A synthetic ion channel with anisotropic ligand response

Spatially Controlled Self-Assembly of Supramolecular Hydrogels Enabled by Light-Triggered Catalysis

Spatial control over supramolecular self-assembly prevails in living system, yet remains difficult to replicate in synthetic scenarios. Here, on the basis of a hydrazone formation-mediated supramolecular hydrogelation system, access to patterning of supramolecular hydrogels is demonstrated via a light-triggered catalysis strategy. A photoacid generator that can produce protons in aqueous solutions upon irradiation is employed. The generated protons lead to a drop in pH of around three units (initial pH 7.0), effectively accelerating the formation and self-assembly of the hydrazone gelators. Because of the light-triggered catalysis, the hydrogelation samples in the presence of photoacid generator show lower critical gelation concentration, higher stiffness, and denser networks. Importantly, by performing selective irradiation using differently shaped masks, various spatially resolved supramolecular hydrogels following the shapes of the masks are fabricated. The concept of using light-triggered catalysis to realize spatial control over supramolecular self-assembly provides an alternative approach toward bottom-up fabrication of structured soft materials for various applications such as tissue engineering, single cell manipulation, and biosensing.

Aromatic Anion Carrier via Self-Assembly with Imidazolium-Fused Aromatic Amphiphiles

The transport of anions across cell membranes is difficult because of the negatively charged outer surfaces of cell membranes. To overcome this limitation, herein, we report a system for transporting aromatic anions across cellular membranes via self-assembly using a synthetic imidazolium-fused aromatic amphiphile. The amphiphile with cationic and aromatic groups in close proximity to each other could interact with anionic pyranine via electrostatic and aromatic interactions to form supramolecular vesicles. Supramolecular vesicles based on the synthetic imidazolium-fused aromatic amphiphile and pyranine complex transport anionic aromatic pyranine across the membranes of live MCF-7 cells without cytotoxicity.

Real-time visualization reveals Mycobacterium tuberculosis ESAT-6 disrupts phagosome-like compartment via fibril-mediated vesiculation

Mycobacterium tuberculosis (Mtb) evades host defense by hijacking and rupturing the phagosome. ESAT-6, a secreted virulence protein of Mtb, is known to be critical for phagosome rupture. However, the mechanism of ESAT-6-mediated disruption of the phagosomal membrane remains unknown. Using in vitro reconstitution, live-cell imaging, and numerical simulations, we discover that ESAT-6 polymerization forces remodeling and vesiculation of the phagosome-like compartment both in vitro and in vivo. Shallow insertion of ESAT-6 leads to tubular and bud-like deformations on the membrane facilitated by a reduction in membrane tension. Growing fibrils generate both radial and tangential forces causing local remodeling and shape transition of the membrane into buds. The ESAT-6-bound tensed membrane undergoes local changes in membrane curvature and lipid phase separation that assist the subsequent fission. Overall, the findings provide mechanistic insights into the long-standing question of phagosome disruption by Mtb for its escape.

Transmembrane Delivery of an Aryl Azopyrazole Photo-switchable Ion Transporter Relay

Stimuli-responsive synthetic ionophores allow for spatial and temporal control over ion transport, with promise for applications in targeted therapy. Relay transporters have emerged as a new class of ion transporters - these are anchored carriers that sit in both leaflets of the bilayer and mediate transport across the membrane by passing ions between them. The relays are themselves membrane impermeable, and so must be incorporated into the membrane during vesicle preparation. Here we show that relay transporters can be delivered to both sides of the membrane of vesicles using a synthetic flippase. By incorporating an aryl azopyrazole photo-switch into the movable arm of the relay transporters the ion transport activity can be very efficiently and reversibly switched between off and on states. This control is achieved by extension and contraction of their movable arms via photo-isomerization of the central aryl azopyrazole moiety, hence modulating the ability of the relays to pass ions across the membrane.

Synthetic ion channels in biomembranes

Ion transport across cell membranes is crucial in maintaining ion homeostasis in cells. Synthetic molecules that can mimic the functions of natural ion channel proteins would possess great potential as therapeutic agents by promoting apoptosis or interfering with autophagic processes through perturbing the intracellular pH or inducing oxidative and osmotic stresses. However, little is known about the underlying mechanisms in terms of direct correlation between ion transport and biological functions. This review summarizes recent progress in the area of synthetic transmembrane ion transport systems, focusing on the channel type, with an emphasis on their bioapplications as anticancer agents.

