Reversible Ligand‐Gated Ion Channel via Interconversion between Hollow Single Helix and Intertwined Double Helix

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.

Concentration-Driven Evolution of Adaptive Artificial Ion Channels or Nanopores with Specific Anticancer Activities

In nature, ceramides are a class of sphingolipids possessing a unique ability to self-assemble into protein-permeable channels with intriguing concentration-dependent adaptive channel cavities. However, within the realm of artificial ion channels, this interesting phenomenon is scarcely represented. Herein, we report on a novel class of adaptive artificial channels, Pn-TPPs, based on PEGylated cholic acids bearing triphenylphosphonium (TPP) groups as anion binding motifs. Interestingly, the molecules self-assemble into chloride ion channels at low concentrations while transforming into small molecule-permeable nanopores at high concentrations. Moreover, the TPP groups endow the molecules with mitochondria-targeting properties, enabling them to selectively drill holes on the mitochondrial membrane of cancer cells and subsequently trigger the caspase 9 apoptotic pathway. The anticancer efficacies of Pn-TPPs correlate with their abilities to form nanopores. Significantly, the most active ensembles formed by P5-TPP exhibits impressive anticancer activity against human liver cancer cells, with an IC50 value of 3.8 μM. While demonstrating similar anticancer performance to doxorubicin, P5-TPP exhibits a selectivity index surpassing that of doxorubicin by a factor of 16.8.

Noncovalent interaction network of chalcogen, halogen and hydrogen bonds for supramolecular β-sheet organization

An alanine-based bilateral building block, linked by 2,5-thiophenediamide motifs and equipped with C-terminal 4-iodoaniline groups, was designed, allowing a noncovalent interaction network consisting of intramolecular chalcogen bonds and intermolecular halogen/hydrogen bonds, which cooperatively maintain a supramolecular β-sheet organization in the solid state, as well as in dilute CH3CN solution with a high g factor of -0.017.

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Design of Chiral β-Double Helices from γ-Peptide Foldamers

Chirality is ubiquitous in nature, and homochirality is manifested in many biomolecules. Although β-double helices are rare in peptides and proteins, they consist of alternating L- and D-amino acids. No peptide double helices with homochiral amino acids have been observed. Here, we report chiral β-double helices constructed from γ-peptides consisting of alternating achiral (E)-α,β-unsaturated 4,4-dimethyl γ-amino acids and chiral (E)-α,β-unsaturated γ-amino acids in both single crystals and in solution. The two independent strands of the same peptide intertwine to form a β-double helix structure, and it is stabilized by inter-strand hydrogen bonds. The peptides with chiral (E)-α,β-unsaturated γ-amino acids derived from α-L-amino acids adopt a (P)-β-double helix, whereas peptides consisting of (E)-α,β-unsaturated γ-amino acids derived from α-D-amino acids adopt an (M)-β-double helix conformation. The circular dichroism (CD) signature of the (P) and (M)-β-double helices and the stability of these peptides at higher temperatures were examined. Furthermore, ion transport studies suggested that these peptides transport ions across membranes. Even though the structural analogy suggests that these new β-double helices are structurally different from those of the α-peptide β-double helices, they retain ion transport activity. The results reported here may open new avenues in the design of functional foldamers.

Cooperativity in Photofoldamer Chloride Double Helices Turned On with Sequences and Solvents, Around with Guests, and Off with Light

Photofoldamers are sequence-defined receptors capable of switching guest binding on and off. When two foldamer strands wrap around the guest into 2:1 double helical complexes, cooperativity emerges, and with it comes the possibility to switch cooperativity with light and other stimuli. We use lessons from nonswitchable sequence isomers of aryl-triazole foldamers to guide how to vary the sequence location of azobenzenes from the end (F_END) to the interior (F_IN) and report their impact on the cooperative formation of 2:1 complexes with Cl^-. This sequence change produces a 125-fold increase from anti-cooperative (α = 0.008) for F_END to non-cooperative with F_IN (α = 1.0). Density functional theory (DFT) studies show greater H-bonding and a more relaxed double helix for F_IN. The solvent and guest complement the synthetic designs. Use of acetonitrile to enhance solvophobicity further enhances cooperativity in F_IN (α = 126) but lowers the difference in cooperativity between sequences. Surprisingly, the impact of the sequence on cooperativity is inverted when the guest size is increased from Cl^- (3.4 Å) to BF₄^- (4.1 Å). While photoconversion of interior azobenzenes was poor, the cis-cis isomer forms 1:1 complexes around chloride consistent with switching cooperativity. The effect of the guest, solvent, and light on the double-helix cooperativity depends on the sequence.

