Conformationally controlled oligocholate membrane transporters: learning through water play

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.

Herringbone Helical Foldamers from Aromatic Ether Derived ϵ-Amino Acid Peptides

Oligomers based on an aromatic ether derived ϵ-amino acid peptides folded into herringbone helical structures, induced by successive NH-O-NH & O-NH-O bifurcated hydrogen bonding interactions and reinforced by π-π stacking between aryls from adjacent layers. The diaryl ether bonds -O- worked both as structural units to provide turn motifs for changing the amplitude of the slope along the axis of helix for herringbone formation, and also as acceptors for hydrogen bonding. Attachment of a single chiral carbon to the C-termini of the peptides induced excess of single-handed screw sense and amplification through the chain propagation as exemplified by chain length dependent circular dichroism (CD) investigations.

Selective confinement of potassium, rubidium, or caesium ions in a non-covalent hydroxyproline octamer cage stabilized by cis-hydroxyl locks

While numerous studies have focused on the impact of chirality on some magic amino acid clusters, this article investigates the effects of steric isomerization using 4-hydroxyproline octamers as a model system. Through mass spectrometry, infrared photodissociation spectroscopy, and theoretical calculation, it was demonstrated that the cis-4-hydroxy-L-proline octamer can selectively cage potassium, rubidium, or caesium ions through stable cis-hydroxyl locks, while the trans-form cannot. The results highlight the importance of hydroxyl group orientation in designing biocompatible membrane transporters with high ion-selectivity.

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.

Facially Amphiphilic Skeleton-Derived Antibacterial Cationic Dendrimers

It is urgent to develop biocompatible and high-efficiency antimicrobial agents since microbial infections have always posed serious challenges to human health. Herein, through the marriage of facially amphiphilic skeletons and cationic dendrimers, high-density positively charged dendrimers D-CA₆-N⁺ (G2) and D-CA₂-N⁺ (G1) were designed and synthesized using the "branch" of facially amphiphilic bile acids, followed by their modification with quaternary ammonium charges. Both dendrimers could self-assemble into nanostructured micelles in aqueous solution. D-CA₆-N⁺ displays potent antibacterial activity against Staphylococcus aureus and Escherichia coli, with minimum inhibitory concentrations (MICs) as low as 7.50 and 7.79 μM, respectively, and has an evidently stronger antibacterial activity than D-CA₂-N⁺. Moreover, D-CA₆-N⁺ can kill S. aureus faster than E. coli. The facial amphiphilicity of the bile acid skeleton facilitates the selective destruction of bacterial membranes and endows dendrimers with negligible hemolysis and cytotoxicity even under a high concentration of 16× MIC. In vivo studies show that D-CA₆-N⁺ is much more effective and safer than penicillin G in treating S. aureus infection and promoting wound healing, which suggests facially amphiphilic skeleton-derived cationic dendrimers can be a promising approach to effectively enhance antibacterial activity and biocompatibility of antibacterial agent, simultaneously.

Principles of Small-Molecule Transport through Synthetic Nanopores

Synthetic nanopores made from DNA replicate the key biological processes of transporting molecular cargo across lipid bilayers. Understanding transport across the confined lumen of the nanopores is of fundamental interest and of relevance to their rational design for biotechnological applications. Here we reveal the transport principles of organic molecules through DNA nanopores by synergistically combining experiments and computer simulations. Using a highly parallel nanostructured platform, we synchronously measure the kinetic flux across hundreds of individual pores to obtain rate constants. The single-channel transport kinetics are close to the theoretical maximum, while selectivity is determined by the interplay of cargo charge and size, the pores' sterics and electrostatics, and the composition of the surrounding lipid bilayer. The narrow distribution of transport rates implies a high structural homogeneity of DNA nanopores. The molecular passageway through the nanopore is elucidated via coarse-grained constant-velocity steered molecular dynamics simulations. The ensemble simulations pinpoint with high resolution and statistical validity the selectivity filter within the channel lumen and determine the energetic factors governing transport. Our findings on these synthetic pores' structure-function relationship will serve to guide their rational engineering to tailor transport selectivity for cell biological research, sensing, and drug delivery.

