Chloride Transport through Supramolecular Barrel-Rosette Ion Channels: Lipophilic Control and Apoptosis-Inducing Activity

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.

G-quadruplex-Based Artificial Transmembrane Channels Induce Cancer Cell Apoptosis by Perturbing Potassium Ion Homeostasis

Transmembrane ion transport modality has received a widespread attention due to its apoptotic activation toward anticancer cell activities. In this study, G-quadruplex-based potassium-specific transmembrane channels have been developed to facilitate the intracellular K+ efflux, which perturbs the cellular ion homeostasis thereby inducing cancer cell apoptosis. Cholesterol-tag, a lipophilic anchor moiety, serves as a rudiment for the G-quadruplex immobilization onto the membrane, while G-quadruplex channel structure as a transport module permits ion binding and migration along the channels. A c-Myc sequence tagged with two-cholesterol is designed as a representative lipophilic G-quadruplex, which forms intramolecular parallel G-quadruplex with three stacks of G-quartets (Ch2-Para3). Fluorescence transport assay demonstrates Ch2-Para3 a high transport activity (EC50 = 10.9 × 10-6 m) and an ion selectivity (K+/Na+ selectivity ratio of 84). Ch2-Para3 mediated K+ efflux in cancer cells is revealed to purge cancer cells through K+ efflux-mediated cell apoptosis, which is confirmed by monitoring the changes in membrane potential of mitochondria, leakage of cytochrome c, reactive oxygen species yield, as well as activation of a family of caspases. The lipophilic G-quadruplex exhibits obvious antitumor activity in vivo without systemic toxicity. This study provides a functional scheme aimed at generating DNA-based selective artificial membrane channels for the purpose of regulating cellular processes and inducing cell apoptosis, which shows a great promising for anticancer therapy in the future.

Platinum-based metal complexes as chloride transporters that trigger apoptosis

In this paper we demonstrate that Pt(ii) complexes can function as efficient transmembrane chloride transporters. A series of Pt(ii) metal complexes with urea-appended isoquinoline ligands were synthesised and operate via classical hydrogen bonding interactions rather than ligand exchange. A number of the complexes exhibited potent transmembrane chloride activity in vesicle studies, while also showing strong antiproliferative activity in cisplatin-resistant cell lines via induction of apoptosis and inhibition of intracellular ROS.

Captopril's influence on Danio rerio embryonic development: Unveiling significant toxic outcomes at environmentally relevant concentrations

Anticipating a global increase in cardiovascular diseases, there is an expected surge in the use of angiotensin-converting enzyme inhibitors, notably captopril (CAP). This heightened usage raises significant environmental apprehensions, mainly due to limited knowledge regarding CAP's toxic effects on aquatic species. In response to these concerns, the current study aimed to tackle this knowledge gap by evaluating the potential influence of nominal concentrations of CAP (0.2-2000 μg/L) on the embryonic development of Danio rerio. The findings revealed that CAP at all concentrations, even at concentrations considered environmentally significant (0.2 and 2 μg/L), induced various malformations in the embryos, ultimately leading to their mortality. Main malformations included pericardial edema, craniofacial malformation, scoliosis, tail deformation, and yolk sac deformation. In addition, CAP significantly altered the antioxidant activity of superoxide dismutase and catalase across all concentrations. Simultaneously, it elevated lipid peroxidation levels, hydroperoxides, and carbonylic proteins in the embryos, eliciting a substantial oxidative stress response. Likewise, CAP, at all concentrations, exerted significant modulatory effects on the expression of genes associated with apoptosis (bax, bcl2, p53, and casp3), organogenesis (tbx2a, tbx2b, and irx3b), and ion exchange (slc12a1 and kcnj1) in Danio rerio embryos. Both augmentation and reduction in the expression levels of these genes characterized this modulation. The Pearson correlation analysis indicated a close association between oxidative damage biomarkers and the expression patterns of all examined genes with the elevated incidence of malformations and mortality in the embryos. In summary, it can be deduced that CAP poses a threat to aquatic species. Nevertheless, further research is imperative to enhance our understanding of the environmental implications of this pharmaceutical compound.

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.

