Exploiting peptide nanostructures to construct functional artificial ion channels

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.

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.

An Exceptionally Active and Highly Selective Perchlorate Transporter Containing a Trimesic Amide Scaffold

We report here a series of alkyl group-modified trimesic amide molecules (TAs) with excellent anion transport activities. Among them, TA6, with the highest ion transport activity and excellent selectivity, efficiently transports anions across the membrane in the order of ClO4- > I- > NO3- > Br- > Cl-, with an EC50 value as low as 17.6 nM (0.022 mol% relative to lipid molecules) for ClO4-, which outperforms other anions by 5- to 22-folds and manifests as the best perchlorate transporter ever reported.

Non-Covalently Stapled H+ /Cl- Ion Channels Activatable by Visible Light for Targeted Anticancer Therapy

The development of stimuli-responsive artificial H+ /Cl- ion channels, capable of specifically disturbing the intracellular ion homeostasis of cancer cells, presents an intriguing opportunity for achieving high selectivity in cancer therapy. Herein, we describe a novel family of non-covalently stapled self-assembled artificial channels activatable by biocompatible visible light at 442 nm, which enables the co-transport of H+ /Cl- across the membrane with H+ /Cl- transport selectivity of 6.0. Upon photoirradiation of the caged C4F-L for 10 min, 90 % of ion transport efficiency can be restored, giving rise to a 10.5-fold enhancement in cytotoxicity against human colorectal cancer cells (IC50 =8.5 μM). The mechanism underlying cancer cell death mediated by the H+ /Cl- channels involves the activation of the caspase 9 apoptosis pathway as well as the scarcely reported disruption of the autophagic processes. In the absence of photoirradiation, C4F-L exhibits minimal toxicity towards normal intestine cells, even at a concentration of 200 μM.

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.

Design of Chiral β-Double Helices from γ-Peptide Foldamers

Chirality is ubiquitous in nature, and homochirality is manifested in many biomolecules. Although β-double helices are rare in peptides and proteins, they consist of alternating L- and D-amino acids. No peptide double helices with homochiral amino acids have been observed. Here, we report chiral β-double helices constructed from γ-peptides consisting of alternating achiral (E)-α,β-unsaturated 4,4-dimethyl γ-amino acids and chiral (E)-α,β-unsaturated γ-amino acids in both single crystals and in solution. The two independent strands of the same peptide intertwine to form a β-double helix structure, and it is stabilized by inter-strand hydrogen bonds. The peptides with chiral (E)-α,β-unsaturated γ-amino acids derived from α-L-amino acids adopt a (P)-β-double helix, whereas peptides consisting of (E)-α,β-unsaturated γ-amino acids derived from α-D-amino acids adopt an (M)-β-double helix conformation. The circular dichroism (CD) signature of the (P) and (M)-β-double helices and the stability of these peptides at higher temperatures were examined. Furthermore, ion transport studies suggested that these peptides transport ions across membranes. Even though the structural analogy suggests that these new β-double helices are structurally different from those of the α-peptide β-double helices, they retain ion transport activity. The results reported here may open new avenues in the design of functional foldamers.

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.

Transmembrane Transporter Constructed from PlatinumMetal-organic Cage

A perylene diimide-based metal-organic cage (MOC4c) was found to be an efficient transmembrane transporter for ions and small molecules through the internal cavity of the cage. MOC4c could selectively transport different anions, as evidenced by vesicle-based fluorescenceassays and planar lipid bilayer-based current recordings.Furthermore, MOC4c appears tofacilitate calcein transport across the lipid bilayer membrane of a livingcell, suggesting that MOC4c could be used as a biologicaltool for small molecule drugstransmembrane transportation.

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.

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.

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.

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.

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.

Guest-Driven Control of Folding in a Crown-Ether-Functionalized ortho-Phenylene

A crown-ether-functionalized o-phenylene tetramer has been synthesized and coassembled with monotopic and ditopic, achiral and chiral secondary ammonium ion guests. NMR spectroscopy shows that the o-phenylene forms both 1:1 and 1:2 complexes with monotopic guests while remaining well-folded. Binding of an elongated ditopic guest, however, forces the o-phenylene to misfold by pulling the terminal rings apart. A chiral ditopic guest biases the o-phenylene twist sense.