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.

Short-chain chlorinated paraffins induce liver injury in mice through mitochondrial disorders and disruption of cholesterol-bile acid pathway

Short-chain chlorinated paraffins (SCCPs) are pervasive organic pollutants recognized for their persistence and bio-toxicity. This study investigated the hepatotoxic mechanisms of SCCPs at environmentally relevant concentration (0.7 μg/kg). The results showed that SCCPs exposure in mice resulted in dysregulated blood and liver lipids, marked by elevated cholesterol levels. Additionally, liver function was compromised, as indicated by increased levels of aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase. Histopathological examination of liver tissue post-SCCPs exposure revealed hepatocyte enlargement, vacuolar degeneration, and mild ballooning degeneration. Mechanistically, SCCPs induced mitochondrial abnormalities, evidenced by heightened Hoechst 33258 fluorescence, and augmented reactive oxygen species and malondialdehyde levels in liver tissue. This was accompanied by a reduction in total antioxidant capacity, culminating in elevated apoptosis markers, including cytochrome C and caspase-3. Moreover, SCCPs perturbed hepatocellular energy metabolism, characterized by increased glycolysis, lactic acid, and fatty acid oxidation, alongside a disruption in the tricarboxylic acid cycle and a decline in mitochondrial energy metabolic function. Furthermore, SCCPs exposure downregulated the expression of genes involved in bile acid synthesis (cyp27a1, fxr, and shp), thereby precipitating the cholesterol-bile acid metabolism disorders and cholesterol accumulation. Collectively, these findings underscore that SCCPs, even at environmentally relevant levels, can induce lipid dysregulation, mitochondrial disorders and cholesterol deposition in the hepatocytes, contributing to liver damage. The study's insights contribute to a comprehension of SCCPs-induced hepatotoxicity and may inform potential preventative and treatment targets for hepatic damage associated with SCCPs exposure.

Structural basis for hepatitis B virus restriction by a viral receptor homologue

Macaque restricts hepatitis B virus (HBV) infection because its receptor homologue, NTCP (mNTCP), cannot bind preS1 on viral surface. To reveal how mNTCP loses the viral receptor function, we here solve the cryo-electron microscopy structure of mNTCP. Superposing on the human NTCP (hNTCP)-preS1 complex structure shows that Arg158 of mNTCP causes steric clash to prevent preS1 from embedding onto the bile acid tunnel of NTCP. Cell-based mutation analysis confirms that only Gly158 permitted preS1 binding, in contrast to robust bile acid transport among mutations. As the second determinant, Asn86 on the extracellular surface of mNTCP shows less capacity to restrain preS1 from dynamic fluctuation than Lys86 of hNTCP, resulting in unstable preS1 binding. Additionally, presence of long-chain conjugated-bile acids in the tunnel induces steric hindrance with preS1 through their tailed-chain. This study presents structural basis in which multiple sites in mNTCP constitute a molecular barrier to strictly restrict HBV.

Spontaneous Transition between Multiple Conductance States and Rectifying Behaviors in an Artificial Single-Molecule Funnel

It has long been an aspirational goal to create artificial channel structures that replicate the feat achieved by ion channel proteins. Biological ion channels occasionally demonstrate multiple conductance states (known as subconductance), remaining a challenging property to achieve in artificial channel molecules. We report a funnel-shaped single-molecule channel constructed by an electron-deficient macrocycle and two electron-deficient aromatic imide arms. Planar lipid bilayer measurements reveal distinct current recordings, including a closed state, two conducting states, and spontaneous transitions between the three states, resembling the events seen in biological ion channels. The transitions result from conformational changes induced by chloride transport in the channel molecule. Both opening states show a non-linear and rectifying I-V relationship, indicating voltage-dependent transport due to the asymmetrical channel structure. This work could enhance our understanding of ion permeation and channel opening mechanism.