Bio-Inspired Subnanofluidics: Advanced Fabrication and Functionalization

Biological ion channels can realize high-speed and high-selective ion transport through the protein filter with the sub-1-nanometer channel. Inspired by biological ion channels, various kinds of artificial subnanopores, subnanochannels, and subnanoslits with improved ion selectivity and permeability are recently developed for efficient separation, energy conversion, and biosensing. This review article discusses the advanced fabrication and functionalization methods for constructing subnanofluidic pores, channels, tubes, and slits, which have shown great potential for various applications. Novel fabrication methods for producing subnanofluidics, including top-down techniques such as electron beam etching, ion irradiation, and electrochemical etching, as well as bottom-up approaches starting from advanced microporous frameworks, microporous polymers, lipid bilayer embedded subnanochannels, and stacked 2D materials are well summarized. Meanwhile, the functionalization methods of subnanochannels are discussed based on the introduction of functional groups, which are classified into direct synthesis, covalent bond modifications, and functional molecule fillings. These methods have enabled the construction of subnanochannels with precise control of structure, size, and functionality. The current progress, challenges, and future directions in the field of subnanofluidic are also discussed.

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.

Sulfur-Containing Foldamer-Based Artificial Lithium Channels

Unlike many other biologically relevant ions (Na+ , K+ , Ca2+ , Cl- , etc) and protons, whose cellular concentrations are closely regulated by highly selective channel proteins, Li+ ion is unusual in that its concentration is well tolerated over many orders of magnitude and that no lithium-specific channel proteins have so far been identified. While one naturally evolved primary pathway for Li+ ions to traverse across the cell membrane is through sodium channels by competing with Na+ ions, highly sought-after artificial lithium-transporting channels remain a major challenge to develop. Here we show that sulfur-containing organic nanotubes derived from intramolecularly H-bonded helically folded aromatic foldamers of 3.6 Å in hollow cavity diameter could facilitate highly selective and efficient transmembrane transport of Li+ ions, with high transport selectivity factors of 15.3 and 19.9 over Na+ and K+ ions, respectively.

Photocontrolled activation of doubly o-nitrobenzyl-protected small molecule benzimidazoles leads to cancer cell death

Artificial biomimetic chloride anionophores have shown promising applications as anticancer scaffolds. Importantly, stimuli-responsive chloride transporters that can be selectively activated inside the cancer cells to avoid undesired toxicity to normal, healthy cells are very rare. Particularly, light-responsive systems promise better applicability for photodynamic therapy because of their spatiotemporal controllability, low toxicity, and high tunability. Here, in this work, we report o-nitrobenzyl-linked, benzimidazole-based singly and doubly protected photocaged protransporters 2a, 2b, 3a, and 3b, respectively, and benzimidazole-2-amine-based active transporters 1a-1d. Among the active compounds, trifluoromethyl-based anionophore 1a showed efficient ion transport activity (EC50 = 1.2 ± 0.2 μM). Detailed mechanistic studies revealed Cl-/NO3- antiport as the main ion transport process. Interestingly, double protection with photocages was found to be necessary to achieve the complete "OFF-state" that could be activated by external light. The procarriers were eventually activated inside the MCF-7 cancer cells to induce phototoxic cell death.

Metallacarborane Cluster Anions of the Cobalt Bisdicarbollide-Type as Chaotropic Carriers for Transmembrane and Intracellular Delivery of Cationic Peptides

Cobalt bisdicarbollides (COSANs) are inorganic boron-based anions that have been previously reported to permeate by themselves through lipid bilayer membranes, a propensity that is related to their superchaotropic character. We now introduce their use as selective and efficient molecular carriers of otherwise impermeable hydrophilic oligopeptides through both artificial and cellular membranes, without causing membrane lysis or poration at low micromolar carrier concentrations. COSANs transport not only arginine-rich but also lysine-rich peptides, whereas low-molecular-weight analytes such as amino acids as well as neutral and anionic cargos (phalloidin and BSA) are not transported. In addition to the unsubstituted isomers (known as ortho- and meta-COSAN), four derivatives bearing organic substituents or halogen atoms have been evaluated, and all six of them surpass established carriers such as pyrenebutyrate in terms of activity. U-tube experiments and black lipid membrane conductance measurements establish that the transport across model membranes is mediated by a molecular carrier mechanism. Transport experiments in living cells showed that a fluorescent peptide cargo, FITC-Arg8, is delivered into the cytosol.

