Molecular Ion Fishers as Highly Active and Exceptionally Selective K+ Transporters

Light-Powered Propeller-like Transporter for Boosted Transmembrane Ion Transport

Membrane-active molecular machines represent a recently emerging, yet important line of expansion in the field of artificial transmembrane transporters. Their hitherto demonstrated limited types (molecular swing, ion fishers, shuttlers, rotors, etc.) certainly call for new inspiring developments. Here, we report a very first motorized ion-transporting carrier-type transporter, i.e., a modularly tunable, light-powered propeller-like transporter derived from Feringa's molecular motor for consistently boosting transmembrane ion transport under continuous UV light irradiation. Based on the EC50 values, the molecular propeller-mediated ion transport activities under UV light irradiation for 300 s are 2.31, 1.74, 2.29, 2.80, and 2.92 times those values obtained without irradiation for Li+, Na+, K+, Rb+, and Cs+ ions, respectively, with EC50 value as low as 0.71 mol % for K+ ion under light irradiation.

Acylhydrazone-based reversibly photoswitchable ion pair transporter with OFF-ON cotransport activity

The cellular membrane transport of physiologically important cations and anions is omnipresent and regulates different physiological functions. Whereas a notable number of cation-anion transporters are being developed to transport salts across the membrane, developing an artificial cation-anion symporter with stimulus-responsive activities is an immense obstacle. Herein, for the first time, we report reversibly photoswitchable acylhydrazone-based transporter 2 that has distinctive OFF-ON cation-anion co-transport abilities. The substituent was modified in 1a-1c and 2, to change the to-and-fro movement of the transporter to enhance the ion transport efficiency. Ion transport experiments across the lipid bilayer membrane demonstrate that 1a has the highest transport activity among the series with irreversible photoisomerization properties, whereas 2 has a unique reversible photoisomerization property. A detailed transport study indicated that the E-conformer of compound 2 facilitates Na+/Cl- transport via the symport process by following the carrier mode of ion transport. 23Na NMR and chloride selective electrode assays confirmed the OFF and ON state of ion transport of compound 2 with photoirradiation. An assembly of [(2 E )2 + NaCl] was subjected to geometry optimization to understand the responsible ion binding motif. Geometry optimization followed by the natural bond orbital analysis of 1a Z and 2 Z demonstrated that 1a Z forms comparatively stronger intramolecular H-bonding than 2 Z , making it accessible for reversible photoisomerization.

Using Nanoscale Passports To Understand and Unlock Ion Channels as Gatekeepers of the Cell

Natural ion channels are proteins embedded in the cell membrane that control many aspects of cell and human physiology by acting as gatekeepers, regulating the flow of ions in and out of cells. Advances in nanotechnology have influenced the methods for studying ion channels in vitro, as well as ways to unlock the delivery of therapeutics by modulating them in vivo. This review provides an overview of nanotechnology-enabled approaches for ion channel research with a focus on the synthesis and applications of synthetic ion channels. Further, the uses of nanotechnology for therapeutic applications are critically analyzed. Finally, we provide an outlook on the opportunities and challenges at the intersection of nanotechnology and ion channels. This work highlights the key role of nanoscale interactions in the operation and modulation of ion channels, which may prompt insights into nanotechnology-enabled mechanisms to study and exploit these systems in the near future.

Synthetic cation channel: reconstructing the ion permeation pathway of TRPA1 in an artificial system

A novel artificial cation channel was developed by rebuilding the ion permeation pathway of the natural channel protein (TRPA1) in a synthetic system. This tubular molecule can effectively embed into lipid bilayers and form transmembrane channels, thereby mediating cation transport. Furthermore, due to its carboxyl-modified ion permeation pathway, the transport activity of this artificial channel can be modulated by the pH of the buffer solution.

Length dependent reversible off-on activation of photo-switchable relay anion transporters

A homologous series of azobenzene-derived photo-switchable ion relay transporters is reported. We reveal that both the length and geometry of the relay strongly affect transport rate, allowing the relative activity of the E and Z isomers to be reversed and hence the wavelengths of light used for on and off switching to be exchanged.

A non-B DNA binding peptidomimetic channel alters cellular functions

DNA binding transcription factors possess the ability to interact with lipid membranes to construct ion-permeable pathways. Herein, we present a thiazole-based DNA binding peptide mimic TBP2, which forms transmembrane ion channels, impacting cellular ion concentration and consequently stabilizing G-quadruplex DNA structures. TBP2 self-assembles into nanostructures, e.g., vesicles and nanofibers and facilitates the transportation of Na+ and K+ across lipid membranes with high conductance (~0.6 nS). Moreover, TBP2 exhibits increased fluorescence when incorporated into the membrane or in cellular nuclei. Monomeric TBP2 can enter the lipid membrane and localize to the nuclei of cancer cells. The coordinated process of time-dependent membrane or nuclear localization of TBP2, combined with elevated intracellular cation levels and direct G-quadruplex (G4) interaction, synergistically promotes formation and stability of G4 structures, triggering cancer cell death. This study introduces a platform to mimic and control intricate biological functions, leading to the discovery of innovative therapeutic approaches.