Nanohoops in membranes: confined supramolecular spaces within phospholipid bilayer membranes

Nanohoops, an exciting class of fluorophores with supramolecular binding abilities, have the potential to become innovative tools within biological imaging and sensing. Given the biological importance of cell membranes, incorporation of macrocyclic materials with the dual capability of fluorescence emission and supramolecular complexation would be particularly interesting. A series of different-sized nanohoops-ethylene glycol-decorated [n]cyclo-para-pyrenylenes (CPYs) (n = 4-8)-were synthesised via an alternate synthetic route which implements a stannylation-based precursor, producing purer material than the previous borylation approach, enabling the growth of single-crystals of the Pt-macrocycle. Reductive elimination of these single-crystals achieved significantly higher selectivity and yields towards smaller ring-sized nanohoops (n = 4-6). The supramolecular binding capabilities of these CPYs were then explored through host-guest studies with a series of polycyclic (aromatic)hydrocarbons, revealing the importance of molecular size, shape, and CH-π contacts for efficient binding. CPYs were incorporated within the hydrophobic layer of lipid bilayer membranes, as confirmed by microscopic imaging and emission spectroscopy, which also demonstrated the size-preferential incorporation of the five-fold nanohoop. Molecular dynamics simulations revealed the position and orientation within the membrane, as well as the unique non-covalent threading interaction between nanohoop and phospholipid.

Constructing artificial gap junctions to mediate intercellular signal and mass transport

Natural gap junctions are a type of channel protein responsible for intercellular signalling and mass communication. However, the scope of applications for these proteins is limited as they cannot be prepared at a large scale and are unable to spontaneously insert into cell membranes in vitro. The construction of artificial gap junctions may provide an alternative strategy for preparing analogues of the natural proteins and bottom-up building blocks necessary for the synthesis of artificial cells. Here we show the construction of artificial gap junction channels from unimolecular tubular molecules consisting of alternately arranged positively and negatively charged pillar[5]arene motifs. These molecules feature a hydrophobic-hydrophilic-hydrophobic triblock structure that allows them to efficiently insert into two adjacent plasma membranes and stretch across the gap between the two membranes to form gap junctions. Similar to natural gap junction channels, the synthetic channels could mediate intercellular signal coupling and reactive oxygen species transmission, leading to cellular activity.

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

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

Supramolecular Channels Assembled within Intercellular Gaps

Tubular structures exist broadly in biological systems and exhibit important functions including mediating cellular communications. The construction of artificial analogues in living cells would provide a new strategy for chemotherapy. In this report, a kind of supramolecular channel has been constructed within intercellular gaps by mimicking the assembly process and structure of natural gap junctional channels, which consist of hydrophobic tubular modules located in the adjacent cell membranes and hydrophilic modules within the extracellular space. The assembly of the channels was driven by electrostatic interactions. The channels could inhibit tumor cell invasion by preventing cell migration.

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.

Lipid vesicle-based molecular robots

A molecular robot, which is a system comprised of one or more molecular machines and computers, can execute sophisticated tasks in many fields that span from nanomedicine to green nanotechnology. The core parts of molecular robots are fairly consistent from system to system and always include (i) a body to encapsulate molecular machines, (ii) sensors to capture signals, (iii) computers to make decisions, and (iv) actuators to perform tasks. This review aims to provide an overview of approaches and considerations to develop molecular robots. We first introduce the basic technologies required for constructing the core parts of molecular robots, describe the recent progress towards achieving higher functionality, and subsequently discuss the current challenges and outlook. We also highlight the applications of molecular robots in sensing biomarkers, signal communications with living cells, and conversion of energy. Although molecular robots are still in their infancy, they will unquestionably initiate massive change in biomedical and environmental technology in the not too distant future.

Beyond Natural Channel Proteins: Recent Advances in Fluorinated Nanochannels

Inspired by the structures and functions of natural channel proteins that selectively permeate ions and molecules across biological membranes, synthetic molecules capable of self-assembling into supramolecular nanotubes within the hydrophobic layer of the membranes have been designed and their material permeation properties have been studied. More recently, synthetic chemists have ventured to incorporate fluorine atoms, elements rarely found in natural proteins, into the structure of synthetic channels and discovered anomalous transmembrane material permeation properties. In this Perspective, the author provides a brief overview of recent advances in the development of fluorinated nanochannels and possible directions for the future.

