive self-assembly behavior in solution

A pillar[6]arene-based [2]pseudorotaxane was constructed in solution and studied in the solid state, and its photo-responsive self-assembly behavior in solution was investigated.

Redox-Responsive Amphiphilic Macromolecular [2]Pseudorotaxane Constructed from a Water-Soluble Pillar[5]arene and a Paraquat-Containing Homopolymer

Here we report a redox-responsive host-guest complex between a new water-soluble pillar[5]arene (WP5) and a paraquat derivative. Compared with the neutral form of the paraquat derivative that binds WP5 weakly, its dication form binds WP5 much more strongly. Furthermore, we utilize this new water-soluble redox-responsive molecular recognition motif to construct the first pillararene-based amphiphilic macromolecular [2]pseudorotaxane, which self-assembles into redox-responsive polymeric vesicles in water. Such pillararene-based supramolecular vesicles were further used to construct a drug delivery system to encapsulate and controlled release DOX·HCl, an anticancer drug. The uptake of these DOX·HCl-loaded supramolecular vesicles by cancer cells was studied with confocal laser scanning microscopy. Meanwhile, DOX·HCl-loaded supramolecular vesicles showed anticancer activity in vitro comparable to free DOX·HCl under the examined conditions.

Zn(2+)-Triggered Drug Release from Biocompatible Zirconium MOFs Equipped with Supramolecular Gates

A new theranostic nanoplatform, comprising of monodisperse zirconium metal-organic frameworks (MOFs) as drug carriers and carboxylatopillar[5]arene-based supramolecular switches as gating entities, is constructed, and controlled drug release triggered by bio-friendly Zn(2+) ions (abundant in synaptic vesicles) and auxiliary thermal stimulus is realized. This on-command drug delivery system exhibits large pore sizes for drug encapsulation, excellent biodegradability and biocompatibility, extremely low cytotoxicity and premature drug release, and superior dual-stimuli responsiveness, opening a new avenue in targeted drug delivery and controlled release of therapeutic agents, especially in the treatment of central nervous system diseases.

Tubular Unimolecular Transmembrane Channels: Construction Strategy and Transport Activities

Lipid bilayer membranes separate living cells from their environment. Membrane proteins are responsible for the processing of ion and molecular inputs and exports, sensing stimuli and signals across the bilayers, which may operate in a channel or carrier mechanism. Inspired by these wide-ranging functions of membrane proteins, chemists have made great efforts in constructing synthetic mimics in order to understand the transport mechanisms, create materials for separation, and develop therapeutic agents. Since the report of an alkylated cyclodextrin for transporting Cu(2+) and Co(2+) by Tabushi and co-workers in 1982, chemists have constructed a variety of artificial transmembrane channels by making use of either the multimolecular self-assembly or unimolecular strategy. In the context of the design of unimolecular channels, important advances have been made, including, among others, the tethering of natural gramicidin A or alamethicin and the modification of various macrocycles such as crown ethers, cyclodextrins, calixarenes, and cucurbiturils. Many of these unimolecular channels exhibit high transport ability for metal ions, particularly K(+) and Na(+). Concerning the development of artificial channels based on macrocyclic frameworks, one straightforward and efficient approach is to introduce discrete chains to reinforce their capability to insert into bilayers. Currently, this approach has found the widest applications in the systems of crown ethers and calixarenes. We envisioned that for macrocycle-based unimolecular channels, control of the arrangement of the appended chains in the upward and/or downward direction would favor the insertion of the molecular systems into bilayers, while the introduction of additional interactions among the chains would further stabilize a tubular conformation. Both factors should be helpful for the formation of new efficient channels. In this Account, we discuss our efforts in designing new unimolecular artificial channels from tubular pillar[n]arenes by extending their lengths with various ester, hydrazide, and short peptide chains. We have utilized well-defined pillar[5]arene and pillar[6]arene as rigid frameworks that allow the appended chains to afford extended tubular structures. We demonstrate that the hydrazide and peptide chains form intramolecular N-H···O═C hydrogen bonds that enhance the tubular conformation of the whole molecule. The new pillar[n]arene derivatives have been successfully applied as unimolecular channels for the selective transport of protons, water, and amino acids and the voltage-gated transport of K(+). We also show that aromatic hydrazide helices and macrocycles appended with peptide chains are able to mediate the selective transport of NH4(+).

