Photo- and Redox-Regulated Transmembrane Ion Transporters

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

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

Controlling the transmembrane transport of chloride by dynamic covalent chemistry with azines

Stimuli-responsive transmembrane ion transport has become a prominent area of research due to its fundamental importance in cellular processes and potential therapeutic applications. Commonly used stimuli include pH, light, and reduction or oxidation agents. This paper presents the use of dynamic covalent chemistry to activate and modulate the transmembrane transport of chloride in liposomes. An active chloride transporter was obtained in situ within the lipid bilayer by dynamic azine metathesis. The transport activity was further tuned by changing the structure of the added azines, while the dynamic covalent chemistry could be activated by lowering the pH. This dynamic covalent chemistry opens a new approach towards controlling transmembrane transport.

Artificial Lithium Channels Built from Polymers with Intrinsic Microporosity

In sharp contrast to numerous artificial potassium channels developed over the past decade, the study of artificial lithium-transporting channels has remained limited. We demonstrate here the use of an interesting class of polymers with intrinsic microporosity (PIM) for constructing artificial lithium channels. These PIM-derived lithium channels show exceptionally efficient (γLi +>40 pS) and highly selective transport of Li+ ions, with selectivity factors of>10 against both Na+ and K+. By simply adjusting the initial reaction temperature, we can tune the transport property in a way that PIMs synthesized at initial reaction temperatures of 60 °C and 80 °C exhibit improved transport efficiency and selectivity, respectively, in the dioleoyl phosphatidylcholine membrane.

Control of Bilayer Transport through a Photoswitchable Membrane-Stiffening Agent

The mobility of proteins in the bilayer membrane is affected by (local) changes in lipid environment, which is important to their biological functioning. Artificial molecular systems that-to some extent-imitate tasks of membrane-embedded proteins are increasingly developed, however, they are usually controlled through responsive units in their core structure. Here we present an alternative approach based on an amphiphilic stiff-stilbene derivative that enables control of membrane fluidity by light. The fluidity increase upon E-to-Z isomerization is shown to enhance the activity of known synthetic anion transporters as a result of a higher mobility. The photoisomerization process is studied by UV/Vis and 1H NMR spectroscopy in solution and in POPC vesicles, where the light-induced changes in fluidity and hence, activity of anion transporters, are monitored by fluorescence spectroscopy. Dynamic light-scattering (DLS) and cryo-EM studies show that vesicle integrity is not impaired by photoswitching. Our work introduces a versatile approach to control solute transport by carrier molecules. Moreover, the photocontrol over membrane fluidity and, with that, mobility could eventually be used for directed motion, which we expect to be key in achieving active transport in the future.

Transmembrane Ion Channels: From Natural to Artificial Systems

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

Transient Photoactivation of Anionophores by Using Redshifted Fast-Relaxing Azobenzenes

Photo-regulated transmembrane ionophores enable spatial and temporal control over activity, offering promise as targeted therapeutics. Key to such applications is control using bio-compatible visible light. Herein, we report red-shifted azobenzene-derived synthetic anionophores that use amber or red light to trigger (E)-(Z) photoisomerisation and activation of transmembrane chloride transport. We demonstrate that by tuning the thermal half-life of the more active, but thermodynamically unstable, Z isomer to relax on the timescale of minutes, transient activation of ion transport can be achieved by activating solely with visible light and deactivating by thermal relaxation.

Aromatic foldamer-derived transmembrane transporters

This review is the first to focus on transmembrane transporters derived from aromatic foldamers, with most studies reported over the past decade. These foldamers have made significant strides in mimicking the essential functions of natural ion channel proteins. With their aromatic backbones rigidified by intramolecular hydrogen bonds or differential repulsive forces, this innovative family of molecules stands out for its structural diversity and functional adaptability. They achieve efficient and selective ion and molecule transport across lipid bilayers via carefully designed helical structures and tunable large cavities. Recent developments in this field highlight the transformative potential of foldamers in therapeutic applications and biomaterial engineering. Key advances include innovative molecular engineering strategies that enable highly selective ion transport by fine-tuning structural and functional attributes. Specific modifications to macrocyclic or helical foldamer structures have allowed precise control over ion selectivity and transport efficiency, with notable selectivity for K+, Li+, H+ and water molecules. Although challenges remain, future directions may focus on more innovative molecular designs, optimizing synthetic methods, improving membrane transport properties, integrating responsive designs that adapt to environmental stimuli, and fostering interdisciplinary collaborations. By emphasizing the pivotal role of aromatic foldamers in modern chemistry, this review aims to inspire further development, offering new molecular toolboxes and strategies to address technological and biological challenges in chemistry, biology, medicine, and materials science.