Structural basis for ligand recognition and signaling of hydroxy-carboxylic acid receptor 2

Hydroxycarboxylic acid receptors (HCAR1, HCAR2, and HCAR3) transduce Gi/o signaling upon biding to molecules such as lactic acid, butyric acid and 3-hydroxyoctanoic acid, which are associated with lipolytic and atherogenic activity, and neuroinflammation. Although many reports have elucidated the function of HCAR2 and its potential as a therapeutic target for treating not only dyslipidemia but also neuroimmune disorders such as multiple sclerosis and Parkinson's disease, the structural basis of ligand recognition and ligand-induced Gi-coupling remains unclear. Here we report three cryo-EM structures of the human HCAR2-Gi signaling complex, each bound with different ligands: niacin, acipimox or GSK256073. All three agonists are held in a deep pocket lined by residues that are not conserved in HCAR1 and HCAR3. A distinct hairpin loop at the HCAR2 N-terminus and extra-cellular loop 2 (ECL2) completely enclose the ligand. These structures also reveal the agonist-induced conformational changes propagated to the G-protein-coupling interface during activation. Collectively, the structures presented here are expected to help in the design of ligands specific for HCAR2, leading to new drugs for the treatment of various diseases such as dyslipidemia and inflammation.

CHARMM-GUI Membrane Builder: Past, Current, and Future Developments and Applications

Molecular dynamics simulations of membranes and membrane proteins serve as computational microscopes, revealing coordinated events at the membrane interface. As G protein-coupled receptors, ion channels, transporters, and membrane-bound enzymes are important drug targets, understanding their drug binding and action mechanisms in a realistic membrane becomes critical. Advances in materials science and physical chemistry further demand an atomistic understanding of lipid domains and interactions between materials and membranes. Despite a wide range of membrane simulation studies, generating a complex membrane assembly remains challenging. Here, we review the capability of CHARMM-GUI Membrane Builder in the context of emerging research demands, as well as the application examples from the CHARMM-GUI user community, including membrane biophysics, membrane protein drug-binding and dynamics, protein-lipid interactions, and nano-bio interface. We also provide our perspective on future Membrane Builder development.

Cell-Free Synthesis and Electrophysiological Analysis of Multipass Voltage-Gated Ion Channels Tethered in Microsomal Membranes

Cell-free protein synthesis (CFPS) has emerged as a powerful tool for the rapid synthesis and analysis of various structurally and functionally distinct proteins. These include 'difficult-to-express' membrane proteins such as large multipass ion channel receptors. Owing to their membrane localization, eukaryotic CFPS supplemented with endoplasmic reticulum (ER)-derived microsomal vesicles has proven to be an efficient system for the synthesis of functional membrane proteins. Here we demonstrate the applicability of the eukaryotic cell-free systems based on lysates from the mammalian Chinese Hamster Ovary (CHO) and insect Spodoptera frugiperda (Sf21) cells. We demonstrate the efficiency of the systems in the de novo cell-free synthesis of the human cardiac ion channels: ether-a-go-go potassium channel (hERG) KV11.1 and the voltage-gated sodium channel hNaV1.5.

Artificial Cells: Past, Present and Future

Artificial cells are constructed to imitate natural cells and allow researchers to explore biological process and the origin of life. The construction methods for artificial cells, through both top-down or bottom-up approaches, have achieved great progress over the past decades. Here we present a comprehensive overview on the development of artificial cells and their properties and applications. Artificial cells are derived from lipids, polymers, lipid/polymer hybrids, natural cell membranes, colloidosome, metal-organic frameworks and coacervates. They can be endowed with various functions through the incorporation of proteins and genes on the cell surface or encapsulated inside of the cells. These modulations determine the properties of artificial cells, including producing energy, cell growth, morphology change, division, transmembrane transport, environmental response, motility and chemotaxis. Multiple applications of these artificial cells are discussed here with a focus on therapeutic applications. Artificial cells are used as carriers for materials and information exchange and have been shown to function as targeted delivery systems of personalized drugs. Additionally, artificial cells can function to substitute for cells with impaired function. Enzyme therapy and immunotherapy using artificial cells have been an intense focus of research. Finally, prospects of future development of cell-mimic properties and broader applications are highlighted.

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.