Molecular Motor-Driven Light-Controlled Logic-Gated K+ Channel for Cancer Cell Apoptosis

Developing artificial ion transport systems, which process complicated information and step-wise regulate properties, is essential for deeply comprehending the subtle dynamic behaviors of natural channel proteins (NCPs). Here a photo-controlled logic-gated K+ channel based on single-chain random heteropolymers containing molecular motors, exhibiting multi-core processor-like properties to step-wise control ion transport is reported. Designed with oxygen, deoxygenation, and different wavelengths of light as input signals, complicated logical circuits comprising "YES", "AND", "OR" and "NOT" gate components are established. Implementing these logical circuits with K+ transport efficiencies as output signals, multiple state transitions including "ON", "Partially OFF" and "Totally OFF" in liposomes and cancer cells are realized, further causing step-wise anticancer treatments. Dramatic K+ efflux in the "ON" state (decrease by 50% within 7 min) significantly induces cancer cell apoptosis. This integrated logic-gated strategy will be expanded toward understanding the delicate mechanism underlying NCPs and treating cancer or other diseases is expected.

Fluorogenic Chemical Probes for Wash-free Imaging of Cell Membrane Damage in Ferroptosis, Necrosis, and Axon Injury

Selectively labeling cells with damaged membranes is needed not only for identifying dead cells in culture, but also for imaging membrane barrier dysfunction in pathologies in vivo. Most membrane permeability stains are permanently colored or fluorescent dyes that need washing to remove their non-uptaken extracellular background and reach good image contrast. Others are DNA-binding environment-dependent fluorophores, which lack design modularity, have potential toxicity, and can only detect permeabilization of cell volumes containing a nucleus (i.e., cannot delineate damaged volumes in vivo nor image non-nucleated cell types or compartments). Here, we develop modular fluorogenic probes that reveal the whole cytosolic volume of damaged cells, with near-zero background fluorescence so that no washing is needed. We identify a specific disulfonated fluorogenic probe type that only enters cells with damaged membranes, then is enzymatically activated and marks them. The esterase probe MDG1 is a reliable tool to reveal live cells that have been permeabilized by biological, biochemical, or physical membrane damage, and it can be used in multicolor microscopy. We confirm the modularity of this approach by also adapting it for improved hydrolytic stability, as the redox probe MDG2. We conclude by showing the unique performance of MDG probes in revealing axonal membrane damage (which DNA fluorogens cannot achieve) and in discriminating damage on a cell-by-cell basis in embryos in vivo. The MDG design thus provides powerful modular tools for wash-free in vivo imaging of membrane damage, and indicates how designs may be adapted for selective delivery of drug cargoes to these damaged cells: offering an outlook from selective diagnosis toward therapy of membrane-compromised cells in disease.

Artificial transmembrane potassium transporters: designs, functions, mechanisms and applications

Potassium channels represent the most prevalent class of ion channels, exerting regulatory control over numerous vital biological processes, including muscle contraction, neurotransmitter release, cell proliferation, and apoptosis. The seamless integration of astonishing functions into a sophisticated structure, as seen in these protein channels, inspires the chemical community to develop artificial versions, gearing toward simplifying their structure while replicating their key functions. In particular, over the past ten years or so, a number of elegant artificial potassium transporters have emerged, demonstrating high selectivity, high transport efficiency or unprecedented transport mechanisms. In this review, we will provide a detailed exposition of these artificial potassium transporters that are derived from a single molecular backbone or self-assembled from multiple components, with their respective structural designs, channel functions, transport mechanisms and biomedical applications thoroughly reviewed.

Molecular Machines For The Control Of Transmembrane Transport

Nature embeds some of its molecular machinery, including ion pumps, within lipid bilayer membranes. This has inspired chemists to attempt to develop synthetic analogues to exploit membrane confinement and transmembrane potential gradients, much like their biological cousins. In this perspective, we outline the various strategies by which molecular machines─molecular systems in which a nanomechanical motion is exploited for function─have been designed to be incorporated within lipid membranes and utilized to mediate transmembrane ion transport. We survey molecular machines spanning both switches and motors, those that act as mobile carriers or that are anchored within the membrane, mechanically interlocked molecules, and examples that are activated in response to external stimuli.