Pnictogen-Bonding Catalysis and Transport Combined: Polyether Transporters Made In Situ

The combination of catalysis and transport across lipid bilayer membranes promises directional access to a solvent-free and structured nanospace that could accelerate, modulate, and, at best, enable new chemical reactions. To elaborate on these expectations, anion transport and catalysis with pnictogen and tetrel bonds are combined with polyether cascade cyclizations into bioinspired cation transporters. Characterized separately, synergistic anion and cation transporters of very high activity are identified. Combined for catalysis in membranes, cascade cyclizations are found to occur with a formal rate enhancement beyond one million compared to bulk solution and product formation is detected in situ as an increase in transport activity. With this operational system in place, intriguing perspectives open up to exploit all aspects of this unique nanospace for important chemical transformations.

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.

Synthesis, Biological Evaluation, and Molecular Modeling of Aza-Crown Ethers

Synthetic and natural ionophores have been developed to catalyze ion transport and have been shown to exhibit a variety of biological effects. We synthesized 24 aza- and diaza-crown ethers containing adamantyl, adamantylalkyl, aminomethylbenzoyl, and ε-aminocaproyl substituents and analyzed their biological effects in vitro. Ten of the compounds (8, 10–17, and 21) increased intracellular calcium ([Ca²⁺]_i) in human neutrophils, with the most potent being compound 15 (N,N’-bis[2-(1-adamantyl)acetyl]-4,10-diaza-15-crown-5), suggesting that these compounds could alter normal neutrophil [Ca²⁺]_i flux. Indeed, a number of these compounds (i.e., 8, 10–17, and 21) inhibited [Ca²⁺]_i flux in human neutrophils activated by N-formyl peptide (fMLF). Some of these compounds also inhibited chemotactic peptide-induced [Ca²⁺]_i flux in HL60 cells transfected with N-formyl peptide receptor 1 or 2 (FPR1 or FPR2). In addition, several of the active compounds inhibited neutrophil reactive oxygen species production induced by phorbol 12-myristate 13-acetate (PMA) and neutrophil chemotaxis toward fMLF, as both of these processes are highly dependent on regulated [Ca²⁺]_i flux. Quantum chemical calculations were performed on five structure-related diaza-crown ethers and their complexes with Ca²⁺, Na⁺, and K⁺ to obtain a set of molecular electronic properties and to correlate these properties with biological activity. According to density-functional theory (DFT) modeling, Ca²⁺ ions were more effectively bound by these compounds versus Na⁺ and K⁺. The DFT-optimized structures of the ligand-Ca²⁺ complexes and quantitative structure-activity relationship (QSAR) analysis showed that the carbonyl oxygen atoms of the N,N’-diacylated diaza-crown ethers participated in cation binding and could play an important role in Ca²⁺ transfer. Thus, our modeling experiments provide a molecular basis to explain at least part of the ionophore mechanism of biological action of aza-crown ethers.

Foldamer-Based Potassium Channels with High Ion Selectivity and Transport Activity

Small molecules that independently perform natural channel-like functions show greatly potential in the treatment of human diseases. Taking advantage of aromatic helical scaffolds, we develop a kind of foldamer-based ion channels with lumen size varying from 3.8 to 2.3 Å through a sequence substitution strategy. Our results clearly elucidate the importance of channel size in ion transport selectivity in molecular detail, eventually leading to the discoveries of the best artificial K⁺ channel by far and a rare sodium-preferential channel as well. High K⁺ selectivity and transport activity together make foldamers promising in therapeutic applications.

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.

Ligand Induced CuII Transport Restricts Cancer and Mycobacterial Growth: Towards a Plug-and-Select Ion Channel Scaffold

Synthetic channels with high ion selectivity are attractive drug targets for diseases involving ion dysregulation. Achieving selective transport of divalent ions is highly challenging due their high hydration energies. A small tripeptide amphiphilic scaffold installed with a pybox ligand selectively transports Cu^II ions across membranes. The peptide forms stable dimeric pores in the membrane and transports ions by a Cu²⁺ /H⁺ antiport mechanism. The ligand-induced excellent Cu^II selectivity as well as high membrane permeability of the peptide is exploited to promote cancer cell death. The peptide's ability to restrict mycobacterial growth serves as seeds to evolve antibacterial strategies centred on selectively modulating ion homeostasis in pathogens. This simple peptide can potentially function as a universal, yet versatile, scaffold wherein the ion selectivity can be precisely controlled by modifying the ligand at the C terminus.

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 %).

