Molecular Swings as Highly Active Ion Transporters

Crown Ether-Based Ion Transporters in Bilayer Membranes

Bilayer membranes that enhance the stability of the cell are essential for cell survival, separating and protecting the interior of the cell from its external environment. Membrane-based channel proteins are crucial for sustaining cellular activities. However, dysfunction of these proteins would induce serial channelopathies, which could be substituted by artificial ion channel analogs. Crown ethers (CEs) are widely studied in the area of artificial ion channels owing to their intrinsic host-guest interaction with different kinds of organic and inorganic ions. Other advantages such as lower price, chemical stability, and easier modification also make CE a research hotspot in the field of synthetic transmembrane nanopores. And numerous CEs-based membrane-active synthetic ion channels were designed and fabricated in the past decades. Herein, the recent progress of CEs-based synthetic ion transporters has been comprehensively summarized in this review, including their design principles, functional mechanisms, controllable properties, and biomedical applications. Furthermore, this review has been concluded by discussing the future opportunities and challenges facing this research field. It is anticipated that this review could offer some inspiration for the future fabrication of novel CEs-derived ion transporters with more advanced structures, properties, and practical applications.

Monofunctionalized Fluorinated Bambusurils and Their Conjugates for Anion Transport and Extraction

Bambusurils are macrocyclic molecules that are known for their high binding affinity and selectivity toward anions. Here, we present the preparation of two bambusurils bearing fluorinated substituents and one carboxylic function. These monofunctionalized bambusurils were conjugated with crown ether and cholesterol units. The resulting conjugates were successfully tested in liquid-liquid extraction of inorganic salts and chloride/bicarbonate transport across lipid bilayers.

A Light-Driven Molecular Machine Controls K+ Channel Transport and Induces Cancer Cell Apoptosis

The design of artificial ion channels with high activity, selectivity and gating function is challenging. Herein, we designed the light-driven motor molecule MC2, which provides new design criteria to overcome these challenges. MC2 forms a selective K⁺ channel through a single molecular transmembrane mechanism, and the light-driven rotary motion significantly accelerates ion transport, which endows the irradiated motor molecule with excellent cytotoxicity and cancer cell selectivity. Mechanistic studies reveal that the rotary motion of MC2 promotes K⁺ efflux, generates reactive oxygen species and eventually activates caspase-3-dependent apoptosis in cancer cells. Combined with the spatiotemporally controllable advantages of light, we believe this strategy can be exploited in the structural design and application of next-generation synthetic cation transporters for the treatment of cancer and other diseases.

Organoplatinum Compounds as Anion‐Tuneable Uphill Hydroxide Transporters

Active transport of ions uphill, creating a concentration gradient across a cell membrane, is essential for life. It remains a significant challenge to develop synthetic systems that allow active uphill transport. Here, a transport process fuelled by organometallic compounds is reported that creates a pH gradient. The hydrolysis reaction of Pt^II complexes results in the formation of aqua complexes that established rapid transmembrane movement (“flip‐flop”) of neutral Pt−OH species, leading to protonation of the OH group in the inner leaflet, generating OH⁻ ions, and so increasing the pH in the intravesicular solution. The organoplatinum complex effectively transports bound hydroxide ions across the membrane in a neutral complex. The initial net flow of the Pt^II complex into the vesicles generates a positive electric potential that can further drive uphill transport because the electric potential is opposed to the chemical potential of OH⁻. The OH⁻ ions equilibrate with this transmembrane electric potential but cannot remove it due to the relatively low permeability of the charged species. As a result, effective hydroxide transport against its concentration gradient can be achieved, and multiple additions can continuously drive the generation of OH⁻ against its concentration gradient up to ΔpH>2. Moreover, the external addition of different anions can control the generation of OH⁻ depending on their anion binding affinity. When anions displayed very high binding affinities towards Pt^II compounds, such as halides, the external anions could dissipate the pH gradient. In contrast, a further pH increase was observed for weak binding anions, such as sulfate, due to the increase of positive electric potential.

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.

Self-Assembly of Size-Controlled m-Pyridine-Urea Oligomers and Their Biomimetic Chloride Ion Channels

The m-pyridine urea (mPU) oligomer was constructed by using the intramolecular hydrogen bond formed by the pyridine nitrogen atom and the NH of urea and the intermolecular hydrogen bond of the terminal carbonyl group and the NH of urea. Due to the synergistic effect of hydrogen bonds, mPU oligomer folds and exhibits strong self-assembly behaviour. Affected by folding, mPU oligomer generates a twisted plane, and one of its important features is that the carbonyl group of the urea group orientates outwards from the twisted plane, while the NHs tend to direct inward. This feature is beneficial to NH attraction for electron-rich species. Among them, the trimer self-assembles into helical nanotubes, and can efficiently transport chloride ions. This study provides a novel and efficient strategy for constructing self-assembled biomimetic materials for electron-rich species transmission.

Ultrasensitive liposome-based assay for the quantification of fundamental ion channel properties

One of the most widely used approaches to characterize transmembrane ion transport through nanoscale synthetic or biological channels is a straightforward, liposome-based assay that monitors changes in ionic flux across the vesicle membrane using pH- or ion-sensitive dyes. However, failure to account for the precise experimental conditions, in particular the complete ionic composition on either side of the membrane and the inherent permeability of ions through the lipid bilayer itself, can prevent quantifications and lead to fundamentally incorrect conclusions. Here we present a quantitative model for this assay based on the Goldman-Hodgkin-Katz flux theory, which enables accurate measurements and identification of optimal conditions for the determination of ion channel permeability and selectivity. Based on our model, the detection sensitivity of channel permeability is improved by two orders of magnitude over the commonly used experimental conditions. Further, rather than obtaining qualitative preferences of ion selectivity as is typical, we determine quantitative values of these parameters under rigorously controlled conditions even when the experimental results would otherwise imply (without our model) incorrect behavior. We anticipate that this simply employed ultrasensitive assay will find wide application in the quantitative characterization of synthetic or biological ion channels.