Voltage-Driven Flipping of Zwitterionic Artificial Channels in Lipid Bilayers to Rectify Ion Transport

Metal-Directed Self-Assembly of Minimal Heterochiral Peptides into Metallo-Supramolecular β-Helical Tubules for Artificial Transmembrane Water Channels

Transmembrane selective transport of metabolites controls essential biological functions. During the last two decades, artificial channels have been developed and cyclic peptides have emerged as ideal platforms for efficient ion, sugar, and nucleic acid channel translocation. Despite these tremendous developments, cyclic peptides have eluded selective water transport. Herein, we report the formation of narrow artificial β-helical tubules with diameters ranging from 2.80 to 3.25 Å that selectively control the water translocation, akin to natural aquaporin channels. The tubular assemblies resulted from the metal-driven folding and assembly of minimal heterochiral metal-binding 3-pyridyl-terminated peptides. The bent ultrashort peptide ligand coordinates with Ag+ metal ions in a head-to-tail manner, which undergoes subsequent polymerization into a β-helical tubular structure stabilized by interstrand hydrogen bonds (H-bonds) between the β-strands and π-π staking interactions between terminal pyridyl moieties. Furthermore, sequence engineering of the heterochiral peptide and subsequent Ag+ ion coordination of the tailored peptides enabled the formation of distinct synthetic double β-barrel and artificial β-helical tubular assemblies, with water molecules encapsulated in the hydrophilic core of the tubes. These water-encapsulated tubes were further explored as artificial water channels in lipid bilayers. Our findings suggest that such β-helical tubular channels achieve a single-channel permeability of 106 water molecules/second/channel, which is within 1-2 orders of magnitude lower than that of aquaporins, with a rather good ability to sterically reject ions and prevent proton transport. These assemblies present significant potential for engineering efficient membranes for water purification and separation sciences.

Lanthanide-Selective Artificial Channels

Lanthanides serve as essential elements for modern technology, playing critical roles in batteries, wind turbines, portable electronics, and energy-efficient lighting. Purifying lanthanides from ores and recycling them from end-of-life consumer materials are costly and damaging to the environment due to inefficient separation technologies. In this study, we present a new approach for lanthanide separations using supramolecular membrane channel nanopores based on a pillar[5]arene scaffold with appended diphenylphosphine oxide (DPP) ligands. These channels show high transport selectivity (>18:1) of the middle lanthanides, europium (Eu3+) and terbium (Tb3+) ions, over monovalent K+ ions and also excluded other common mono- and divalent metal ions (Na+, Ca2+, and Mg2+) including protons. These membrane channels also have high lanthanide-lanthanide transport selectivity with Eu3+/La3+ selectivity of >40 and Eu3+/Yb3+ selectivity of ∼30. Additionally, they demonstrated significantly higher selectivities between middle lanthanides and both light and heavy lanthanides: Tb3+/La3+ (∼140), Tb3+/Yb3+ (∼72), Tb3+/Nd3+ (∼58), and Eu3+/Nd3+ (∼17), which are considerably higher than selectivities reported in studies using traditional solvent extraction methods. Molecular dynamics simulations indicate that the high selectivity observed is due to specific water-mediated interactions between the hydrated ions and the channel. Our findings could contribute to ongoing efforts to improve lanthanide separation efficiency and reduce the environmental impact associated with current methods.