Transmembrane Ion Channels Formed by a Star of David [2]Catenane and a Molecular Pentafoil Knot

A (Fe^II)₆-coordinated triply interlocked ("Star of David") [2]catenane (6₁² link) and a (Fe^II)₅-coordinated pentafoil (5₁) knot are found to selectively transport anions across phospholipid bilayers. Allostery, topology, and building block stoichiometry all play important roles in the efficacy of the ionophoric activity. Multiple Fe^II cation coordination by the interlocked molecules is crucial: the demetalated catenane exhibits no anion binding in solution nor any transmembrane ion transport properties. However, the topologically trivial, Lehn-type cyclic hexameric Fe^II helicates-which have similar anion binding affinities to the metalated Star of David catenane in solution-also display no ion transport properties. The unanticipated difference in behavior between the open- and closed-loop structures may arise from conformational restrictions in the linking groups that likely enhances the rigidity of the channel-forming topologically complex molecules. The (Fe^II)₆-coordinated Star of David catenane, derived from a hexameric cyclic helicate, is 2 orders of magnitude more potent in terms of ion transport than the (Fe^II)₅-coordinated pentafoil knot, derived from a cyclic pentamer of the same building block. The reduced efficacy is reminiscent of multisubunit protein ion channels assembled with incorrect monomer stoichiometries.

Supramolecular Chemistry in the Biomembrane

The combination of supramolecular functional systems with biomolecular chemistry has been a fruitful exercise for decades, leading to a greater understanding of biomolecules and to a great variety of applications, for example, in drug delivery and sensing. Within these developments, the phospholipid bilayer membrane, surrounding live cells, with all its functions has also intrigued supramolecular chemists. Herein, recent efforts from the supramolecular chemistry community to mimic natural functions of lipid membranes, such as sensing, molecular recognition, membrane fusion, signal transduction, and gated transport, are reviewed.

Selective Proton-Mediated Transport by Electrogenic K+-Binding Macrocycles

Synthetic K+-binding macrocycles have potential as therapeutic agents for diseases associated with KcsA K+ channel dysfunction. We recently discovered that artificial self-assembled n-alkyl-benzoureido-15-crown-5-ether form selective ion-channels for K+ cations, which are highly preferred to Na+ cations. Here, we describe an impressive selective activation of the K+ transport via electrogenic macrocycles, stimulated by the addition of the carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP) proton carrier. The transport performances show that both the position of branching or the size of appended alkyl arms favor high transport activity and selectivity SK+/Na+ up to 48.8, one of the best values reported up to now. Our study demonstrates that high K+/Na+ selectivity obtained with natural KcsA K+ channels is achievable using simpler artificial macrocycles displaying constitutional functions.

Conformational Control in Main Group Phosphazane Anion Receptors and Transporters

Anion binding by receptor molecules is a central field of modern chemistry which impacts areas of catalysis as well as biological and materials chemistry. As binding often requires high chemical stability under aerobic and aqueous conditions for practical applications, carbon-based anion receptors have dominated this field, with main group element analogues receiving far less attention. The recent observation that the air- and moisture-stable amino-cyclophosph(V)azanes of the type [RN(E)P(μ-NR)]₂ (E = O, S, Se) can exhibit halide binding that is competitive with topologically related organic receptors (such as squaramides and thioureas) has motivated us here to explore how the binding properties of phosphazane receptors can be enhanced further. Coordination of transition metals by the two P,N metal coordination sites of the phosph(III)azane dimer [(2-py)NHP(μ-N^tBu)]₂ not only activates the receptor for anion binding (by fixing the optimum exo-exo conformation and polarizing the endocyclic N-H substituents) but also stabilizes the P₂N₂ ring to hydrolysis and oxidation. We show how the binding properties of these receptors can be modulated by the coordinated metal fragments and that they can bind chloride 1 to 2 orders of magnitude stronger than the related squaramides and thioureas. These features can be utilized in anion transport through phospholipid bilayers under aqueous conditions for which transport can be improved by 1 order of magnitude compared to the previous best phosphazane and thiourea transporters. This study demonstrates how careful design of inorganic systems can result in potent supramolecular functionality, beyond that observed for organic counterparts.

Light-controlled switchable complexation by a non-photoresponsive hydrogen-bonded amide macrocycle

A light controlled switchable host–guest system based on a non-photoresponsive H-bonded macrocycle and pyridinium salts was developed using a photoacid.

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.