Protein-Mimicking Nanoparticles in Biosystems

Proteins are essential elements for almost all life activities. The emergence of nanotechnology offers innovative strategies to create a diversity of nanoparticles (NPs) with intrinsic capacities of mimicking the functions of proteins. These artificial mimics are produced in a cost-efficient and controllable manner, with their protein-mimicking performances comparable or superior to those of natural proteins. Moreover, they can be endowed with additional functionalities that are absent in natural proteins, such as cargo loading, active targeting, membrane penetrating, and multistimuli responding. Therefore, protein-mimicking NPs have been utilized more and more often in biosystems for a wide range of applications including detection, imaging, diagnosis, and therapy. To highlight recent progress in this broad field, herein, representative protein-mimicking NPs that fall into one of the four distinct categories are summarized: mimics of enzymes (nanozymes), mimics of fluorescent proteins, NPs with high affinity binding to specific proteins or DNA sequences, and mimics of protein scaffolds. This review covers their subclassifications, characteristic features, functioning mechanisms, as well as the extensive exploitation of their great potential for biological and biomedical purposes. Finally, the challenges and prospects in future development of protein-mimicking NPs are discussed.

Structure of SARS-CoV-2 membrane protein essential for virus assembly

The coronavirus membrane protein (M) is the most abundant viral structural protein and plays a central role in virus assembly and morphogenesis. However, the process of M protein-driven virus assembly are largely unknown. Here, we report the cryo-electron microscopy structure of the SARS-CoV-2 M protein in two different conformations. M protein forms a mushroom-shaped dimer, composed of two transmembrane domain-swapped three-helix bundles and two intravirion domains. M protein further assembles into higher-order oligomers. A highly conserved hinge region is key for conformational changes. The M protein dimer is unexpectedly similar to SARS-CoV-2 ORF3a, a viral ion channel. Moreover, the interaction analyses of M protein with nucleocapsid protein (N) and RNA suggest that the M protein mediates the concerted recruitment of these components through the positively charged intravirion domain. Our data shed light on the M protein-driven virus assembly mechanism and provide a structural basis for therapeutic intervention targeting M protein.

Supramolecular Mechanosensitive Potassium Channel Formed by Fluorinated Amphiphilic Cyclophane

Inspired by mechanosensitive potassium channels found in nature, we developed a fluorinated amphiphilic cyclophane composed of fluorinated rigid aromatic units connected via flexible hydrophilic octa(ethylene glycol) chains. Microscopic and emission spectroscopic studies revealed that the cyclophane could be incorporated into the hydrophobic layer of the lipid bilayer membranes and self-assembled to form a supramolecular transmembrane ion channel. Current recording measurements using cyclophane-containing planer lipid bilayer membranes successfully demonstrated an efficient transmembrane ion transport. We also demonstrated that the ion transport property was sensitive to the mechanical forces applied to the membranes. In addition, ion transport assays using pH-sensitive fluorescence dye revealed that the supramolecular channel possesses potassium ion selectivity. We also performed all-atom hybrid quantum-mechanical/molecular mechanical simulations to assess the channel structures at atomic resolution and the mechanism of selective potassium ion transport. This research demonstrated the first example of a synthetic mechanosensitive potassium channel, which would open a new door to sensing and manipulating biologically important processes and purification of key materials in industries.

A reversibly gated protein-transporting membrane channel made of DNA

Controlled transport of biomolecules across lipid bilayer membranes is of profound significance in biological processes. In cells, cargo exchange is mediated by dedicated channels that respond to triggers, undergo a nanomechanical change to reversibly open, and thus regulate cargo flux. Replicating these processes with simple yet programmable chemical means is of fundamental scientific interest. Artificial systems that go beyond nature's remit in transport control and cargo are also of considerable interest for biotechnological applications but challenging to build. Here, we describe a synthetic channel that allows precisely timed, stimulus-controlled transport of folded and functional proteins across bilayer membranes. The channel is made via DNA nanotechnology design principles and features a 416 nm2 opening cross-section and a nanomechanical lid which can be controllably closed and re-opened via a lock-and-key mechanism. We envision that the functional DNA device may be used in highly sensitive biosensing, drug delivery of proteins, and the creation of artificial cell networks.

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.