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.

Membrane-Suspended Nanopores in Microchip Arrays for Stochastic Transport Recording and Sensing

The transport of nutrients, xenobiotics, and signaling molecules across biological membranes is essential for life. As gatekeepers of cells, membrane proteins and nanopores are key targets in pharmaceutical research and industry. Multiple techniques help in elucidating, utilizing, or mimicking the function of biological membrane-embedded nanodevices. In particular, the use of DNA origami to construct simple nanopores based on the predictable folding of nucleotides provides a promising direction for innovative sensing and sequencing approaches. Knowledge of translocation characteristics is crucial to link structural design with function. Here, we summarize recent developments and compare features of membrane-embedded nanopores with solid-state analogues. We also describe how their translocation properties are characterized by microchip systems. The recently developed silicon chips, comprising solid-state nanopores of 80 nm connecting femtoliter cavities in combination with vesicle spreading and formation of nanopore-suspended membranes, will pave the way to characterize translocation properties of nanopores and membrane proteins in high-throughput and at single-transporter resolution.

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.

Cholate Conjugated Polymeric Amphiphiles as Efficient Artificial Ionophores

A family of amphiphilic copolymers containing hydrophobic cholate pendants has been prepared by copolymerization of cholic acid-based monomer 2-(methacryloxy)-ethyl cholate (MAECA) with polyethylene glycol methyl ether methacrylate (PEGMA). The polymers differ for the content of MAECA that increases from 0 to 35%. The copolymers partition within liposomes and display potent ionophoric activity forming large pores in the membrane and allowing the leakage of small inorganic ions (H+, Na+) and of large polar organic molecules (calcein). Their activity is strictly correlated to the content of cholic acid subunits, increasing as the fraction of cholate moiety increases.

Physiology and Physical Chemistry of Bile Acids

Bile acids (BAs) are facial amphiphiles synthesized in the body of all vertebrates. They undergo the enterohepatic circulation: they are produced in the liver, stored in the gallbladder, released in the intestine, taken into the bloodstream and lastly re-absorbed in the liver. During this pathway, BAs are modified in their molecular structure by the action of enzymes and bacteria. Such transformations allow them to acquire the chemical-physical properties needed for fulling several activities including metabolic regulation, antimicrobial functions and solubilization of lipids in digestion. The versatility of BAs in the physiological functions has inspired their use in many bio-applications, making them important tools for active molecule delivery, metabolic disease treatments and emulsification processes in food and drug industries. Moreover, moving over the borders of the biological field, BAs have been largely investigated as building blocks for the construction of supramolecular aggregates having peculiar structural, mechanical, chemical and optical properties. The review starts with a biological analysis of the BAs functions before progressively switching to a general overview of BAs in pharmacology and medicine applications. Lastly the focus moves to the BAs use in material science.

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+.

Effect of charge status on the ion transport and antimicrobial activity of synthetic channels

The charge status of channels formed by pillararene–gramicidin hybrid molecules has a significant impact on their trans-membrane transport properties, membrane-association abilities and antimicrobial activities.

Enhancing K+ transport activity and selectivity of synthetic K+ channels via electron-donating effects

Electron-withdrawing groups enhance ion transport activity by 160% and selectivity by >50%, leading to high K⁺/Na⁺ selectivity of 14.0.

Pyridine/Oxadiazole-Based Helical Foldamer Ion Channels with Exceptionally High K+ /Na+ Selectivity

Protein channels are characterized by high transport selectivity, which is essential for maintaining cellular function. Efforts to reproduce such high selectivity over the past four decades have not been very successful. We report a novel series of aromatic foldamer-based polymeric channels where the backbone is stabilized by differential electrostatic repulsions among heteroatoms helically arrayed along the helical backbone. Nanotubes averaging 2.3 and 2.7 nm in length mediate highly efficient transport of K⁺ ions as a consequence of hydrophilic electron-rich hollow cavities that are 3 Å in diameter. Exceptionally high K⁺ and Na⁺ selectivity values of 16.3 and 12.6, respectively, are achieved.

