pH-Gated Chloride Transport by a Triazine-Based Tripodal Semicage

Supramolecular chemical biology: designed receptors and dynamic chemical systems

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

Stimulus-Controlled Anion Binding and Transport by Synthetic Receptors

Anionic species are omnipresent and involved in many important biological processes. A large number of artificial anion receptors has therefore been developed. Some of these are capable of mediating transmembrane transport. However, where transport proteins can respond to stimuli in their surroundings, creation of synthetic receptors with stimuli-responsive functions poses a major challenge. Herein, we give a full overview of the stimulus-controlled anion receptors that have been developed thus far, including their application in membrane transport. In addition to their potential operation as membrane carriers, the use of anion recognition motifs in forming responsive membrane-spanning channels is discussed. With this review article, we intend to increase interest in transmembrane transport among scientists working on host-guest complexes and dynamic functional systems in order to stimulate further developments.

Progress and prospects toward supramolecular bioactive ion transporters

The majority of cellular physiological processes depend on natural ion channels, which are pore-forming membrane-embedded proteins that let ions flow across the cell membranes selectively. This selective movement of ions across the membranes balances the osmolality within and outside the cell. However, mutations in the genes that encode essential membrane transport proteins or structural reorganisation of these proteins can cause life-threatening diseases like cystic fibrosis. Artificial ion transport systems have opened up a way to replace dysfunctional natural ion channels to cure such diseases through channel replacement therapy. Moreover, recent research has also demonstrated the ability of these systems to kill cancer cells, reigniting interest in the field among scientists. Our contributions to the recent progress in the design and development of artificial chloride ion transporters and their effect on biological systems have been discussed in this review. This review would provide current vistas and future directions toward the development of novel ion transporters with improved biocompatibility and desired anti-cancer properties. Additionally, it strongly emphasises stimuli-responsive ion transport systems, which are crucial for obtaining target-specificity and may speed up the application of these systems in clinical therapeutics.

Formation of supramolecular channels by reversible unwinding-rewinding of bis(indole) double helix via ion coordination

Stimulus-responsive reversible transformation between two structural conformers is an essential process in many biological systems. An example of such a process is the conversion of amyloid-β peptide into β-sheet-rich oligomers, which leads to the accumulation of insoluble amyloid in the brain, in Alzheimer's disease. To reverse this unique structural shift and prevent amyloid accumulation, β-sheet breakers are used. Herein, we report a series of bis(indole)-based biofunctional molecules, which form a stable double helix structure in the solid and solution state. In presence of chloride anion, the double helical structure unwinds to form an anion-coordinated supramolecular polymeric channel, which in turn rewinds upon the addition of Ag⁺ salts. Moreover, the formation of the anion-induced supramolecular ion channel results in efficient ion transport across lipid bilayer membranes with excellent chloride selectivity. This work demonstrates anion-cation-assisted stimulus-responsive unwinding and rewinding of artificial double-helix systems, paving way for smart materials with better biomedical applications.

Halide-selective, proton-coupled anion transport by phenylthiosemicarbazones

Phenylthiosemicarbazones (PTSCs) are proton-coupled anion transporters with pH-switchable behaviour known to be regulated by an imine protonation equilibrium. Previously, chloride/nitrate exchange by PTSCs was found to be inactive at pH 7.2 due to locking of the thiourea anion binding site by an intramolecular hydrogen bond, and switched ON upon imine protonation at pH 4.5. The rate-determining process of the pH switch, however, was not examined. We here develop a new series of PTSCs and demonstrate their conformational behaviour by X-ray crystallographic analysis and pH-switchable anion transport properties by liposomal assays. We report the surprising finding that the protonated PTSCs are extremely selective for halides over oxyanions in membrane transport. Owing to the high chloride over nitrate selectivity, the pH-dependent chloride/nitrate exchange of PTSCs originates from the rate-limiting nitrate transport process being inhibited at neutral pH.

Photomodulation of Transmembrane Transport and Potential by Stiff-Stilbene Based Bis(thio)ureas

Membrane transport proteins fulfill important regulatory functions in biology with a common trait being their ability to respond to stimuli in the environment. Various small-molecule receptors, capable of mediating transmembrane transport, have been successfully developed. However, to confer stimuli-responsiveness on them poses a fundamental challenge. Here we demonstrate photocontrol of transmembrane transport and electric potential using bis(thio)ureas derived from stiff-stilbene. UV-vis and 1H NMR spectroscopy are used to monitor E-Z photoisomerization of these bis(thio)ureas and 1H NMR titrations reveal stronger binding of chloride to the (Z)-form than to the (E)-form. Additional insight into the binding properties is provided by single crystal X-ray crystallographic analysis and DFT geometry optimization. Importantly, the (Z)-isomers are much more active in transmembrane transport than the respective (E)-isomers as shown through various assays. As a result, both membrane transport and depolarization can be modulated upon irradiation, opening up new prospects toward light-based therapeutics as well as physiological and optopharmacological tools for studying anion transport-associated diseases and to stimulate neuronal activity, respectively.