Mechanosensitive and pH-Gated Butterfly-Shaped Artificial Ion Channel for High-Selective K+ Transport and Cancer Cell Apoptosis

To advance the exploration of mechanisms underlying natural multi-gated ion channels, a novel butterfly-shaped biomimetic K+ channel GnC7 (n = 3, 4) is developed with dual mechanical and pH responsiveness, exhibiting unprecedented K+/Na+ selectivity (G3C7: 34.4; G4C7: 41.3). These channels constructed from poly(propylene imine) dendrimer and benzo-21-crown-7-ethers achieve high K+ transport activity (EC50: 0.72 µm for G3C7; 0.9 µm for G4C7) due to their arc-like mechanical rotation. The dynamic mode relies on butterfly-shaped topology derived from the highly symmetrical core and multiple intramolecular hydrogen bonds. GnC7 can sense mechanical stimulus applied to liposomes/cells and then adapt the K+ transport rate accordingly. Furthermore, reversible ON/OFF switching of K+ transport is realized through the pH-controllable host-guest complexation. G4C7-induced ultrafast cellular K+ efflux (70% within only 9 min) efficiently triggers mitochondrial-dependent apoptosis of cancer cells by provoking endoplasmic reticulum stress accompanied by drastic Ca2+ sparks. This work embodies a multi-dimensional regulation of channel functions; it will provide insights into the dynamic behaviors of biological analogs and promote the innovative design of artificial ion channels and therapeutic agents.

Engineered Reactive Oxygen Species (ROS)-Responsive Artificial H+/Cl- Ion Channels for Targeted Cancer Treatment

Reactive oxygen species (ROS)-responsive ion channels regulate the ion flow across the membranes in response to alterations in the cellular redox state, playing a crucial role in cellular adaptation to oxidative stress. Despite their significance, replicating ROS-responsive functionality in artificial ion channels remains elusive. In this study, we introduce a novel class of artificial H+/Cl- ion channels activatable by elevated ROS levels in cancer cells. ROS-induced decaging of the phenylboronate group triggers the rapid release of the channel-forming units, leading to self-assembly of the H-bonded cascades facilitating the synergistic transport of H+ and Cl- ions, with H+/Cl- ion transport selectivity of 7.7. Upon activation, ROS-C-Cl exhibits significant apoptotic activity against human breast cancer cells, achieving an IC50 of 2.8 μM, comparable to that of paclitaxel. Exploiting the intrinsic oxidative microenvironment of cancer cells, along with the enhanced oxidative stress arising from H+/Cl- co-transport, ROS-C-Cl demonstrates exceptional selectivity in targeting cancer cells with a selectivity index of 10.2 over normal breast cells, outperforming that of paclitaxel by 19.4 folds.

Synthetic ion channels in biomembranes

Ion transport across cell membranes is crucial in maintaining ion homeostasis in cells. Synthetic molecules that can mimic the functions of natural ion channel proteins would possess great potential as therapeutic agents by promoting apoptosis or interfering with autophagic processes through perturbing the intracellular pH or inducing oxidative and osmotic stresses. However, little is known about the underlying mechanisms in terms of direct correlation between ion transport and biological functions. This review summarizes recent progress in the area of synthetic transmembrane ion transport systems, focusing on the channel type, with an emphasis on their bioapplications as anticancer agents.

Transmembrane Ion Channels: From Natural to Artificial Systems

Natural channel proteins allow the selective permeation of ions, water or other nutritious entities across bilayer membranes, facilitating various essential physiological functions in living systems. Inspired by nature, chemists endeavor to simulate the structural features and transport behaviors of channel proteins through biomimetic strategies. In this review, we start from introducing the inherent traits of channel proteins such as their crystal structures, functions and mechanisms. Subsequently, different kind of synthetic ion channels including their design principles, dynamic regulations and therapeutic applications were carefully reviewed. Finally, the potential challenges and opportunities in this research field were also carefully discussed. It is anticipated that this review could provide some inspiring ideas and future directions towards the construction of novel bionic ion channels with higher-level structures, properties, functions and practical applications.

Bioinspired Ion Host with Buried and Consecutive Binding Sites for Controlled Ion Dislocation

This study presents a bioinspired ion host featuring continuous binding sites arranged in a tunnel-like structure, closely resembling the selectivity filter of natural ion channels. Our investigation reveals that ions traverse these sites in a controlled, sequential manner due to the structural constraints, effectively mimicking the ion translocation process observed in natural channels. Unlike systems with open binding sites, our model facilitates sequential ion recognition state transitions, enabled by the deliberate design of the tunnel. Notably, we observe dual ion release kinetics, highlighting the system's capacity to maintain ion balance in complex environments and adapt to changing conditions. Additionally, we demonstrate selective binding of two different ions-a challenging task for systems lacking structured tunnels.