Bile Salts: Natural Surfactants and Precursors of a Broad Family of Complex Amphiphiles

Bile salts (BSs) are naturally occurring rigid surfactants with a steroidal skeleton and specific self-assembly and interface behaviors. Using bile salts as precursors, derivatives can be synthesized to obtain molecules with specific functionalities and amphiphilic structure. Modifications on single molecules are normally performed by substituting the least-hindered hydroxyl group on carbon C-3 of the steroidal A ring or at the end of the lateral chain. This leads to monosteroidal rigid building blocks that are often able to self-organize into 1D structures such as tubules, twisted ribbons, and fibrils with helical supramolecular packing. Tubular aggregates are of particular interest, and they are characterized by cross-section inner diameters spanning a wide range of values (3-500 nm). They can form through appealing pH- or temperature-responsive aggregation and in mixtures of bile salt derivatives to provide mixed tubules with tunable charge and size. Other derivatives can be prepared by covalently linking two or more bile salt molecules to provide complex systems such as oligomers, dendrimers, and polymeric materials. The unconventional amphiphilic molecular structure imparts specific features to BSs and derivatives that can be exploited in the formulation of capsules, drug carriers, dispersants, and templates for the synthesis of nanomaterials.

A Temperature-Gated Nanovalve Self-Assembled from DNA to Control Molecular Transport across Membranes

Nanopores are powerful nanodevices that puncture semifluid membranes to enable transport of molecular matter across biological or synthetic thin layers. Advanced nanopores featuring more complex functions such as ambient sensing and reversible channel opening are of considerable scientific and technological interest but challenging to achieve with classical building materials. Here we exploit the predictable assembly properties of DNA to form a multifunctional nanovalve that senses temperature for controlled channel opening and tunable transport. The barrel-shaped valve is formed from solely seven oligonucleotides and is closed at ambient temperatures. At >40 °C a programmable thermosensitive lid opens the barrel to allow transport of small molecules across the membrane. The multifunctional DNA nanodevice may be used to create logic ionic networks or to achieve controlled drug delivery from vesicles.

Artificial K+ Channels Formed by Pillararene-Cyclodextrin Hybrid Molecules: Tuning Cation Selectivity and Generating Membrane Potential

A class of artificial K⁺ channels formed by pillararene-cyclodextrin hybrid molecules have been designed and synthesized. These channels efficiently inserted into lipid bilayers and displayed high selectivity for K⁺ over Na⁺ in fluorescence and electrophysiological experiments. The cation transport selectivity of the artificial channels is tunable by varying the length of the linkers between pillararene and cyclodexrin. The shortest channel showed specific transmembrane transport preference for K⁺ over all alkali metal ions (selective sequence: K⁺ > Cs⁺ > Rb⁺ > Na⁺ > Li⁺ ), and is rarely observed for artificial K⁺ channels. The high selectivity of this artificial channel for K⁺ over Na⁺ ensures specific transmembrane translocation of K⁺ , and generated stable membrane potential across lipid bilayers.

Controllable synthetic ion channels

The controllable synthetic ion channels with voltage-, ligand- light- and mechano-gating, as well as rectifying behaviours are discussed in regarding to their construction strategies and functions.

Tuning the biomimetic performances of 4-hydroxyproline-containing cyclic peptoids

Five new cyclic peptoids containing (2S,4R)-4-hydroxyproline (Hyp) residues have been designed and synthesized using a mixed “submonomer/monomer” approach.

A synthetic channel that efficiently inserts into mammalian cell membranes and destroys cancer cells

A tubular molecule with terminal positively charged amino groups that displays a strong ability to insert into the membrane of mammalian cells.

Reversible conversion between a pleated oligo-tetrathiafulvalene radical foldamer and folded donor-acceptor [3]pseudorotaxane under redox conditions

Linear tetrathiafulvalene (TTF) oligomers were synthesized, which could not only form a pleated TTF˙⁺ radical cation foldamer under oxidation conditions, but also interlocked with CBPQT⁴⁺ to form folded donor-acceptor [3]pseudorotaxane in the neutral state of TTF. Moreover, switchable transformation between these two folded supramolecular structures was achieved under the alternative regulation of the redox states of TTF units.