Conformationally Flexible Cleft Receptor for Chloride Anion Transport

The design and synthesis of a cleft-shaped bis-diarylurea receptor for chloride anion transport is reported in this work. The receptor is based on the foldameric nature of N,N'-diphenylurea upon its dimethylation. The bis-diarylurea receptor exhibits a strong and selective affinity for chloride over bromide and iodide anions. A nanomolar quantity of the receptor efficiently transports the chloride across a lipid bilayer membrane as a 1:1 complex (EC₅₀ = 5.23 nm). The work demonstrates the utility of the N,N'-dimethyl-N,N'-diphenylurea scaffold in anion recognition and transport.

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.

Is it really safe to replace decabromodiphenyl ether (BDE209) with decabromodiphenyl ethane (DBDPE)?: A perspective from hepatotoxicity

In this paper, the hepatocytotoxicity and aryl hydrocarbon receptor (AHR) activity of decabromodiphenyl ethane (DBDPE), decabromodiphenyl ether (BDE209) and other 18 analogues were evaluated in vitro using human normal liver cell L02. These dioxin-like compounds showed differential hepatocytotoxicity (EC₅₀  = 0.38-17.87 mg/L) and AHR activity (EROD activity = 4.53-46.35 U/μg). In silico study indicated the distance of π-π bonds between the benzene ring of compounds and residue Phe234 of AHR played a key role in the binding of AHR, and the substituents on the benzene ring also influenced the activity. Combining molecular biology and bioomics, the comprehensive investigations on the hepatotoxic mechanisms have demonstrated the AHR signaling pathway was the key mediation mechanism for the hepatotoxicity of DBDPE/BDE209. The cytochrome P450s (CYP2 family) mediated formation of reactive oxygenated intermediates might be the dominant toxic mechanism, which could produce oxidative stress or cause genotoxicity. Although the experimental toxicity of DBDPE was smaller relative to BDE209, the health risk of DBDPE may be much greater than we expected, due to the high potential to form a variety of dioxin-like intermediates by microbial oxidation of ethyl group. Therefore, whether it is really safe to replace BDE209 with DBDPE is a debatable question, and more ecotoxicological and health data are needed to clarify this issue.

Nontoxic Artificial Chloride Channel Formation in Epithelial Cells by Isophthalic Acid-Based Small Molecules

Artificial channels capable of facilitating the transport of Cl^- ions across cell membranes while being nontoxic to the cells are rare. Such synthetic ion channels can mimic the functions of membrane transport proteins and, therefore, have the potential to treat channelopathies by replacing defective ion channels. Here we report isophthalic acid-based structurally simple molecules 1 a and 2 a, which self-assemble to render supramolecular nanochannels that allow selective transport of Cl^- ions. As evident from the single-crystal X-ray diffraction analysis, the self-assembly is governed by intermolecular hydrogen bonding and π-π stacking interactions. The MD simulation studies for both 1 a and 2 a confirmed the formation of stable Cl^- channel assembly in the lipid membrane and Cl^- transport through them. The MQAE assay showed the efficacy of the compounds in delivering Cl^- ions into cells, and the MTT assays proved that the compounds are nontoxic to cells even at a concentration of 100 μM.

Progress and prospects toward supramolecular bioactive ion transporters

The majority of cellular physiological processes depend on natural ion channels, which are pore-forming membrane-embedded proteins that let ions flow across the cell membranes selectively. This selective movement of ions across the membranes balances the osmolality within and outside the cell. However, mutations in the genes that encode essential membrane transport proteins or structural reorganisation of these proteins can cause life-threatening diseases like cystic fibrosis. Artificial ion transport systems have opened up a way to replace dysfunctional natural ion channels to cure such diseases through channel replacement therapy. Moreover, recent research has also demonstrated the ability of these systems to kill cancer cells, reigniting interest in the field among scientists. Our contributions to the recent progress in the design and development of artificial chloride ion transporters and their effect on biological systems have been discussed in this review. This review would provide current vistas and future directions toward the development of novel ion transporters with improved biocompatibility and desired anti-cancer properties. Additionally, it strongly emphasises stimuli-responsive ion transport systems, which are crucial for obtaining target-specificity and may speed up the application of these systems in clinical therapeutics.

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.