A Helical Polypeptide-Based Potassium Ionophore Induces Endoplasmic Reticulum Stress-Mediated Apoptosis by Perturbing Ion Homeostasis

Perturbation of potassium homeostasis can affect various cell functions and lead to the onset of programmed cell death. Although ionophores have been intensively used as an ion homeostasis disturber, the mechanisms of cell death are unclear and the bioapplicability is limited. In this study, helical polypeptide-based potassium ionophores are developed to induce endoplasmic reticulum (ER) stress-mediated apoptosis. The polypeptide-based potassium ionophores disturb ion homeostasis and then induce prolonged ER stress in the cells. The ER stress results in oxidative environments that accelerate the activation of mitochondria-dependent apoptosis. Moreover, ER stress-mediated apoptosis is triggered in a tumor-bearing mouse model that suppresses tumor proliferation. This study provides the first evidence showing that helical polypeptide-based potassium ionophores trigger ER stress-mediated apoptosis by perturbation of potassium homeostasis.

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

We report here a unique ion-fishing mechanism as an alternative to conventional carrier or channel mechanisms for mediating highly efficient and exceptionally selective transmembrane K⁺ flux. The molecular framework, underlying the fishing mechanism and comprising a fishing rod, a fishing line and a fishing bait/hook, is simple yet modularly modifiable. This feature enables rapid construction of a series of molecular ion fishers with distinctively different ion transport patterns. While more efficient ion transports are generally achieved by using 18-crown-6 as the fishing bait/hook, ion transport selectivity (K⁺/Na⁺) critically depends on the length of the fishing line, with the most selective MF6-C14 exhibiting exceptionally high selectivity (K⁺/Na⁺ = 18) and high activity ( EC₅₀ = 1.1 mol % relative to lipid).

Molecular Swings as Highly Active Ion Transporters

Ions are transported across membrane mostly via carrier or channel mechanisms. Herein, a unique class of molecular-machine-inspired membrane transporters, termed molecular swings is reported that utilize a previously unexplored swing mechanism for promoting ion transport in a highly efficient manner. In particular, the molecular swing, which carries a 15-crown-5 unit as the ion-binding and transporting unit, exhibits extremely high ion-transport activities with EC₅₀ values of 46 nm (a channel:lipid molar ratio of 1:4800 or 0.021 mol % relative to lipid) and 110 nm for K⁺ and Na⁺ ions, respectively. Remarkably, such ion transport activities remain high in a cholesterol-rich environment, with EC₅₀ values of 130 (0.045 mol % relative to lipid/cholesterol) and 326 nm for K⁺ and Na⁺ ions, respectively.

Thermosensitive Microgels-Decorated Magnetic Graphene Oxides for Specific Recognition and Adsorption of Pb(II) from Aqueous Solution

Herein, we report a novel type of smart graphene oxide nanocomposites (MGO@PNB) with excellent magnetism and high thermosensitive ion-recognition selectivity of lead ions (Pb2+). The MGO@PNB are fabricated by immobilizing superparamagnetic Fe3O4 nanoparticles (NPs) and poly(N-isopropylacrylamide-co-benzo-18-crown-6 acrylamide) thermosensitive microgels (PNB) onto graphene oxide (GO) nanosheets using a simple one-step solvothermal method and mussel-inspired polydopamine chemistry. The PNB are composed of cross-linked poly(N-isopropylacrylamide) (PNIPAM) chains with numerous appended 18-crown-6 units. The 18-crown-6 units serve as hosts that can selectively recognize and capture Pb2+ from aqueous solution, and the PNIPAM chains act as a microenvironmental actuator for the inclusion constants of 18-crown-6/Pb2+ host-guest complexes. The loaded Fe3O4 NPs endow the MGO@PNB with convenient magnetic separability. The fabricated MGO@PNB demonstrate remarkably high ion-recognition selectivity of Pb2+ among the coexisting metal ions because of the formation of stable 18-crown-6/Pb2+ inclusion complexes. Most interestingly, the MGO@PNB show excellent thermosensitive adsorption ability toward Pb2+ due to the incorporation of PNIPAM functional chains on the GO. Further thermodynamic studies indicate that the adsorption of Pb2+ onto the MGO@PNB is a spontaneous and endothermic process. The adsorption kinetics and isotherm data can be well described by the pseudo-second-order kinetic model and the Langmuir isotherm model, respectively. Most importantly, the Pb2+-loaded MGO@PNB can be more easily regenerated by alternatively washing with hot/cold water than the commonly used regeneration methods. Such multifunctional graphene oxide nanocomposites could be used for specific recognition and removal of Pb2+ from water environment.