Surprisingly High Selectivity and High Affinity in Mercury Recognition by H-Bonded Cavity-Containing Aromatic Foldarands

In the absence of macrocyclic ring constraints, few synthetic systems, possessing a mostly solvent-independent well-folded conformation that is predisposed for highly selective and high affinity recognition of metal ions, have been demonstrated. We report here such a unique class of conformationally robust modularly tunable folding molecules termed foldarands that can recognize Hg²⁺ ions surprisingly well over 22 other metal ions. Despite the lack of sulfur atoms and having only oxygen-donor atoms in its structure, the best foldarand molecule, i.e., tetramer 4, exhibits a selectivity factor of at least 19 in differentiating the most tightly bound Hg²⁺ ion from all other metal ions, and a binding capacity that is ≥18 times that of thio-crown ethers. These two noteworthy binding characters make possible low level removal of Hg²⁺ ions. With a [4]:[Hg²⁺] molar ratio of 5:1 and a single biphasic solvent extraction, the concentration of Hg²⁺ ions could be reduced drastically by 98% (from 200 to 4 ppb) in pure water. 4 could also effect a highly efficient reduction in mercury content by 98% (from 500 to 10 ppb) in artificial groundwater via multiple successive extractions with an overall consumption of 4 being 9:1 in terms of [4]:[Hg²⁺] molar ratio.

Functionalized hydrazide macrocycle ion channels showing pH-sensitive ion selectivities

The protonation and deprotonation of multiple amines and carboxyls in channels change the charge distribution, which leads to pH-sensitive ion selectivity.

A unimolecular channel formed by dual helical peptide modified pillar[5]arene: correlating transmembrane transport properties with antimicrobial activity and haemolytic toxicity

Five unimolecular channels with different lengths are presented. The varying length of these channels has significant impact on their transport properties.

One-pot formation of hydrazide macrocycles with modified cavities: an example of pH-sensitive unimolecular cation channels

The number and position of carboxyls in the channel have a significant impact on the membrane-incorporation ability, ion selectivity and NH₄⁺ transport activity of the macrocyclic channels.

Reversible photo-gated transmembrane channel assembled from an acylhydrazone-containing crown ether triad

Ion channel transport could be reversibly regulated by irradiation of the lipid bilayer with alternating 320/365 nm light.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

A Halogen-Bond-Induced Triple Helicate Encapsulates Iodide

The self-assembly of higher-order anion helicates in solution remains an elusive goal. Herein, we present the first triple helicate to encapsulate iodide in organic and aqueous media as well as the solid state. The triple helicate self-assembles from three tricationic arylethynyl strands and resembles a tubular anion channel lined with nine halogen bond donors. Eight strong iodine⋅⋅⋅iodide halogen bonds and numerous buried π-surfaces endow the triplex with remarkable stability, even at elevated temperatures. We suggest that the natural rise of a single-strand helix renders its linear halogen-bond donors non-convergent. Thus, the stringent linearity of halogen bonding is a powerful tool for the synthesis of multi-strand anion helicates.

Aza-Bambusurils En Route to Anion Transporters

Previous calculations of anion binding with various bambusuril analogs predicted that the replacement of oxygen by nitrogen atoms to produce semiaza-bambus[6]urils would award these new cavitands with multiple anion binding properties. This study validates the hypothesis by efficient synthesis, crystallography, thermogravimetric analysis and calorimetry. These unique host molecules are easily accessible from the corresponding semithio-bambusurils in a one-pot reaction, which converts a single anion receptor into a potential anion channel. Solid-state structures exhibit simultaneous accommodation of three anions, linearly positioned within the cavity along the main symmetry axis. The ability to hold anions at a short distance of about 4 Å is reminiscent of natural chloride channels in E. coli, which exhibit similar distances between their adjacent anion binding sites. The calculated transition-state energy for double-anion movement through the channel suggests that although these host-guest complexes are thermodynamically stable they enjoy high kinetic flexibility to render them efficient anion channels.