Formation of supramolecular channels by reversible unwinding-rewinding of bis(indole) double helix via ion coordination

Stimulus-responsive reversible transformation between two structural conformers is an essential process in many biological systems. An example of such a process is the conversion of amyloid-β peptide into β-sheet-rich oligomers, which leads to the accumulation of insoluble amyloid in the brain, in Alzheimer's disease. To reverse this unique structural shift and prevent amyloid accumulation, β-sheet breakers are used. Herein, we report a series of bis(indole)-based biofunctional molecules, which form a stable double helix structure in the solid and solution state. In presence of chloride anion, the double helical structure unwinds to form an anion-coordinated supramolecular polymeric channel, which in turn rewinds upon the addition of Ag⁺ salts. Moreover, the formation of the anion-induced supramolecular ion channel results in efficient ion transport across lipid bilayer membranes with excellent chloride selectivity. This work demonstrates anion-cation-assisted stimulus-responsive unwinding and rewinding of artificial double-helix systems, paving way for smart materials with better biomedical 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.

Synthesis and mechanism of biological action of morpholinyl-bearing arylsquaramides as small-molecule lysosomal pH modulators

Lysosomal pH is an important modulator for many cellular processes. An agent that is capable of regulating lysosomal pH may find a wide range of potential applications in the field of biomedicine. In this study, we describe the synthesis of a family of morpholinyl-bearing arylsquaramides as small-molecule lysosomal pH modulators. These compounds are able to efficiently facilitate the transmembrane transport of chloride anions as mobile carriers across vesicular and cellular phospholipid membranes. They are capable of specifically alkalizing liposomes, disrupting the homeostasis of lysosomal pH and inactivivating lysosomal Cathepsin B enzyme. Anion transport is considered as the probable mechanism of action for the high efficiency of these compounds to modulate lysosomal pH. The present findings present a novel means to efficiently regulate lysosomal pH, which is in contrast to the methods shown by conventional lysosomal pH modulators that generally function by either acting as a weak base/acid, or releasing a basic/acidic component in lysosomal environments to change lysosomal pH.

Chloride Transport across Liposomes and Cells by Nontoxic 3-(1H-1,2,3-Triazol-1-yl)benzamides

Synthetic anion transmembrane transporters are adding new aspirations for treating channelopathies by replacing defective ion channels. The availability of such suitable candidates is still infrequent due to the associated toxicity. Here, we report 3-(1H-1,2,3-triazol-1-yl)benzamides as transmembrane anion carriers, nontoxic to cells. The selective and electrogenic chloride transport activity was established by fluorescence and ion selective electrode-based assays. MQAE assay confirmed the chloride uptake into the cells by the nontoxic compounds.

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.

Ion Channels and Transporters as Therapeutic Agents: From Biomolecules to Supramolecular Medicinal Chemistry

Ion channels and transporters typically consist of biomolecules that play key roles in a large variety of physiological and pathological processes. Traditional therapies include many ion-channel blockers, and some activators, although the exact biochemical pathways and mechanisms that regulate ion homeostasis are yet to be fully elucidated. An emerging area of research with great innovative potential in biomedicine pertains the design and development of synthetic ion channels and transporters, which may provide unexplored therapeutic opportunities. However, most studies in this challenging and multidisciplinary area are still at a fundamental level. In this review, we discuss the progress that has been made over the last five years on ion channels and transporters, touching upon biomolecules and synthetic supramolecules that are relevant to biological use. We conclude with the identification of therapeutic opportunities for future exploration.

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.

Bis(cholyl)-based chloride channels with oxalamide and hydrazide selectivity filters

We report the development of supramolecular bis(cholyl) ion channels by using oxalamide and hydrazide as selectivity filters. The hydrazide system displayed superior chloride transport activity than oxalamide via the formation...

Molecular Self-Assembly as a Tool to Construct Transmembrane Supramolecular Ion Channels

Self-assembly has become a powerful tool for building various supramolecular architectures with applications in material science, environmental science, and chemical biology. One such area is the development of artificial transmembrane ion channels that mimic naturally occurring channel-forming proteins to unveil various structural and functional aspects of these complex biological systems, hoping to replace the defective protein channels with these synthetically accessible moieties. This account describes our recent approaches to construct supramolecular ion channels using synthetic molecules and their applications in medicinal chemistry.