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.

Roles of Metal Ions in Foldamers and Other Conformationally Flexible Supramolecular Systems

Conformational control is a key prerequisite for much molecular function. As chemists seek to create complex molecules that have applications beyond the academic laboratory, correct spatial positioning is critical. This is particularly true of flexible systems. Conformationally flexible molecules show potential because they resemble in many cases naturally occurring analogues such as the secondary structures found in proteins and peptides such as α-helices and β-sheets. One of the ways in which conformation can be controlled in these molecules is through interaction with or coordination to metal ions. This review explores how secondary structure (i.e., controlled local conformation) in foldamers and other conformationally flexible systems can be enforced or modified through coordination to metal ions. We hope to provide examples that illustrate the power of metal ions to influence this structure toward multiple different outcomes.

A Photo-responsive Transmembrane Anion Transporter Relay

Ion transport across lipid membranes in biology is controlled by stimuli-responsive membrane channels and molecular machine ion pumps such as ATPases. Here, we report a synthetic molecular machine-like ion transport relay, in which transporters on opposite sides of a lipid bilayer membrane facilitate transport by passing ions between them. By incorporating a photo-responsive telescopic arm into the relay design, this process is reversibly controlled in response to irradiation with blue and green light. Transport occurs only in the extended state when the length of the arm is sufficient to pass the anion between transporters located on opposite sides of the membrane. In contrast, the contracted state of the telescopic arm is too short to mediate effective transport. The system acts as a stimuli-responsive ensemble of machine-like components, reminiscent of robotic arms in a factory assembly line, working cooperatively to mediate ion transport. This work points to new prospects for using lipid bilayer membranes as scaffolds for confining, orientating, and controlling the relative positions of molecular machines, thus enabling multiple components to work in concert and opening up new applications in biological contexts.

Supramolecular helices from helical building blocks via head-to-tail intermolecular interactions

Supramolecular helices from helical building blocks represent an emerging analogue of the α-helix. In cases where the helicity of the helical building block is well propagated, the head-to-tail intermolecular interactions that lead to the helix could be enhanced to promote the formation and the stability of the supramolecular helix, wherein homochiral elongation dominates and functional helical channel structures could also be generated. This feature article outlines the supramolecular helices built from helical building blocks, i.e., helical aromatic foldamers and helical short peptides that are held together by intermolecular π-π stacking, hydrogen/halogen/chalcogen bonding, metal coordination, dynamic covalent bonding and solvophobic interactions, with emphasis on the influence of efficient propagation of helicity during assembly, favouring homochirality and channel functions.

Discrete Self-Assembled Metallo-Foldamers with Heteroleptic Sequence Specificity

Discrete and structurally diverse foldamer sequences are constructed in both natural and abiotic systems primarily using inert connectivity with irreversible organic covalent bonds, serving to preserve the identity of the sequence. The formation of sequences under thermodynamic control using labile coordination bonds would be attractive for synthetic ease and modular capability, but this presents issues regarding sequence preservation. Here is presented an approach integrating palladium(II) metal ions into the sequence itself, with fidelity maintained through use of complementary pairings of ligand arrangements at the metal centre. This is accomplished using sites of different denticity and/or hydrogen bonding capability. In this fashion, discrete and ordered metallo-sequences are formed as thermodynamic products in a single step, and these then fold into defined conformations due to π-π interactions between electron-rich and -poor aromatic regions of the combined componentry.

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.

Engineering of stimuli-responsive lipid-bilayer membranes using supramolecular systems

The membrane proteins found in nature control many important cellular functions, including signal transduction and transmembrane ion transport, and these, in turn, are regulated by external stimuli, such as small molecules, membrane potential and light. Membrane proteins also find technological applications in fields ranging from optogenetics to synthetic biology. Synthetic supramolecular analogues have emerged as a complementary method to engineer functional membranes. This Review describes stimuli-responsive supramolecular systems developed for the control of ion transport, signal transduction and catalysis in lipid-bilayer-membrane systems. Recent advances towards achieving spatio-temporal control over activity in artificial and living cells are highlighted. Current challenges, the scope, limitations and future potential to exploit supramolecular systems for engineering stimuli-responsive lipid-bilayer membranes are discussed.