Spontaneous Transition between Multiple Conductance States and Rectifying Behaviors in an Artificial Single-Molecule Funnel

It has long been an aspirational goal to create artificial channel structures that replicate the feat achieved by ion channel proteins. Biological ion channels occasionally demonstrate multiple conductance states (known as subconductance), remaining a challenging property to achieve in artificial channel molecules. We report a funnel-shaped single-molecule channel constructed by an electron-deficient macrocycle and two electron-deficient aromatic imide arms. Planar lipid bilayer measurements reveal distinct current recordings, including a closed state, two conducting states, and spontaneous transitions between the three states, resembling the events seen in biological ion channels. The transitions result from conformational changes induced by chloride transport in the channel molecule. Both opening states show a non-linear and rectifying I-V relationship, indicating voltage-dependent transport due to the asymmetrical channel structure. This work could enhance our understanding of ion permeation and channel opening mechanism.

Naphthalene Diimide-Based Metallacage as an Artificial Ion Channel for Chloride Ion Transport

Developing synthetic molecular devices for controlling ion transmembrane transport is a promising research field in supramolecular chemistry. These artificial ion channels provide models to study ion channel diseases and have huge potential for therapeutic applications. Compared with self-assembled ion channels constructed by intermolecular weak interactions between smaller molecules or cyclic compounds, metallacage-based ion channels have well-defined structures and can exist as single components in the phospholipid bilayer. A naphthalene diimide-based artificial chloride ion channel is constructed through efficient subcomponent self-assembly and its selective ion transport activity in large unilamellar vesicles and the planar lipid bilayer membrane by fluorescence and ion-current measurements is investigated. Molecular dynamics simulations and density functional theory calculations show that the metallacage spans the entire phospholipid bilayer as an unimolecular ion transport channel. This channel transports chloride ions across the cell membrane, which disturbs the ion balance of cancer cells and inhibits the growth of cancer cells at low concentrations.

Redox-Regulated Synthetic Channels: Enabling Reversible Ion Transport by Modulating the Ion-Permeation Pathway

Natural redox-regulated channel proteins often utilize disulfide bonds as redox sensors for adaptive regulation of channel conformations in response to diverse physiological environments. In this study, we developed novel synthetic ion channels capable of reversibly switching their ion-transport capabilities by incorporating multiple disulfide bonds into artificial systems. X-ray structural analysis and electrophysiological experiments demonstrated that these disulfide-bridged molecules possess well-defined tubular cavities and can be efficiently inserted into lipid bilayers to form artificial ion channels. More importantly, the disulfide bonds in these molecules serve as redox-tunable switches to regulate the formation and disruption of ion-permeation pathways, thereby achieving a transition in the transmembrane transport process between the ON and OFF states.

Concentration-Driven Evolution of Adaptive Artificial Ion Channels or Nanopores with Specific Anticancer Activities

In nature, ceramides are a class of sphingolipids possessing a unique ability to self-assemble into protein-permeable channels with intriguing concentration-dependent adaptive channel cavities. However, within the realm of artificial ion channels, this interesting phenomenon is scarcely represented. Herein, we report on a novel class of adaptive artificial channels, Pn-TPPs, based on PEGylated cholic acids bearing triphenylphosphonium (TPP) groups as anion binding motifs. Interestingly, the molecules self-assemble into chloride ion channels at low concentrations while transforming into small molecule-permeable nanopores at high concentrations. Moreover, the TPP groups endow the molecules with mitochondria-targeting properties, enabling them to selectively drill holes on the mitochondrial membrane of cancer cells and subsequently trigger the caspase 9 apoptotic pathway. The anticancer efficacies of Pn-TPPs correlate with their abilities to form nanopores. Significantly, the most active ensembles formed by P5-TPP exhibits impressive anticancer activity against human liver cancer cells, with an IC50 value of 3.8 μM. While demonstrating similar anticancer performance to doxorubicin, P5-TPP exhibits a selectivity index surpassing that of doxorubicin by a factor of 16.8.

