Trimodal Control of Ion-Transport Activity on Cyclo-oligo-(1→6)-β-D-glucosamine-Based Artificial Ion-Transport Systems

Recent Advances in Artificial Anion Channels and Their Selectivity

Nature performs critical physiological functions using a series of structurally and functionally diverse membrane proteins embedded in cell membranes, in which native ion protein channels modify the electrical potential inside and outside the cell membrane through charged ion movements. Consequently, the cell responds to external stimuli, playing an essential role in various life activities, such as nerve excitation conduction, neurotransmitter release, muscle movement, and control of cell differentiation. Supramolecular artificial channels, which mimic native protein channels in structure and function, adopt unimolecular or self-assembled structures, such as crown ethers, cyclodextrins, cucurbiturils, column arenes, cyclic peptide nanotubes, and metal-organic artificial channels, in channel construction strategies. Owing to the various driving forces involved, artificial synthetic ion channels can be divided into artificial cation and anion channels in terms of ion selectivity. Cation selectivity usually originates from ion coordination, whereas anion selectivity is related to hydrogen bonding, ion pairing, and anion-dipole interactions. Several studies have been conducted on artificial cation channels, and several reviews have summarized them in detail; however, the research on anions is still in the initial stages, and related reviews have rarely been reported. Hence, this article primarily focuses on the recent research on anion channels.

Probing and evaluating transmembrane chloride ion transport in double walled trifluorophenyl/phthalimide extended calix[4]pyrrole-based supramolecular receptors

Therapeutic applications have sparked increased interest in the use of synthetic anion receptors for ion transport across lipid membranes. In this context, the construction of synthetic transmembrane transporters for the physiologically important chloride ion is currently of enormous interest. As a result, considerable effort is being devoted to the design and synthesis of artificial transmembrane chloride ion transporters. However, only inadequate progress has been made in developing macrocyclic chloride ion transporters using the fundamental principles of supramolecular chemistry, and hence this field entails fostering investigations. In this investigation, the synthesis of two new double walled trifluorophenyl/phthalimide extended calix[4]pyrrole (C4P) receptors (3 and 7) has been successfully reported. 1H-NMR titration and HRMS studies confirmed the 1 : 1 binding stoichiometry of the chloride ion with these receptors in the solution phase (only receptor 3b was studied by 1H-NMR). Regarding ion transport of 3b and 7, when studied in the HPTS-based vesicular system, 3b showed better activity with an EC50 value of 0.39 μM. The detailed ion transport studies on 3b have revealed that ion transport occurs through the Cl-/NO3- antiport mode.

Photocontrolled activation of doubly o-nitrobenzyl-protected small molecule benzimidazoles leads to cancer cell death

Artificial biomimetic chloride anionophores have shown promising applications as anticancer scaffolds. Importantly, stimuli-responsive chloride transporters that can be selectively activated inside the cancer cells to avoid undesired toxicity to normal, healthy cells are very rare. Particularly, light-responsive systems promise better applicability for photodynamic therapy because of their spatiotemporal controllability, low toxicity, and high tunability. Here, in this work, we report o-nitrobenzyl-linked, benzimidazole-based singly and doubly protected photocaged protransporters 2a, 2b, 3a, and 3b, respectively, and benzimidazole-2-amine-based active transporters 1a-1d. Among the active compounds, trifluoromethyl-based anionophore 1a showed efficient ion transport activity (EC50 = 1.2 ± 0.2 μM). Detailed mechanistic studies revealed Cl-/NO3- antiport as the main ion transport process. Interestingly, double protection with photocages was found to be necessary to achieve the complete "OFF-state" that could be activated by external light. The procarriers were eventually activated inside the MCF-7 cancer cells to induce phototoxic cell death.

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.