Mining anion-aromatic interactions in the Protein Data Bank

Mutual positioning and non-covalent interactions in anion–aromatic motifs are crucial for functional performance of biological systems. In this context, regular, comprehensive Protein Data Bank (PDB) screening that involves various scientific points of view and individual critical analysis is of utmost importance. Analysis of anions in spheres with radii of 5 Å around all 5- and 6-membered aromatic rings allowed us to distinguish 555 259 unique anion–aromatic motifs, including 92 660 structures out of the 171 588 structural files in the PDB. The use of a scarcely exploited (x, h) coordinate system led to (i) identification of three separate areas of motif accumulation: A – over the ring, B – over the ring-substituent bonds, and C – roughly in the plane of the aromatic ring, and (ii) unprecedented simultaneous comparative description of various anion–aromatic motifs located in these areas. Of the various residues considered, i.e. aminoacids, nucleotides, and ligands, the latter two exhibited a considerable tendency to locate in region Avia archetypal anion–π contacts. The applied model not only enabled statistical quantitative analysis of space around the ring, but also enabled discussion of local intermolecular arrangements, as well as detailed sequence and secondary structure analysis, e.g. anion–π interactions in the GNRA tetraloop in RNA and protein helical structures. As a purely practical issue of this work, the new code source for the PDB research was produced, tested and made freely available at https://github.com/chemiczny/PDB_supramolecular_search.

The comprehensive analysis of non-redundant PDB macromolecular structures investigating anion distributions around all aromatic molecules in available biosystems is presented.

Bis(cholyl)-based chloride channels with oxalamide and hydrazide selectivity filters

We report the development of supramolecular bis(cholyl) ion channels by using oxalamide and hydrazide as selectivity filters. The hydrazide system displayed superior chloride transport activity than oxalamide via the formation...

Advances in anion transport and supramolecular medicinal chemistry

Advances in anion transport by synthetic supramolecular systems are discussed in this article. Developments in the design of discrete molecular carriers for anions and supramolecular anion channels are reviewed followed by an overview of the use of these systems in biological systems as putative treatments for diseases such as cystic fibrosis and cancer.

Macrocyclic Transmembrane Anion Transporters via a One-Pot Condensation Reaction

Synthetic chloride transporters are potential therapeutic agents for cystic fibrosis and cancer. Reported herein are macrocyclic transmembrane chloride transporters prepared by a one-pot condensation reaction. The most efficient macrocycle possesses a fine balance of hydrophobicity for membrane permeation and hydrophilicity for ion recognition. The macrocycle transports chloride ions by forming channels in the membrane. Hydrogen bonds and anion-π interactions assist chloride transport.

Anion Transporters Based on Noncovalent Balance including Anion-π, Hydrogen, and Halogen Bonding

Anion transmembrane transport mediated by novel noncovalent interactions is of central interest in supramolecular chemistry. In this work, a series of oxacalix[2]arene[2]triazine-derived transporters 1 and 2 bearing anion-π-, hydrogen-, and halogen-bonding sites in rational proximity were designed and synthesized by a one-pot strategy starting from gallic acid ester derivatives and mono- or di-halogen-substituted triazines. ¹H NMR titrations demonstrated efficient binding of 1 and 2 toward Cl^- and Br^- in solution, giving association constants in the range of 10²-10⁴ M^-1. Cooperation of anion-π, hydrogen, and halogen bonding was revealed as a driving force for anion binding by single-crystal structures of two complexes and density functional theory calculations. Fluorescence assays indicated that compounds 1 are efficient chloride transporters with effective concentrations (EC₅₀) falling in the range of 3.1-7.4 μM and following an order of 1a > 1b > 1c > 1d. The contribution of halogen bonding and cooperative noncovalent bonds to ion transport was then discussed. Significantly, transporters 1 exhibit high anticancer activity. In the presence of 1 and KCl (60 mM), the cell survival of HCT116 reduces to 11.9-24.9% with IC₅₀ values in the range of 52.3-66.4 μM.

Triaminopyrimidine derivatives as transmembrane HCl transporters

A series of triaminopyrimidine-based anion transporters has been described, capable of diminishing proton gradients across lipid bilayers at physiologically relevant pH.

Tripodal cyanurates as selective transmembrane Cl− transporters

Tris-carboxyamide and tris-sulfonamide-based anion receptors with cyanuric acid core are developed for transmembrane chloride transport.