Structurally simple trimesic amides as highly selective anion channels

Trimesic amide molecules, which contain simple alkyl chains in their periphery, exhibit interesting anion-transport functions. The most active and highly selective channel TA12 efficiently transports ClO4- anions across membranes, with other anions conducted in the order of I- > NO3- > Br- > Cl-.

Solid-Phase Total Synthesis and Dual Mechanism of Action of the Channel-Forming 48-mer Peptide Polytheonamide B

Polytheonamide B (1) is a unique peptide natural product because of its extremely complex structure, a channel-forming ability in vitro, and the extremely potent cytotoxicity. The 48-mer sequence of 1 comprises alternating d,l-amino acids and possesses an array of sterically bulky β-tetrasubstituted and hydrogen bond forming residues. These unusual structural features are believed to drive 1 to fold into a 4.5 nm long tube, form a transmembrane ion channel at the plasma membrane, and exert cytotoxicity. Despite its potential biological application, however, multiple substitutions by these unusual residues significantly heightened the synthetic challenges, impeding the solid-phase peptide synthesis (SPPS) of 1. In this study, we first addressed the synthesis problem by extensive optimization of various factors of the SPPS. Adaptation of a new protective group strategy allowed for elongation of a 37-mer peptide on resin, to which an N-terminal 11-mer fragment was condensed. Removal of the 18 protective groups and resin gave rise to 1 in excellent overall yield (4.5%, 76 steps from 17). The SPPS protocol is operationally simple and was proven easily amenable to total synthesis of the fluorescent 48-mer probe 2. Synthetic 1 and 2 were utilized for analysis of their cellular behavior. Reflecting its ion-channel function, the addition of 1 to MCF-7 cells rapidly diminished a potential across the plasma membrane. Furthermore, fluorescence imaging study revealed that 1 and 2 were also internalized into the cells, accumulating in acidic lysosomes and neutralizing the lysosomal pH gradient. These new findings indicated that 1 is capable of exerting two functions upon causing apoptotic cell death of mammalian cells: It induces free cation transport across the plasma as well as lysosomal membranes. The present chemical and biological studies provide valuable information for the design and synthesis of polytheonamide-based molecules with more potent and selective biological activities.

Structure-Driven Selection of Adaptive Transmembrane Na+ Carriers or K+ Channels

Self-assembled alkyl-ureido-benzo-15-crown-5-ethers are selective ionophores for K⁺ cations, which are preferred to Na⁺ cations. The transport mechanism is determined by the optimal coordination rather than classical dimensional compatibility between the crown ether hole and the cation diameter. Herein, we demonstrate that systematic changes of the structure lead to unexpected modifications in the cation-transport activity and suffice to produce adaptive selection. We show that the main contribution to performance arises from optimal constraints on the conformational freedom, which are determined by the binding macrocycles, the nature of the hydrogen-bonding groups, and the hydrophobic tails. Simple changes to the flexible 15-crown-5-ether lead to selective carriers for Na⁺ . Hydrophobic stabilization of the channels through mutual interactions between lipids and variable hydrophobic tails appears to be an important cause of increased activity. Oppositely, restricted translocation is achieved when constrained hydrogen-bonded macrocyclic relays are less dynamic in a pore superstructure.

Quantified structural speciation in self-sorted CoII6L4 cage systems

The molecular components of biological systems self-sort in different ways to function cooperatively and to avoid interfering with each other. Understanding the driving forces behind these different sorting modes enables progressively more complex self-assembling synthetic systems to be designed. Here we show that subtle ligand differences engender distinct M₆L₄ cage geometries - an S₄-symmetric scalenohedron, or pseudo-octahedra having T point symmetry. When two different ligands were simultaneously employed during self-assembly, a mixture of homo- and heteroleptic cages was generated. Each set of product structures represents a unique sorting regime: biases toward specific geometries, preferential incorporation of one ligand over another, and the amplification of homoleptic products were all observed. The ligands' geometries, electronic properties, and flexibility were found to influence the sorting regime adopted, together with templation effects. A new method of using mass spectrometry to quantitatively analyse mixtures of self-sorted assemblies was developed to assess individual outcomes. Product distributions in complex, dynamic mixtures were thus quantified by non-chromatographic methods.