Designing artificial ion channels with strict K+/Na+ selectivity toward next-generation electric-eel-mimetic ionic power generation

A biological potassium channel is >1000 times more permeable to K+ than to Na+ and exhibits a giant permeation rate of ∼108 ions/s. It is a great challenge to construct artificial potassium channels with such high selectivity and ion conduction rate. Herein, we unveil a long-overlooked structural feature that underpins the ultra-high K+/Na+ selectivity. By carrying out massive molecular dynamics simulation for ion transport through carbonyl-oxygen-modified bi-layer graphene nanopores, we find that the twisted carbonyl rings enable strict potassium selectivity with a dynamic K+/Na+ selectivity ratio of 1295 and a K+ conduction rate of 3.5 × 107 ions/s, approaching those of the biological counterparts. Intriguingly, atomic trajectories of K+ permeation events suggest a dual-ion transport mode, i.e. two like-charged potassium ions are successively captured by the nanopores in the graphene bi-layer and are interconnected by sharing one or two interlayer water molecules. The dual-ion behavior allows rapid release of the exiting potassium ion via a soft knock-on mechanism, which has previously been found only in biological ion channels. As a proof-of-concept utilization of this discovery, we propose a novel way for ionic power generation by mixing KCl and NaCl solutions through the bi-layer graphene nanopores, termed potassium-permselectivity enabled osmotic power generation (PoPee-OPG). Theoretically, the biomimetic device achieves a very high power density of >1000 W/m2 with graphene sheets of <1% porosity. This study provides a blueprint for artificial potassium channels and thus paves the way toward next-generation electric-eel-mimetic ionic power generation.

Supramolecular Barrel-Rosette Ion Channel Based on 3,5-Diaminobenzoic Acid for Cation-Anion Symport

The structural tropology and functions of natural cation-anion symporting channels have been continuously investigated due to their crucial role in regulating various physiological functions. To understand the physiological functions of the natural symporter channels, it is vital to develop small-molecule-based biomimicking systems that can provide mechanistic insights into the ion-binding sites and the ion-translocation pathways. Herein, we report a series of bis((R)-(-)-mandelic acid)-linked 3,5-diaminobenzoic acid based self-assembled ion channels with distinctive ion transport ability. Ion transport experiment across the lipid bilayer membrane revealed that compound 1 b exhibits the highest transport activity among the series, and it has interesting selective co-transporting functions, i.e., facilitates K+ /ClO4 - symport. Electrophysiology experiments confirmed the formation of supramolecular ion channels with an average diameter of 6.2±1 Å and single channel conductance of 57.3±1.9 pS. Selectivity studies of channel 1 b in a bilayer lipid membrane demonstrated a permeability ratio ofP C l - / P K + = 0 . 053 ± 0 . 02 ${{P}_{{Cl}^{-}}/{P}_{{K}^{+}}=0.053\pm 0.02}$ ,P C l O 4 - / P C l - = 2 . 1 ± 0 . 5 ${{P}_{{ClO}_{4}^{-}}/{P}_{{Cl}^{-}}=2.1\pm 0.5}$ , andP K + / P N a + = 1 . 5 ± 1 , ${{P}_{{K}^{+}}/{P}_{{Na}^{+}}=1.5\pm 1,}$ indicating the higher selectivity of the channel towards KClO4 over KCl salt. A hexameric assembly of a trimeric rosette of 1 b was subjected to molecular dynamics simulations with different salts to understand the supramolecular channel formation and ion selectivity pattern.

Molecular Complementarity of Proteomimetic Materials for Target-Specific Recognition and Recognition-Mediated Complex Functions

As biomolecules essential for sustaining life, proteins are generated from long chains of 20 different α-amino acids that are folded into unique 3D structures. In particular, many proteins have molecular recognition functions owing to their binding pockets, which have complementary shapes, charges, and polarities for specific targets, making these biopolymers unique and highly valuable for biomedical and biocatalytic applications. Based on the understanding of protein structures and microenvironments, molecular complementarity can be exhibited by synthesizable and modifiable materials. This has prompted researchers to explore the proteomimetic potentials of a diverse range of materials, including biologically available peptides and oligonucleotides, synthetic supramolecules, inorganic molecules, and related coordination networks. To fully resemble a protein, proteomimetic materials perform the molecular recognition to mediate complex molecular functions, such as allosteric regulation, signal transduction, enzymatic reactions, and stimuli-responsive motions; this can also expand the landscape of their potential bio-applications. This review focuses on the recognitive aspects of proteomimetic designs derived for individual materials and their conformations. Recent progress provides insights to help guide the development of advanced protein mimicry with material heterogeneity, design modularity, and tailored functionality. The perspectives and challenges of current proteomimetic designs and tools are also discussed in relation to future applications.