Efficient synthetic supramolecular channels and their light-deactivated ion transport in bilayer lipid membranes

Efficient ion transport and photo-deactivation was achieved from the self-assembled channel of o-nitrobenzyl-based amphiphilic small molecules.

Hydrazide macrocycles as effective transmembrane channels for ammonium

The cavity of aromatic hydrazide macrocycles is tuned by appended Phe peptide chains to form deformable channels for efficient transport of NH₄⁺.

A non-symmetric pillar[5]arene based on triazole-linked 8-oxyquinolines as a sequential sensor for thorium(iv) followed by fluoride ions

A novel non-symmetric pillar[5]arene bearing triazole-linked 8-oxyquinolines at one rim was synthesized and demonstrated as a sequential fluorescence sensor for thorium(iv) followed by fluoride ions with high sensitivity and selectivity.

An anthracene-appended 2:3 copillar[5]arene: synthesis, computational studies, and application in highly selective fluorescence sensing for Fe(iii) ions

A novel anthracene-appended 2:3 copillar[5]arene was prepared and it was further used as a Fe³⁺-selective fluorescent chemosensor over a wide range of metal ions. The binding mode was modeled by DFT and molecular dynamics computational calculations.

Ditopic CMPO-pillar[5]arenes as unique receptors for efficient separation of americium(iii) and europium(iii)

A unique host–guest recognition process involving a new class of homoditopic CMPO-pillar[5]arenes and lanthanides was revealed to proceed in a stepwise manner, and correlated to the efficient and selective separation of americium(iii) from europium(iii) under acidic feed conditions.

Solid-state assembly of carboxylic acid substituted pillar[5]arene and its host–guest complex with tetracaine

The first examples of carboxylic acid substituted pillar[5]arene structures in the form of its inclusion complexes with ethanol and the anesthetic drug tetracaine are reported.

Stimuli-responsive metal-organic frameworks gated by pillar[5]arene supramolecular switches

Spurred on by recent advances in materials chemistry and drug delivery, a new stimuli-responsive theranostic hybrid platform, based on mechanized monodisperse nano metal-organic frameworks (NMOFs) gated by carboxylatopillar[5]arene (CP5) switches with bio-friendly pH-triggered cargo release capabilities, has been constructed for the first time. This nanoscale smart cargo delivery system showed pH- and/or competitive binding agent-triggered controlled cargo release with negligible premature release, large pore sizes for drug encapsulation, low cytotoxicity, good biodegradability and biocompatibility, and potential application in cell imaging, which offers a new tool in targeted drug delivery and the controlled release of therapeutic agents.

Dual photo- and pH-responsive supramolecular nanocarriers based on water-soluble pillar[6]arene and different azobenzene derivatives for intracellular anticancer drug delivery

Two novel types of supramolecular nanocarriers fabricated by the amphiphilic host-guest inclusion complex formed from water-soluble pillar[6]arene (WP6) and azobenzene derivatives G1 or G2 have been developed, in which G1 is structurally similar to G2 but has an extra phenoxy group in its hydrophobic region. Supramolecular micelles can be initially formed by WP6 with G1, which gradually transform into layered structures with liquid-crystalline properties, whereas stable supramolecular vesicles are obtained from WP6 and G2, which exhibit dual photo- and pH-responsiveness. Notably, the resulting WP6⊃G2 vesicles can efficiently encapsulate anticancer drug mitoxantrone (MTZ) to achieve MTZ-loaded vesicles, which maintain good stability in a simulated normal physiological environment, whereas in an acid environment similar to that of tumor cells or with external UV irradiation, the encapsulated drug is promptly released. More importantly, cytotoxicity assay indicates that such vesicles have good biocompatibility and the MTZ-loaded vesicles exhibit comparable anticancer activity to free MTZ, especially with additional UV stimulus, whereas its cytotoxicity for normal cells was remarkably reduced. Flow cytometric analysis further confirms that the cancer cell death caused by MTZ-loaded vesicles is associated with apoptosis. Therefore, the dual pH- and UV-responsive supramolecular vesicles are a potential platform for controlled release and targeted anticancer drug delivery.

Synthesis of various supramolecular hybrid nanostructures based on pillar[6]arene modified gold nanoparticles/nanorods and their application in pH- and NIR-triggered controlled release

Organic/inorganic supramolecular hybrid micelles, onion-like disks and vesicles were obtained by the self-assembly of water-soluble pillar[6]arene stabilized gold nanoparticles/nanorods with different amounts of a hydrophobic chain functionalized paraquat derivative in water.