Recent Advances in Bioactive Artificial Ionophores

Several life-threatening diseases, also known as 'Channelopathies' are linked to irregularities in ion transport proteins. Significant research efforts have fostered the development of artificial transport systems that facilitates to restore the functions of impaired natural transport proteins. Indeed, a few of these artificial ionophores demonstrate the rare combination of transmembrane ion transport and important biological activity, offering early promises of suitability in 'channel replacement therapy'. In this review, structural facets and functions of both cationophores and anionophores are discussed. Ionophores that are toxic to various bacteria and yeast, could be exploited as antimicrobial agent. Nevertheless, few non-toxic ionophores offer the likelihood of treating a wide range of genetic diseases caused by the gene mutations. In addition, their ability to disrupt cellular homeostasis and to alter lysosomal pH endow ionophores as promising candidates for cancer treatment. Overall, critically outlining the advances in artificial ionophores in terms of in vitro ion transport, possible modes of action and biological activities enables us to propose possible future roadmaps in this research area.

Stimuli-Responsive Anion Transport through Acylhydrazone-Based Synthetic Anionophores

Photoswitchable acylhydrazone-based synthetic anionophores are reported. Single-crystal X-ray structure and ¹H NMR titration studies confirmed the chloride binding in solid and solution states. The ion transport activity of 1a was greatly attenuated through a phototriggered E to Z photoisomerization process, and the photoisomerized deactivated state showed high kinetic stability due to an intramolecular hydrogen bond. Switchable "OFF-ON" transport activity was achieved by the application of light and acid-catalyzed reactivation process.

Stimuli-responsive transmembrane anion transport by AIE-active fluorescent probes

Anticancer drug resistance implicates multifunctional mechanisms, and hypoxia is one of the key factors in therapeutic resistance. Hypoxia-specific therapy is considered an extremely effective strategy to fight against cancer. The development of small molecule-based synthetic anion transporters has also recently drawn attention for their potential therapeutic applications against several ion-transport-associated diseases, such as cancer and others. Herein, we describe the development of a hypoxia-responsive proanionophore to trigger controlled transport of anions across membranes under pathogenic conditions. Herein, we report the development of tetraphenylethene (TPE)-based anion transporters. The sulfonium-linked p-nitrobenzyl containing TPE-based proanionophore could be converted into a lipophilic fluorescent Cl^- ion carrier in a hypoxic or reductive environment. Stimuli such as nitroreductase (NTR) and glutathione (GSH) mediated regeneration of the TPE-based active Cl^- ion transporter also showed aggregation-induced emission (AIE) properties. We hypothesize that such hypoxia and reductive stimuli activatable proanionophores have tremendous potential to fight against channelopathies, including cancer.

Supramolecular chemistry in lipid bilayer membranes

Lipid bilayer membranes form compartments requisite for life. Interfacing supramolecular systems, including receptors, catalysts, signal transducers and ion transporters, enables the function of the membrane to be controlled in artificial and living cellular compartments. In this perspective, we take stock of the current state of the art of this rapidly expanding field, and discuss prospects for the future in both fundamental science and applications in biology and medicine.

A Pyridyl-Linked Benzimidazolyl Tautomer Facilitates Prodigious H+/Cl- Symport through a Cooperative Protonation and Chloride Ion Recognition

We report two pyridyl-linked benzimidazolyl hydrazones as HCl cotransporters that are 5 and 2 times superior to prodigiosin, a natural product whose transport efficiency has never been routed by synthetic molecules. These hydrazones provide a suitable HCl binding site through a cooperative protonation and chloride ion recognition. HCl transport by the most active compound induces lysosome deacidification. Viability assays confirmed that the compounds induce cytotoxicity toward human breast cancer MCF-7 cells but are relatively nontoxic toward noncancerous HEK293T cells.

Gramicidin A-based unimolecular channel: cancer cell-targeting behavior and ion transport-induced apoptosis

A series of glycoside-peptide conjugates were prepared by engineering at the N-terminus of the natural peptide gramicidin A. The conjugate containing galactose moiety formed a unimolecular transmembrane channel and mediated ion transport to induce apoptosis of cancer cells. More importantly, it exhibited liver cancer cell-targeting behavior due to the galactose-asialoglycoprotein receptor recognition.