Halogen bonding relay and mobile anion transporters with kinetically controlled chloride selectivity

Selective transmembrane transport of chloride over competing proton or hydroxide transport is key for the therapeutic application of anionophores, but remains a significant challenge. Current approaches rely on enhancing chloride anion encapsulation within synthetic anionophores. Here we report the first example of a halogen bonding ion relay in which transport is facilitated by the exchange of ions between lipid-anchored receptors on opposite sides of the membrane. The system exhibits non-protonophoric chloride selectivity, uniquely arising from the lower kinetic barrier to chloride exchange between transporters within the membrane, compared to hydroxide, with selectivity maintained across membranes with different hydrophobic thicknesses. In contrast, we demonstrate that for a range of mobile carriers with known high chloride over hydroxide/proton selectivity, the discrimination is strongly dependent on membrane thickness. These results demonstrate that the selectivity of non-protonophoric mobile carriers does not arise from ion binding discrimination at the interface, but rather through a kinetic bias in transport rates, arising from differing membrane translocation rates of the anion-transporter complexes.

A K+-selective channel with a record-high K+/Na+ selectivity of 20.1

For compounds each containing a phenylalanine moiety with its two ends amidated to have a 15-crown-5 unit and an alkyl chain, a simple tuning of the alkyl chain length delivered a K⁺-selective channel with a record-high K⁺/Na⁺ selectivity of 20.1.

Cu(II)-Triggered Ion Channel Properties of a 2,2'-Bipyridine-Modified Amphotericin B

The development of stimuli-responsive synthetic channels that open and close in response to physical and chemical changes in the surrounding environment has attracted attention because of their potential bioapplications such as sensing, drug release, antibiotics, and molecular manipulation tools to control membrane transport in cells. Metal coordination is ideal as a stimulus for stimuli-responsive channels because it allows for reversible gating behavior through the addition and removal of metal ions and fine-tuning of channel structure through coordination geometry defined by the type of the metal ion and ligand. We have previously reported on transition metal-ion dependent ion permeability control of Amphotericin B (AmB) modified with a metal coordination site, 2,2'-bipyridine ligand (bpy-AmB). AmB is one of the polyene macrolide antibiotics, and it is known that the interaction between AmB and ergosterol molecules is required for AmB channel formation. In contrast, the Cu²⁺ coordination to the bpy moiety of bpy-AmB induces formation of Ca²⁺ ion-permeable channels in the ergosterol-free POPC membrane. However, the details of bpy-AmB properties such as channel stability, ion selectivity, pore size, and the effect of ergosterol on channel formation remain unclear. Here, we investigate bpy-AmB channels triggered by transition metal coordination in POPC or ergosterol-containing POPC liposomes using an HPTS assay, electrophysiological measurements, and time-resolved UV-vis spectral measurements. These analyses reveal that bpy-AmB channels triggered by Cu²⁺ ions are more stable and have larger pore sizes than the original AmB channels and enable efficient permeation of various cations. We believe that our channel design will lead to the construction of metal coordination-triggered synthetic ion channels.

Crown Ether-Based Ion Transporters in Bilayer Membranes

Bilayer membranes that enhance the stability of the cell are essential for cell survival, separating and protecting the interior of the cell from its external environment. Membrane-based channel proteins are crucial for sustaining cellular activities. However, dysfunction of these proteins would induce serial channelopathies, which could be substituted by artificial ion channel analogs. Crown ethers (CEs) are widely studied in the area of artificial ion channels owing to their intrinsic host-guest interaction with different kinds of organic and inorganic ions. Other advantages such as lower price, chemical stability, and easier modification also make CE a research hotspot in the field of synthetic transmembrane nanopores. And numerous CEs-based membrane-active synthetic ion channels were designed and fabricated in the past decades. Herein, the recent progress of CEs-based synthetic ion transporters has been comprehensively summarized in this review, including their design principles, functional mechanisms, controllable properties, and biomedical applications. Furthermore, this review has been concluded by discussing the future opportunities and challenges facing this research field. It is anticipated that this review could offer some inspiration for the future fabrication of novel CEs-derived ion transporters with more advanced structures, properties, and practical applications.

An artificial sodium-selective subnanochannel

Single-ion selectivity with high precision has long been pursued for fundamental bioinspired engineering and applications such as in ion separation and energy conversion. However, it remains a challenge to develop artificial ion channels to achieve single-ion selectivity comparable to their biological analogs, especially for high Na⁺/K⁺ selectivity. Here, we report an artificial sodium channel by subnanoconfinement of 4'-aminobenzo-15-crown-5 ethers (15C5s) into ~6-Å-sized metal-organic framework subnanochannel (MOFSNC). The resulting 15C5-MOFSNC shows an unprecedented Na⁺/K⁺ selectivity of tens to 10² and Na⁺/Li⁺ selectivity of 10³ under multicomponent permeation conditions, comparable to biological sodium channels. A co-ion-responsive single-file transport mechanism in 15C-MOFSNC is proposed for the preferential transport of Na⁺ over K⁺ due to the synergetic effects of size exclusion, charge selectivity, local hydrophobicity, and preferential binding with functional groups. This study provides an alternative strategy for developing potential single-ion selective channels and membranes for many applications.

