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

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.

Artificial Channels Based on Bottlebrush Polymers: Enhanced Ion Transport Through Polymer Topology Control

Synthetic structures mimicking the transport function of natural ion channel proteins have a wide range of applications, including therapeutic treatments, separation membranes, sensing, and biotechnologies. However, the development of polymer-based artificial channels has been hampered due to the limitation on available models. In this study, we demonstrate the great potential of bottlebrush polymers as accessible and versatile molecular scaffolds for developing efficient artificial ion channels. Adopting the bottlebrush configuration enhanced ion transport activity of the channels compared to their linear analogs. Matching the structure of lipid bilayers, the bottlebrush channel with a hydrophilic-hydrophobic-hydrophilic triblock architecture exhibited the highest activity among the series. Functionalized with urea groups, these channels displayed high anion selectivity. Additionally, we illustrated that the transport properties could be fine-tuned by modifying the chemistry of ion binding sites. This work not only highlights the importance of polymer topology control in channel design, but also reveals the great potential for further developing bottlebrush channels with customized features and diverse functionalities.

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.

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.

Perfect confinement of crown ethers in MOF membrane for complete dehydration and fast transport of monovalent ions

Fast transport of monovalent ions is imperative in selective monovalent ion separation based on membranes. Here, we report the in situ growth of crown ether@UiO-66 membranes at a mild condition, where dibenzo-18-crown-6 (DB18C6) or dibenzo-15-crown-5 is perfectly confined in the UiO-66 cavity. Crown ether@UiO-66 membranes exhibit enhanced monovalent ion transport rates and mono-/divalent ion selectivity, due to the combination of size sieving and interaction screening effects toward the complete monovalent ion dehydration. Specifically, the DB18C6@UiO-66 membrane shows a permeation rate (e.g., K+) of 1.2 mol per square meter per hour and a mono-/divalent ion selectivity (e.g., K+/Mg2+) of 57. Theoretical calculations and simulations illustrate that, presumably, ions are completely dehydrated while transporting through the DB18C6@UiO-66 cavity with a lower energy barrier than that of the UiO-66 cavity. This work provides a strategy to develop efficient ion separation membranes via integrating size sieving and interaction screening and to illuminate the effect of ion dehydration on fast ion transport.

New Polyether Macrocycles as Promising Antitumor Agents-Targeted Synthesis and Induction of Mitochondrial Apoptosis

A series of previously unknown aromatic polyether macrodiolides containing a cis,cis-1,5-diene moiety in the molecule were synthesized in 47-74% yields. Macrocycle compounds were first obtained by intermolecular esterification of aromatic polyether diols with α,ω-alka-nZ,(n+4)Z-dienedioic acids mediated by N-(3-(dimethylamino)propyl)-N'-ethylcarbodiimide hydrochloride (EDC·HCl) and 4-(dimethylamino)pyridine (DMAP). For the synthesized compounds, studies of cytotoxicity on tumor (Jurkat, K562, U937), conditionally normal (HEK293) cell lines, and normal fibroblasts were carried out. CC50 was determined, and the therapeutic selectivity index of cytotoxic action (SI) in comparison with normal fibroblasts was evaluated. With the involvement of modern methods of flow cytometry for the most promising macrocycles, their effect on mitochondria and the cell cycle was investigated. It was found that a new macrocycle exhibits pronounced apoptosis-inducing activity toward Jurkat cells and can retard cell division by blocking at the G1/S checkpoint. Also, it was shown that the synthesized macrodiolides influence mitochondria due to their high ability to penetrate the mitochondrial membrane.

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.

Mitochondrial Artificial K+ Channel Construction Using MPTPP@5F8 Nanoparticles for Overcoming Cancer Drug Resistance via Disrupting Cellular Ion Homeostasis

Mitochondrial potassium ion channels have become a promising target for cancer therapy. However, in malignant tumors, their low expression or inhibitory regulation typically leads to undesired cancer therapy, or even induces drug resistance. Herein, this work develops an in situ mitochondria-targeted artificial K+ channel construction strategy, with the purpose to trigger cancer cell apoptosis by impairing mitochondrial ion homeostasis. Considering the fact that cancer cells have a lower membrane potential than that of normal cells, this strategy can selectively deliver artificial K+ channel molecule 5F8 to the mitochondria of cancer cells, by using a mitochondria-targeting triphenylphosphine (TPP) modified block polymer (MPTPP) as a carrier. More importantly, 5F8 can further specifically form a K+ -selective ion channel through the directional assembly of crown ethers on the mitochondrial membrane, thereby inducing mitochondrial K+ influx and disrupting ions homeostasis. Thanks to this design, mitochondrial dysfunction, including decreased mitochondrial membrane potential, reduced adenosine triphosphate (ATP) synthesis, downregulated antiapoptotic BCL-2 and MCL-1 protein levels, and increased reactive oxygen species (ROS) levels, can further effectively induce the programmed apoptosis of multidrug-resistant cancer cells, no matter in case of pump or nonpump dependent drug resistance. In short, this mitochondria-targeted artificial K+ -selective ion channel construction strategy may be beneficial for potential drug resistance cancer therapy.

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.

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.

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.

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.

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.

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.

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.

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.