Off-On Photo- and Redox-Triggered Anion Transport Using an Indole-Based Hydrogen Bond Switch

A stimulus-responsive indole-based hydrogen bonding switch is reported, which enables off-on activation of transmembrane ion transport in response to photo- and redox triggers. This is achieved by alkylation of an indole-based anionophore, preorganized through intramolecular hydrogen bonding, with o-nitrobenzyl and azobenzene cages. This renders the anionophore inactive through formation of a six-membered intramolecular hydrogen bonding interaction and locking of the anion binding protons. Decaging with biologically relevant light and redox stimuli leads to efficient activation of anion transport across lipid bilayer membranes by unlocking the hydrogen bond donors, such that they are now available for anion binding and transport.

Supramolecular chemical biology: designed receptors and dynamic chemical systems

Supramolecular chemistry focuses on the study of species joined by non-covalent interactions, and therefore on dynamic and relatively ill-defined structures. Despite being a well-developed field, it has to face important challenges when dealing with the selective recognition of biomolecules in highly competitive biomimetic media. However, supramolecular interactions reside at the core of chemical biology systems, since many processes in nature are governed by weak, non-covalent, strongly dynamic contacts. Therefore, there is a natural connection between these two research fields, which are not frequently related or share interests. In this feature article, I will highlight our most recent results in the molecular recognition of biologically relevant species, following different conceptual approaches from the most conventional design of elaborated receptors to the less popular dynamic combinatorial chemistry methodology. Selected illustrative examples from other groups will be also included. The discussion has been focused mainly on systems with potential biomedical applications.

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

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

Responsive Anionophores with AND Logic Multi-Stimuli Activation

Artificial ion transport systems have emerged as an important class of compounds that promise applications in chemotherapeutics as anticancer agents or to treat channelopathies. Stimulus-responsive systems that offer spatiotemporally controlled activity for targeted applications remain rare. Here we utilize dynamic hydrogen bonding interactions of a 4,6-dihydroxy-isophthalamide core to generate a modular platform enabling access to stimuli-responsive ion transporters that can be activated in response to a wide variety of external stimuli, including light, redox, and enzymes, with excellent OFF-ON activation profiles. Alkylation of the two free hydroxyl groups with stimulus-responsive moieties locks the amide bonds through intramolecular hydrogen bonding and hence makes them unavailable for anion binding and transport. Triggering using a particular stimulus to cleave both cages reverses the hydrogen bonding arrangement, to generate a highly preorganized anion binding cavity for efficient transmembrane transport. Integration of two cages that are responsive to orthogonal stimuli enables multi-stimuli activation, where both stimuli are required to trigger transport in an AND logic process. Importantly, the strategy provides a facile method to post-functionalize the highly active transporter core with a variety of stimulus-responsive moieties for targeted activation with multiple triggers.

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.

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

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

Coupling Photoresponsive Transmembrane Ion Transport with Transition Metal Catalysis

Artificial ion transporters have been explored both as tools for studying fundamental ion transport processes and as potential therapeutics for cancer and channelopathies. Here we demonstrate that synthetic transporters may also be used to regulate the transport of catalytic metal ions across lipid membranes and thus control chemical reactivity inside lipid-bound compartments. We show that acyclic lipophilic pyridyltriazoles enable Pd(II) cations to be transported from the external aqueous phase across the lipid bilayer and into the interior of large unilamellar vesicles. In situ reduction generates Pd(0) species, which catalyze the generation of a fluorescent product. Photocaging the Pd(II) transporter allows for photoactivation of the transport process and hence photocontrol over the internal catalysis process. This work demonstrates that artificial transporters enable control over catalysis inside artificial cell-like systems, which could form the basis of biocompatible nanoreactors for applications such as drug synthesis and delivery or to mediate phototargeted catalyst delivery into cells.

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

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

Molecular Machines For The Control Of Transmembrane Transport

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