Guanidinium-Perfunctionalized Polyhedral Oligomeric Silsesquioxanes as Highly Potent Antimicrobials against Planktonic Microbes, Biofilms, and Coronavirus

Supramolecules have been drawing increasing attention recently in addressing healthcare challenges caused by infectious pathogens. We herein report a novel class of guanidinium-perfunctionalized polyhedral oligomeric silsesquioxane (Gua-POSS) supramolecules with highly potent antimicrobial activities. The modular structure of Gua-POSS Tm-Cn consists of an inorganic T10 or T8 core (m = 10 or 8), flexible linear linkers of varying lengths (n = 1 or 3), and peripherally aligned cationic guanidinium groups as the membrane-binding units. Such Gua-POSS supramolecules with spherically arrayed guanidinium cations display high antimicrobial potency against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, as well as fungus (Candida albicans), with the best showing excellently low minimal inhibitory concentrations (MICs) of 1.7-6.8 μM in media, yet with negligible hemolytic activity and low in vitro cytotoxicity to mammalian cells. More significantly, they can inhibit biofilm formation at around their MICs and near-completely break down preestablished difficult-to-break biofilms at 250 μg mL^-1 (∼50 μM). Their strong antiviral efficacy was also experimentally demonstrated against the enveloped murine hepatitis coronavirus as a surrogate of the SARS-CoV species. Overall, this study provides a new design approach to novel classes of sphere-shaped organic-inorganic hybrid supramolecular materials, especially for potent antimicrobial, anti-biofilm, and antiviral applications.

Cholesterol-stabilized membrane-active nanopores with anticancer activities

Cholesterol-enhanced pore formation is one evolutionary means cholesterol-free bacterial cells utilize to specifically target cholesterol-rich eukaryotic cells, thus escaping the toxicity these membrane-lytic pores might have brought onto themselves. Here, we present a class of artificial cholesterol-dependent nanopores, manifesting nanopore formation sensitivity, up-regulated by cholesterol of up to 50 mol% (relative to the lipid molecules). The high modularity in the amphiphilic molecular backbone enables a facile tuning of pore size and consequently channel activity. Possessing a nano-sized cavity of ~ 1.6 nm in diameter, our most active channel Ch-C1 can transport nanometer-sized molecules as large as 5(6)-carboxyfluorescein and display potent anticancer activity (IC₅₀ = 3.8 µM) toward human hepatocellular carcinomas, with high selectivity index values of 12.5 and >130 against normal human liver and kidney cells, respectively.

Bacterial cells utilize cholesterol-enhanced pore formation to specifically target eukaryotic cells. Here, the authors present a class of bio-inspired, cholesterol-enhanced nanopores which display anticancer activities in vitro.

A water-soluble membrane transporter for biologically relevant cations

Synthetic ionophores are promising therapeutic targets, yet poor water solubility limits their potential for translation into the clinic. Here we report a water-soluble, supramolecular self-associating amphiphile that functions as a cation uniporter in synthetic vesicle systems, deriving mechanistic insight through planar bilayer patch clamp experiments.

Selective and rapid water transportation across a self-assembled peptide-diol channel via the formation of a dual water array

Achieving superfast water transport by using synthetically designed molecular artifacts, which exclude salts and protons, is a challenging task in separation science today, as it requires the concomitant presence of a proper water-binding site and necessary selectivity filter for transporting water. Here, we demonstrate the water channel behavior of two configurationally different peptide diol isomers that mimic the natural water channel system, i.e., aquaporins. The solid-state morphology studies showed the formation of a self-assembled aggregated structure, and X-ray crystal structure analysis confirmed the formation of a nanotubular assembly that comprises two distinct water channels. The water permeabilities of all six compounds were evaluated and are found to transport water by excluding salts and protons with a water permeability rate of 5.05 × 108 water molecules per s per channel, which is around one order of magnitude less than the water permeability rate of aquaporins. MD simulation studies showed that the system forms a stable water channel inside the bilayer membrane under ambient conditions, with a 2 × 8 layered assembly, and efficiently transports water molecules by forming two distinct water arrays within the channel.

Monofunctionalized Fluorinated Bambusurils and Their Conjugates for Anion Transport and Extraction

Bambusurils are macrocyclic molecules that are known for their high binding affinity and selectivity toward anions. Here, we present the preparation of two bambusurils bearing fluorinated substituents and one carboxylic function. These monofunctionalized bambusurils were conjugated with crown ether and cholesterol units. The resulting conjugates were successfully tested in liquid-liquid extraction of inorganic salts and chloride/bicarbonate transport across lipid bilayers.