Towards a protein-selective Raman enhancement by a glycopolymer-based composite surface

Surface-enhanced Raman scattering (SERS), which is based on the surface plasmon resonance (LSPR) of noble metal nanostructures, is widely used in the biological field due to its advantages of non-damaging samples and detection up to the molecular level. For biological SERS detection, preparation of substrates with biocompatibility and specific adsorption, leading to selective enhancement of the target biomolecules, are important design strategies. Utilizing the specific interaction between a carbohydrate and protein, a glycopolymer-based composite surface is fabricated to realize specific SERS detection of proteins. Herein, we use N-3,4-dihydroxybenzeneethyl methacrylamide (DMA), 2-deoxy-2-(methacrylamido)glucopyranose (MAG) and methacrylic acid (MAA) as monomers in a sunlight-induced RAFT polymerization to synthesize a dopamine-containing glycopolymer. The glycopolymers are used to prepare a SERS substrate. The composite surface shows specific protein adsorption capacity, and the selective Raman enhancement of specific proteins was successfully achieved between the two different proteins Con A and BSA. This provides a feasible approach to design a SERS surface for protein detection and the study of the interaction between sugar and proteins.

Supramolecular Transmembrane Ion Channels Formed by Multiblock Amphiphiles

Transmembrane proteins located within biological membranes play a crucial role in a variety of important cellular processes, such as energy conversion and signal transduction. Among them, ion channel proteins that can transport specific ions across the biological membranes are particularly important for achieving precise control over those processes. Strikingly, approximately 20% of currently approved drugs are targeted to ion channel proteins within membranes. Thus, synthetic molecules that can mimic the functions of natural ion channel proteins would possess great potential in the sensing and manipulation of biologically important processes, as well as in the purification of key industrial materials.Inspired by the sophisticated structures and functions of natural ion channel proteins, our research group developed a series of multiblock amphiphiles (MAs) composed of a repetitive sequence of flexible hydrophilic oligo(ethylene glycol) chains and rigid hydrophobic oligo(phenylene-ethynylene) units. These MAs can be effectively incorporated into the hydrophobic layer of lipid bilayer membranes and adopt folded conformations, with their hydrophobic units stacked in a face-to-face configuration. Moreover, the folded MAs can self-assemble within the membranes and form supramolecular nanopores that can transport ions across the membranes. In these studies, we focused on the structural flexibility of the MAs and decided to design new molecules able to respond to various external stimuli in order to control their transmembrane ion transport properties. For this purpose, we developed new MAs incorporating sterically bulky groups within their hydrophobic units and demonstrated that their transmembrane ion transport properties could be controlled via mechanical forces applied to the membranes. Moreover, we developed MAs incorporating phosphate ester groups that functioned as ligand-binding sites at the boundary between hydrophilic and hydrophobic units and found that these MAs exhibited transmembrane ion transport properties upon binding with aromatic amine ligands, even within the biological membranes of living cells. We further modified the hydrophobic units of the MAs with fluorine atoms and demonstrated their voltage-responsive transmembrane ion transport properties. These molecular design principles were extended to the development of a transmembrane anion transporter whose transport mechanism was studied by all-atom molecular dynamics simulations.This Account describes the basic principles of the molecular designs of MAs, the characterization of their self-assembled structures within a lipid bilayer, and their transmembrane ion transport properties, including their responsiveness to stimuli. Finally, we discuss future perspectives on the manipulation of biological processes based on the characteristic features of MAs.

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.

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

Voltage-Driven Flipping of Zwitterionic Artificial Channels in Lipid Bilayers to Rectify Ion Transport

We developed a voltage-sensitive artificial transmembrane channel by mimicking the dipolar structure of natural alamethicin channel. The artificial channel featured a zwitterionic structure and could undergo voltage-driven flipping in the lipid bilayers. Importantly, this flipping of the channel could lead to their directional alignment in the bilayers and rectifying behavior for ion transport.

Effective Na+-Binding Ability and Molecular Assembly of an Alkylamide-Substituted Penta(ethylene)glycol Derivative

A new amphiphilic penta(ethylene glycol) derivative (1) bearing two hydrogen-bonding -CONHC₁₄H₂₉ chains was prepared. Compound 1 exhibited ion-recognition abilities for Na⁺ and K⁺, and its properties were compared with those of the macrocyclic [18]crown-6. Although both compound 1 and [18]crown-6 have six ether oxygen atoms (-OC₂H₂-), the Na⁺-binding ability of the former was much higher than that of the latter. K⁺-binding ability of cyclic [18]crown-6 was much higher than its Na⁺-binding ability, while the reverse was true for acyclic compound 1. Single-crystal X-ray structural analysis of Na⁺·1·B(Ph)₄^-·(hexane)₂ at 100 K revealed the existence of a wrapped Na⁺-coordination by six ether and one carbonyl oxygen atoms of 1, which was further stabilized by intramolecular N-H···O═ hydrogen-bonding interactions. The complex phase transition during glass (G) formation and recrystallization was confirmed in the thermal cycle of Na⁺·1·B(Ph)₄^-, whose molten state showed two kinds of liquid phases, Na⁺-complexed (Na⁺·1) + B(Ph)₄^- and completely dissociated Na⁺ + 1 + B(Ph)₄^-. The Na⁺ conductivity of the molten state was 2 orders of magnitude higher than that of the G phase.