Surface-Cross-Linked Micelles as Multifunctionalized Organic Nanoparticles for Controlled Release, Light Harvesting, and Catalysis

Surfactant micelles are dynamic entities with a rapid exchange of monomers. By "clicking" tripropargylammonium-containing surfactants with diazide cross-linkers, we obtained surface-cross-linked micelles (SCMs) that could be multifunctionalized for different applications. They triggered membrane fusion through tunable electrostatic interactions with lipid bilayers. Antenna chromophores could be installed on them to create artificial light-harvesting complexes with efficient energy migration among tens to hundreds of chromophores. When cleavable cross-linkers were used, the SCMs could break apart in response to redox or pH signals, ejecting entrapped contents quickly as a result of built-in electrostatic stress. They served as caged surfactants whose surface activity was turned on by environmental stimuli. They crossed cell membranes readily. Encapsulated fluorophores showed enhanced photophysical properties including improved quantum yields and greatly expanded Stokes shifts. Catalytic groups could be installed on the surface or in the interior, covalently attached or physically entrapped. As enzyme mimics, the SCMs enabled rational engineering of the microenvironment around the catalysts to afford activity and selectivity not possible with conventional catalysts.

"Sticky"-Ends-Guided Creation of Functional Hollow Nanopores for Guest Encapsulation and Water Transport

Commercial uses of water-transporting aquaporins for seawater desalination and wastewater reclamation/reuse are being investigated in both academia and the industry. Presently, structural complexity, stability, scalability, and activity reconstitution of these costly channel proteins still present substantial challenges to scientists and engineers. An attractive strategy is to develop robust synthetic water channels able to mimic the water-transporting function of aquaporins for utility in the making of next generation of water channel-based biomimetic porous membranes for various water purification applications. In sharp contrast to burgeoning development in constructing synthetic ion channels over the past four decades, very limited progress has been made in the area of synthetic water channels. A handful of such examples include the first report by Percec in 2007 (Percec et al. J. Am. Chem. Soc. 2007, 129, 11698-11699), which was followed by Barboiu in 2011 (Barboiu et al. Angew. Chem., Int. Ed. 2011, 50, 11366-11372), Gong and Hou in 2012 (Gong et al. Nat. Commun. 2012, 3, 949; Hou et al. J. Am. Chem. Soc. 2012, 134, 8384-8387), and Zeng in 2014 (Zeng et al. J. Am. Chem. Soc. 2014, 136, 14270-14276). Radically deviating from the fact that the discovery of novel synthetic channel systems with desired transport selectivity is most often empirical and very often serendipitous, we have instead adopted a more rational designer approach whereby molecular building blocks have been carefully designed from scratch to perform their intended built-in functions. Our designer journey started in 2008, two years after I started leading a group at the National University of Singapore. Since then, we have been actively investigating the use of designed water-binding "aquafoldamers" to construct synthetic water channels for the rapid and selective transport of water molecules ideally with the exclusion of all other nonproton molecular species. Toward this goal, we designed and characterized, by an experimental-theoretical synergy, a new class of modular, H-bonded, and crescent-shaped oligopyridine amide foldamers, enclosing a sizable cavity of about 2.8 Å in diameter. Matching well with the diameter of water molecules and decorated by interior-pointing H-bond donors (amide H atoms) and acceptors (pyridine N atoms) for water binding, this sizable cavity experimentally proves to be suitable for water recognition. In particular, helically folded oligomers are found to be capable of binding two water molecules that are vertically aligned in parallel with helical axis. However, the existence of two repulsive groups at the two helical ends prevents the formation of 1D hollow tubular cavity, via self-assembly, for encapsulating 1D water chains. Subsequently, we introduced two electrostatically complementary functional groups that act as "sticky" ends at helical ends. These feeble "sticky" ends faithfully and seamlessly align short cavity-containing helices one-dimensionally to create hollow tubular aquapores. To our delight, these aquapores demonstrate their excellent ability of highly selectively hosting a chain of single file H-bonded water molecules and allow for selective transport of both protons and water molecules with exclusion of metal ions including Na(+) and K(+) ions across the lipid membranes.