A Light-Driven Molecular Machine Controls K+ Channel Transport and Induces Cancer Cell Apoptosis

The design of artificial ion channels with high activity, selectivity and gating function is challenging. Herein, we designed the light-driven motor molecule MC2, which provides new design criteria to overcome these challenges. MC2 forms a selective K⁺ channel through a single molecular transmembrane mechanism, and the light-driven rotary motion significantly accelerates ion transport, which endows the irradiated motor molecule with excellent cytotoxicity and cancer cell selectivity. Mechanistic studies reveal that the rotary motion of MC2 promotes K⁺ efflux, generates reactive oxygen species and eventually activates caspase-3-dependent apoptosis in cancer cells. Combined with the spatiotemporally controllable advantages of light, we believe this strategy can be exploited in the structural design and application of next-generation synthetic cation transporters for the treatment of cancer and other diseases.

Membrane-Active Molecular Machines

Both biological and artificial membrane transporters mediate passive transmembrane ion flux predominantly via either channel or carrier mechanisms, tightly regulating the transport of materials entering and exiting the cell. One early elegant example unclassifiable as carriers or channels was reported by Smith who derivatized a phospholipid molecule into an anion transporter, facilitating membrane transport via a two-station relay mechanism (Smith et al. J. Am. Chem. Soc. 2008, 130, 17274-17275). Our journey toward blurring or even breaking the boundaries defined by the carrier and channel mechanisms starts in January of 2018 when seeing a child swinging on the swing at the playground park. Since then, I have been wondering whether we could build a nanoscale-sized molecular swing able to perform the swing function at the molecular level to induce transmembrane ion flux. Such research journey culminates in several membrane-active artificial molecular machines, including molecular swings, ion fishers, ion swimmers, rotors, tetrapuses and dodecapuses that permeabilize the membrane via swinging, ion-fishing, swimming, rotating, or swing-relaying actions, respectively. Except for molecular ion swimmers, these unconventional membrane transporters in their most stable states readily span across the entire membrane in a way akin to channels. With built-in flexible arms that can swing or bend in the dynamic membrane environment, they transport ions via constantly changing ion permeation pathways that are more defined than carriers but less defined than channels. Applying the same benzo-crown ether groups as the sole ion-binding and -transporting units, these transporters however differ immensely in ion transport property. While the maximal K⁺ transport activity is achieved by the molecular swing also termed "motional channel" that displays an EC₅₀ value of 0.021 mol % relative to lipid and transports K⁺ ions at rate 27% faster than gramicidin A, the highest K⁺/Na⁺ selectivity of 18.3 is attained by the molecular ion fisher, with the highest Na⁺/K⁺ selectivity of 13.7 by the molecular dodecapus. Having EC₅₀ values of 0.49-1.60 mol % and K⁺/Na⁺ values of 1.1-6.3, molecular rotors and tetrapuses are found to be generally active but weakly to moderately K⁺-selective. For molecular ion swimmers that contain 10 to 14 carbon atom alkyl linkers, they all turn out to be highly active (EC₅₀ = 0.18-0.41 mol %) and highly selective (R_K⁺/R_Na⁺ = 7.0-9.5) transporters. Of special note are crown ether-appended molecular dodecapuses that establish the C₆₀-fullerene core as an excellent platform to allow for a direct translation of solution binding affinity to transmembrane ion transport selectivity, providing a de novo basis for rationally designing artificial ion transporters with high transport selectivity. Considering remarkable cytotoxic activities displayed by molecular swings and ion swimmers, the varied types of existing and emerging unconventional membrane transporters with enhanced activities and selectivities eventually might lead to medical benefits in the future.

Angstrom-scale ion channels towards single-ion selectivity

Artificial ion channels with ion permeability and selectivity comparable to their biological counterparts are highly desired for efficient separation, biosensing, and energy conversion technologies. In the past two decades, both nanoscale and sub-nanoscale ion channels have been successfully fabricated to mimic biological ion channels. Although nanoscale ion channels have achieved intelligent gating and rectification properties, they cannot realize high ion selectivity, especially single-ion selectivity. Artificial angstrom-sized ion channels with narrow pore sizes <1 nm and well-defined pore structures mimicking biological channels have accomplished high ion conductivity and single-ion selectivity. This review comprehensively summarizes the research progress in the rational design and synthesis of artificial subnanometer-sized ion channels with zero-dimensional to three-dimensional pore structures. Then we discuss cation/anion, mono-/di-valent cation, mono-/di-valent anion, and single-ion selectivities of the synthetic ion channels and highlight their potential applications in high-efficiency ion separation, energy conversion, and biological therapeutics. The gaps of single-ion selectivity between artificial and natural channels and the connections between ion selectivity and permeability of synthetic ion channels are covered. Finally, the challenges that need to be addressed in this research field and the perspective of angstrom-scale ion channels are discussed.