Membrane Mimetic Chemistry in Artificial Cells

Lipid membranes in cells are fluid structures that undergo constant synthesis, remodeling, fission, and fusion. The dynamic nature of lipid membranes enables their use as adaptive compartments, making them indispensable for all life on Earth. Efforts to create life-like artificial cells will likely involve mimicking the structure and function of lipid membranes to recapitulate fundamental cellular processes such as growth and division. As such, there is considerable interest in chemistry that mimics the functional properties of membranes, with the express intent of recapitulating biological phenomena. We suggest expanding the definition of membrane mimetic chemistry to capture these efforts. In this Perspective, we discuss how membrane mimetic chemistry serves the development of artificial cells. By leveraging recent advances in chemical biology and systems chemistry, we have an opportunity to use simplified chemical and biochemical systems to mimic the remarkable properties of living membranes.

Making Waxy Salts in Water: Synthetic Control of Hydrophobicity for Anion-Induced and Aggregation-Enhanced Light Emission

We show that multipodal polycationic receptors function as anion-responsive light-emitters in water. Prevailing paradigms utilize rigid holes and cavities for ion recognition. We instead built open amphiphilic scaffolds that trigger polar-to-nonpolar environment transitions around cationic fluorophores upon anion complexation. This ion-pairing and aggregation event produces a dramatic enhancement in the emission intensity, as demonstrated by perchlorate as a non-spherical hydrophobic anion model. A synergetic interplay of C-H⋅⋅⋅anion hydrogen bonding and tight anion-π⁺ contacts underpins this supramolecular phenomenon. By changing the aliphatic chain length, we demonstrate that the response profile and threshold of this signaling event can be controlled at the molecular level. With appropriate molecular design, inherently weak, ill-defined, and non-directional van der Waals interaction enables selective, sensitive, and tunable recognition in water.

Calcium-induced reversible assembly of phosphorylated amphiphile within lipid bilayer membranes

Inspired by calcium-induced reversible assembly and disassembly of membrane proteins found in nature, here we developed a phosphorylated amphiphile (PA) that contains an oligo(phenylene-ethynylene) unit as a hydrophobic unit and a phosphate ester group as a hydrophilic calcium-binding unit. We demonstrated that PA can assemble and disassemble in a reversible manner in response to the sequential addition of calcium chloride and ethylene-diaminetetraacetic acid within the lipid bilayer membranes for the first time as a synthetic molecule.

Unimolecular artificial transmembrane channels showing reversible ligand-gating behavior

A series of peptide-appended bisresorcinarenes were synthesized, which adopted tubular conformation induced by intramolecular hydrogen bonds. The derivatives formed unimolecular artificial transmembrane channels in lipid bilayers to enable selective transport of monovalent cations. Importantly, the channels exhibited reversible ligand-gating behavior in response to alkyl amine and Cu2+.

Imidazolinium-based Multiblock Amphiphile as Transmembrane Anion Transporter

Transmembrane anion transport is an important biological process in maintaining cellular functions. Thus, synthetic anion transporters are widely developed for their biological applications. Imidazolinium was introduced as anion recognition site to a multiblock amphiphilic structure that consists of octa(ethylene glycol) and aromatic units. Ion transport assay using halide-sensitive lucigenin and pH-sensitive 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) revealed that imidazolinium-based multiblock amphiphile (IMA) transports anions and showed high selectivity for nitrate, which plays crucial roles in many biological events. Temperature-dependent ion transport assay using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) indicated that IMA works as a mobile carrier. ¹ H NMR titration experiments indicated that the C2 proton of the imidazolinium ring recognizes anions via a (C-H)⁺ ⋅⋅⋅X^- hydrogen bond. Furthermore, all-atom molecular dynamics simulations revealed a dynamic feature of IMA within the membranes during ion transportation.