Host-Guest-Interaction Enhanced Nitric Oxide Photo-Generation within a Pillar[5]arene Cavity for Antibacterial Gas Therapy

Supramolecular macrocycles with intrinsic cavities have been widely explored as containers to fabricate versatile functional materials via specific host-guest recognitions. However, relatively few studies have focused on the modulation of guest reactivity within a macrocyclic cavity. Here, we demonstrate the confinement effect of pillar[5]arene with an electron-rich and precise cavity that can dramatically enhance guest photoactivity and nitric oxide (NO) generation upon visible light irradiation. Mechanism studies reveal that it is achieved through increasing the ground state nitro-aromatic torsion angle, suppressing the intersystem crossing relaxation path of the S1 state, and accelerating the isomerization reaction path of guest molecules. This NO-generating system displays broad-spectrum antibacterial, biofilm inhibition, and dispersal activities. Moreover, it can accelerate the healing of methicillin-resistant Staphylococcus aureus (MRSA)-infected wounds in vivo.

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 K+ Channels Constructed by Rebuilding the Core Modules of Natural K+ Channels in an Artificial System

Different types of natural K⁺ channels share similar core modules and cation permeability characteristics. In this study, we have developed novel artificial K⁺ channels by rebuilding the core modules of natural K⁺ channels in artificial systems. All the channels displayed high selectivity for K⁺ over Na⁺ and exhibited a selectivity sequence of K⁺ ≈Rb⁺ during the transport process, which is highly consistent with the cation permeability characteristics of natural K⁺ channels. More importantly, these artificial channels could be efficiently inserted into cell membranes and mediate the transmembrane transport of K⁺ , disrupting the cellular K⁺ homeostasis and eventually triggering the apoptosis of cells. These findings demonstrate that, by rebuilding the core modules of natural K⁺ channels in artificial systems, the structures, transport behaviors, and physiological functions of natural K⁺ channels can be mimicked in synthetic channels.

Pillararene-Based Supramolecular Polymers for Cancer Therapy

Supramolecular polymers have attracted considerable interest due to their intriguing features and functions. The dynamic reversibility of noncovalent interactions endows supramolecular polymers with tunable physicochemical properties, self-healing, and externally stimulated responses. Among them, pillararene-based supramolecular polymers show great potential for biomedical applications due to their fascinating host-guest interactions and easy modification. Herein, we summarize the state of the art of pillararene-based supramolecular polymers for cancer therapy and illustrate its developmental trend and future perspective.

MXenes Functionalized with Macrocyclic Hosts: From Molecular Design to Applications

Two-dimensional (2D) MXene has aroused wide attention for its excellent physical and chemical properties. The interlayer engineering formed by layer-by-layer stacking of MXene nanosheets can be employed for molecular sieving and water purification by incorporating specific groups onto the exterior surface of MXene. Macrocyclic hosts exhibiting unique structural features and recognition ability can construct smart devices for external stimuli with reversible features between macrocycles and guests. On that basis, macrocyclic hosts can be anchored to MXene to provide numerous insights into their compositions and intercalation states. In this review, the MXene prepared based on macrocyclic hosts from molecular design to applications is highlighted. Various MXenes functionalized with macrocyclic hosts are empowered in functional membrane (including water purification, organic solvent nanofiltration, and electromagnetic shielding), photocatalysis, sensing, and adsorption (interactions with specific guest). Hopefully, this review can bring new inspiration to the design of multifunctional MXene-based materials and improving its practical applications.