Conformational Change from U- to I-Shape of Ion Transporters Facilitates K+ Transport across Lipid Bilayers

We have investigated, at the atomic level, the ion-fishing mechanism underlying the ion transport across membranes mediated by an artificial ion transporter composed of a hydroxyl-rich cholesterol group, a flexible alkyl chain, and a crown ether. Our results show that the transporter can spontaneously insert into the membrane and switch between the folded (U-shaped) and extended (I-shaped) conformations. The free-energy profile associated with the conformational transition indicates that compared with the U-shaped conformation of the transporter, the I-shaped one is thermodynamically more favorable. Furthermore, the free-energy profiles describing the ion translocation reveal that the transporter capturing the ion in U-shape on one side of the membrane and releasing it in I-shape on the other side constitutes a key way for the highly efficient transport of K⁺ ions. We present herewith a rigorous and rational framework to decipher the detailed ion-fishing mechanism of transmembrane ion transport with exceptionally high activity.

Rational ion transport management mediated through membrane structures

Unique membrane structures endow membranes with controlled ion transport properties in both biological and artificial systems, and they have shown broad application prospects from industrial production to biological interfaces. Herein, current advances in nanochannel-structured membranes for manipulating ion transport are reviewed from the perspective of membrane structures. First, the controllability of ion transport through ion selectivity, ion gating, ion rectification, and ion storage is introduced. Second, nanochannel-structured membranes are highlighted according to the nanochannel dimensions, including single-dimensional nanochannels (i.e., 1D, 2D, and 3D) functioning by the controllable geometrical parameters of 1D nanochannels, the adjustable interlayer spacing of 2D nanochannels, and the interconnected ion diffusion pathways of 3D nanochannels, and mixed-dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D) tuned through asymmetric factors (e.g., components, geometric parameters, and interface properties). Then, ultrathin membranes with short ion transport distances and sandwich-like membranes with more delicate nanochannels and combination structures are reviewed, and stimulus-responsive nanochannels are discussed. Construction methods for nanochannel-structured membranes are briefly introduced, and a variety of applications of these membranes are summarized. Finally, future perspectives to developing nanochannel-structured membranes with unique structures (e.g., combinations of external macro/micro/nanostructures and the internal nanochannel arrangement) for mediating ion transport are presented.

Crystal Packing-Guided Construction of Hetero-Oligomeric Peptidic Ensembles as Synthetic 3-in-1 Transporters

Strategies to generate heteromeric peptidic ensembles via a social self-sorting process are limited. Herein, we report a crystal packing-inspired social self-sorting strategy broadly applicable to diverse types of H-bonded peptidic frameworks. Specifically, a crystal structure of H-bonded alkyl chain-appended monopeptides reveals an inter-chain separation distance of 4.8 Å dictated by the H-bonded amide groups, which is larger than 4.1 Å separation distance desired by the tightly packed straight alkyl chains. This incompatibility results in loosely packed alkyl chains, prompting us to investigate and validate the feasibility of applying bulky tert-butyl groups, modified with an anion-binding group, to alternatively interpenetrate the straight alkyl chains, modified with a crown ether group. Structurally, this social self-sorting approach generates highly stable hetero-oligomeric ensembles, having alternated anion- and cation-binding units vertically aligned to the same side. Functionally, these hetero-oligomeric ensembles promote transmembrane transport of cations, anions and more interestingly zwitterionic species such as amino acids.

Nanomachine-Assisted Ion Transport Across Membranes: From Mechanism to Rational Design and Applications

Assisting ion transport across membranes by means of sophisticated molecular machines has promising applications in the treatment of diseases induced by dysregulated ion transport. To develop such nanoscale devices imbued with specific functions, rational de novo design, upstream from costly syntheses, is eminently desirable but would require the atomic detail of the translocation mechanism, which is still largely missing. We have explored the full ion capture-transport-release process over an aggregate simulation time of 60 μs, employing leading-edge enhanced-sampling algorithms to disentangle with unprecedented detail the mechanism that underlies ion transport mediated by a membrane-spanning [2]rotaxane composed of an ion carrier linked to a wheel threaded onto an axle. Beyond validating the reliability of our methodology through careful examination of the clockwork of a documented nanomachine, we put forth an original pH-controlled nano-object that can assist transient unidirectional ion transport across membranes.

Small Molecule-Based Highly Active and Selective K+ Transporters with Potent Anticancer Activities

We report here a novel class of cation transporters with extreme simplicity, opening a whole new dimension of scientific research for finding small molecule-based cation transporters for therapeutic applications. Comprising three modular components (a headgroup, a flexible alkyl chain-derived body, and a crown ether-derived foot for ion binding), these transporters efficiently (EC₅₀ = 0.18-0.41 mol % relative to lipid) and selectively (K⁺/Na⁺ selectivity = 7.0-9.5) move K⁺ ions across the membrane. Importantly, the most active (EC₅₀ = 0.18-0.22 mol %) and highly selective series of transporters A12, B12, and C12 concurrently possess potent anticancer activities with IC₅₀ values as low as 4.35 ± 0.91 and 6.00 ± 0.13 μM toward HeLa and PC3 cells, respectively. Notably, a mere replacement of the 18-crown-6 unit in the structure with 12-crown-4 or 15-crown-5 units completely annihilates the cation-transporting ability.