Multicomponent Self-Assembly of a Giant Heterometallic Polyoxotungstate Supercluster with Antitumor Activity

The hierarchical aggregation of molecular nanostructures from multiple components is a grand synthetic challenge, which requires highly selective linkage control. We demonstrate how two orthogonal linkage groups, that is, organotin and lanthanide cations, can be used to drive the aggregation of a giant molecular metal oxide superstructure. The title compound {[(Sn(CH3 )2 )2 O]4 {[CeW5 O18 ] [TeW4 O16 ][CeSn(CH3 )2 ]4 [TeW8 O31 ]4 }2 }46- (1 a) features dimensions of ca. 2.2×2.3×3.4 nm3 and a molecular weight of ca. 25 kDa. Structural analysis shows the hierarchical aggregation from several independent subunits. Initial biomedical tests show that 1 features an inhibitory effect on the proliferation of HeLa cells based on an apoptosis pathway. In vivo experiments in mice reveal the antiproliferative activity of 1 and open new paths for further development of this new compound class.

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.

A Synthetic Phospholipid Derivative Mediates Ion Transport Across Lipid Bilayers

Inspired by the natural phospholipid structures for cell membrane, a synthetic phospholipid LC with an ion recognition group benzo-18-crown-6 (B18C6) moiety was prepared which has been demonstrated to be able to transport ions across the lipid bilayers. Fluorescent vesicle assay shows that LC has an excellent transport activity, and the EC₅₀ value for K⁺ is 11.2 μM. The voltage clamp measurement exhibits regular square-like current signals with considerably long opening times, which indicates that LC achieves efficient ion transport through a channel mechanism and its single channel conductivity is 17 pS. Both of the vesicle assay and patch clamp tests indicate that LC has selectivity for Rb⁺, whose ionic radius is larger than the cavity of crown ether. It suggests that the sandwich interaction may play a key role in the ion transport across lipid bilayers. All these results help us to speculate that LC transports ions via a channel mechanism with a tetrameric aggregate as the active structure. In addition, LC had obvious toxicity to HeLa cells, and the IC₅₀ was 100.0 μM after coculture for 36 h. We hope that this simple synthetic phospholipid will offer novel perspectives in the development of more efficient and selective ion transporters.

Self-Assembly of Size-Controlled m-Pyridine-Urea Oligomers and Their Biomimetic Chloride Ion Channels

The m-pyridine urea (mPU) oligomer was constructed by using the intramolecular hydrogen bond formed by the pyridine nitrogen atom and the NH of urea and the intermolecular hydrogen bond of the terminal carbonyl group and the NH of urea. Due to the synergistic effect of hydrogen bonds, mPU oligomer folds and exhibits strong self-assembly behaviour. Affected by folding, mPU oligomer generates a twisted plane, and one of its important features is that the carbonyl group of the urea group orientates outwards from the twisted plane, while the NHs tend to direct inward. This feature is beneficial to NH attraction for electron-rich species. Among them, the trimer self-assembles into helical nanotubes, and can efficiently transport chloride ions. This study provides a novel and efficient strategy for constructing self-assembled biomimetic materials for electron-rich species transmission.

Hydroxy Channels-Adaptive Pathways for Selective Water Cluster Permeation

Artificial water channels (AWCs) are known to selectively transport water, with ion exclusion. Similarly to natural porins, AWCs encapsulate water wires or clusters, offering continuous and iterative H-bonding that plays a vital role in their stabilization. Herein, we report octyl-ureido-polyol AWCs capable of self-assembly into hydrophilic hydroxy channels. Variants of ethanol, propanediol, and trimethanol are used as head groups to modulate the water transport permeabilities, with rejection of ions. The hydroxy channels achieve a single-channel permeability of 2.33 × 10⁸ water molecules per second, which is within the same order of magnitude as the transport rates for aquaporins. Depending on their concentration in the membrane, adaptive channels are observed in the membrane. Over increased concentrations, a significant shift occurs, initiating unexpected higher water permeation. Molecular simulations probe that spongelike or cylindrical aggregates can form to generate transient cluster water pathways through the bilayer. Altogether, the adaptive self-assembly is a key feature influencing channel efficiency. The adaptive channels described here may be considered an important milestone contributing to the systematic discovery of artificial water channels for water desalination.