Controlling transmembrane ion transport via photo-regulated carrier mobility

Stimuli-responsive transmembrane ion carriers allow for targeted and controllable transport activity, with potential applications as therapeutics for channelopathies and cancer, and in fundamental studies into ion transport phenomena. These applications require OFF-ON activation from a fully inactive state which does not exhibit background activity, but this remains challenging to achieve with synthetic transport systems. Here we introduce a novel mechanism for photo-gating mobile ion carriers, which involves modulating the mobility of the carriers within the lipid bilayer membrane. By appending a membrane-targeting anchor to the carrier using a photo-cleavable linker, the carrier's ion transport activity is fully switched off by suppressing its ability to shuttle between the two aqueous-membrane interfaces of the bilayer. The system can be reactivated rapidly in situ within the membrane by photo-triggered cleavage of the anchor to release the mobile ion carrier. This approach does not involve direct functionalization of the ion binding site of the carrier, and so does not require the de novo design of novel ion binding motifs to implement the photo-caging of activity. This work demonstrates that controlling the mobility of artificial transport systems enables precise control over activity, opening up new avenues for spatio-temporally targeted ionophores.

Supramolecular Mechanosensitive Potassium Channel Formed by Fluorinated Amphiphilic Cyclophane

Inspired by mechanosensitive potassium channels found in nature, we developed a fluorinated amphiphilic cyclophane composed of fluorinated rigid aromatic units connected via flexible hydrophilic octa(ethylene glycol) chains. Microscopic and emission spectroscopic studies revealed that the cyclophane could be incorporated into the hydrophobic layer of the lipid bilayer membranes and self-assembled to form a supramolecular transmembrane ion channel. Current recording measurements using cyclophane-containing planer lipid bilayer membranes successfully demonstrated an efficient transmembrane ion transport. We also demonstrated that the ion transport property was sensitive to the mechanical forces applied to the membranes. In addition, ion transport assays using pH-sensitive fluorescence dye revealed that the supramolecular channel possesses potassium ion selectivity. We also performed all-atom hybrid quantum-mechanical/molecular mechanical simulations to assess the channel structures at atomic resolution and the mechanism of selective potassium ion transport. This research demonstrated the first example of a synthetic mechanosensitive potassium channel, which would open a new door to sensing and manipulating biologically important processes and purification of key materials in industries.

Charged Tubular Supramolecule Boosting Multivalent Interactions for the Drastic Suppression of Aβ Fibrillation

Anti-Aβ therapy has dominated clinical trials for the prevention and treatment of Alzheimer's disease (AD). However, suppressing Aβ aggregation and disintegrating mature fibrils simultaneously remains a great challenge. In this work, we developed a new strategy using a charged tubular supramolecule (CTS) with pillar[5]arene as the backbone and modifying amino and carboxyl groups at the tubular terminals (noted as CTS-A, CTS-A/C, and CTS-C, respectively) to suppress Aβ fibrillation for the first time. According to the spectroscopic and microscopic characterizations, Aβ₄₀ fibrillation can be efficiently suppressed by CTS-A in a very low inhibitor:peptide (I:P) molar ratio (1:10). A greatly alleviated cytotoxic effect of Aβ peptides after the inhibition or disaggregation process is further disclosed. The well-organized supramolecular structure drives multivalent interaction and gains enhanced efficiency on amyloid fibrillar modulation. These results open a new path for the design of supramolecules in the application of AD treatment.

Adhesion behaviour of bulk supramolecular polymers via pillar[5]arene-based molecular recognition

Pillar[n]arenes were rarely used as the building blocks for supramolecular adhesives. Herein, pillar[5]arene-based supramolecular polymer materials with tough adhesion behaviours on different substrates were prepared, with adhesion strengths up to 4.75 MPa. Strong and long-term dichloromethane-resistant adhesion performances were successfully obtained.