Buckyball-Based Spherical Display of Crown Ethers for De Novo Custom Design of Ion Transport Selectivity

Searching for membrane-active synthetic analogues that are structurally simple yet functionally comparable to natural channel proteins has been of central research interest in the past four decades, yet custom design of the ion transport selectivity still remains a grand challenge. Here we report on a suite of buckyball-based molecular balls (MBs), enabling transmembrane ion transport selectivity to be custom designable. The modularly tunable MBm-Cn (m = 4-7; n = 6-12) structures consist of a C₆₀-fullerene core, flexible alkyl linkers Cn (i.e., C6 for n-C₆H₁₂ group), and peripherally aligned benzo-3m-crown-m ethers (i.e., m = 4 for benzo-12-crown-4) as ion-transporting units. Screening a matrix of 16 such MBs, combinatorially derived from four different crown units and four different Cn linkers, intriguingly revealed that their transport selectivity well resembles the intrinsic ion binding affinity of the respective benzo-crown units present, making custom design of the transport selectivity possible. Specifically, MB4s, containing benzo-12-crown-4 units, all are Li⁺-selective in transmembrane ion transport, with the most active MB4-C10 exhibiting an EC₅₀(Li⁺) value of 0.13 μM (corresponding to 0.13 mol % of the lipid present) while excluding all other monovalent alkali-metal ions. Likewise, the most Na⁺ selective MB5-C8 and K⁺ selective MB6-C8 demonstrate high Na⁺/K⁺ and K⁺/Na⁺ selectivity values of 13.7 and 7.8, respectively. For selectivity to Rb⁺ and Cs⁺ ions, the most active MB7-C8 displays exceptionally high transport efficiencies, with an EC₅₀(Rb⁺) value of 105 nM (0.11 mol %) and an EC₅₀(Cs⁺) value of 77 nM (0.079 mol %).

Engineering of stimuli-responsive lipid-bilayer membranes using supramolecular systems

The membrane proteins found in nature control many important cellular functions, including signal transduction and transmembrane ion transport, and these, in turn, are regulated by external stimuli, such as small molecules, membrane potential and light. Membrane proteins also find technological applications in fields ranging from optogenetics to synthetic biology. Synthetic supramolecular analogues have emerged as a complementary method to engineer functional membranes. This Review describes stimuli-responsive supramolecular systems developed for the control of ion transport, signal transduction and catalysis in lipid-bilayer-membrane systems. Recent advances towards achieving spatio-temporal control over activity in artificial and living cells are highlighted. Current challenges, the scope, limitations and future potential to exploit supramolecular systems for engineering stimuli-responsive lipid-bilayer membranes are discussed.

Mediating K+/H+ Transport on Organelle Membranes to Selectively Eradicate Cancer Stem Cells with a Small Molecule

Molecules that are capable of disrupting cellular ion homeostasis offer unique opportunities to treat cancer. However, previously reported synthetic ion transporters showed limited value, as promiscuous ionic disruption caused toxicity to both healthy cells and cancer cells indiscriminately. Here we report a simple yet efficient synthetic K⁺ transporter that takes advantage of the endogenous subcellular pH gradient and membrane potential to site-selectively mediate K⁺/H⁺ transport on the mitochondrial and lysosomal membranes in living cells. Consequent mitochondrial and lysosomal damages enhanced cytotoxicity to chemo-resistant ovarian cancer stem cells (CSCs) via apoptosis induction and autophagy suppression with remarkable selectivity (up to 47-fold). The eradication of CSCs blunted tumor formation in mice. We believe this strategy can be exploited in the structural design and applications of next-generation synthetic cation transporters for the treatment of cancer and other diseases related to dysfunctional K⁺ channels.

Enhancing K+ transport activity and selectivity of synthetic K+ channels via electron-donating effects

Electron-withdrawing groups enhance ion transport activity by 160% and selectivity by >50%, leading to high K⁺/Na⁺ selectivity of 14.0.

Pyridine/Oxadiazole-Based Helical Foldamer Ion Channels with Exceptionally High K+ /Na+ Selectivity

Protein channels are characterized by high transport selectivity, which is essential for maintaining cellular function. Efforts to reproduce such high selectivity over the past four decades have not been very successful. We report a novel series of aromatic foldamer-based polymeric channels where the backbone is stabilized by differential electrostatic repulsions among heteroatoms helically arrayed along the helical backbone. Nanotubes averaging 2.3 and 2.7 nm in length mediate highly efficient transport of K⁺ ions as a consequence of hydrophilic electron-rich hollow cavities that are 3 Å in diameter. Exceptionally high K⁺ and Na⁺ selectivity values of 16.3 and 12.6, respectively, are achieved.