Molecular Self-Assembly as a Tool to Construct Transmembrane Supramolecular Ion Channels

Self-assembly has become a powerful tool for building various supramolecular architectures with applications in material science, environmental science, and chemical biology. One such area is the development of artificial transmembrane ion channels that mimic naturally occurring channel-forming proteins to unveil various structural and functional aspects of these complex biological systems, hoping to replace the defective protein channels with these synthetically accessible moieties. This account describes our recent approaches to construct supramolecular ion channels using synthetic molecules and their applications in medicinal chemistry.

Synthesis of a cyclic tetramer of 3-amino-3-deoxyallose with axially oriented amino groups

A linear tetramer of β-(1 → 6)-linked 3-azido-3-deoxy-d-allose containing glycosyl donor and glycosyl acceptor functions in the terminal monosaccharide units was prepared starting from 3-azido-3-deoxy-1,2:5,6-di-O-isopropylidene-α-d-allofuranose. Cyclization of the linear tetramer under glycosylation conditions afforded the corresponding cyclic tetrasaccharide in 77% yield; its deprotection and reduction of the azido groups resulted in the formation of the cyclic tetramer of 3-amino-3-deoxy-d-allose with axial amino groups, a potential scaffold for the synthesis of tetravalent functional clusters.

Stimuli-Responsive Anion Transport through Acylhydrazone-Based Synthetic Anionophores

Photoswitchable acylhydrazone-based synthetic anionophores are reported. Single-crystal X-ray structure and ¹H NMR titration studies confirmed the chloride binding in solid and solution states. The ion transport activity of 1a was greatly attenuated through a phototriggered E to Z photoisomerization process, and the photoisomerized deactivated state showed high kinetic stability due to an intramolecular hydrogen bond. Switchable "OFF-ON" transport activity was achieved by the application of light and acid-catalyzed reactivation process.

Tandem Electrospray Mass Spectrometry of Cyclic N-Substituted Oligo-β-(1→6)-D-glucosamines

High-resolution electrospray mass spectra (MS and MS/MS CID) of positive ions of a series of protonated, ammoniated, and metallated molecules of cyclic N-substituted oligo-β-(1→6)-D-glucosamines differing in cycle size and N-acyl substituents were registered and interpreted. It was shown that the main type of fragmentation is a cleavage of glycosidic bonds of a cycle, and in some cases fragmentation of amide side chains is possible. If labile fragments in substituents (e.g., carbohydrate chains) are present, a decay of the cycle and an elimination of labile fragments are of comparable possibility. It was found that in some cases rearrangements with loss of an internal carbohydrate residue (IRL), or an internal part of a side chain, are feasible.

Broadly protective semi-synthetic glycoconjugate vaccine against pathogens capable of producing poly-β-(1→6)-N-acetyl-d-glucosamine exopolysaccharide

Poly-β-(1→6)-N-acetylglucosamine (PNAG) was first discovered as a major component of biofilms formed by Staphylococcus aureus and some other staphylococci but later this exopolysaccharide was also found to be produced by pathogens of various nature. This common antigen is considered as a promising target for construction of a broadly protective vaccine. Extensive studies of PNAG, its de-N-acetylated derivative (dPNAG, containing around 15% of residual N-acetates) and their conjugates with Tetanus Toxoid (TT) revealed the crucial role of de-N-acetylated glucosamine units for the induction of protective immunity. Conjugates of synthetic penta- (5GlcNH₂) and nona-β-(1→6)-d-glucosamines (9GlcNH₂) were tested in vitro and in different animal models and proved to be effective in passive and active protection against different microbial pathogens. Presently conjugate 5GlcNH₂-TT is being produced under GMP conditions and undergoes safety and effectiveness evaluation in humans and economically important animals. Current review summarizes all stages of this long-termed study.

A Sandwich Azobenzene-Diamide Dimer for Photoregulated Chloride Transport

There has been a tremendous evolution for artificial ion transport systems, especially gated synthetic systems, which closely mimic their natural congeners. Herein, we demonstrate a trans-azobenzene-based photoregulatory anionophoric system that transports chloride by forming a sandwich dimeric complex. Further studies confirmed a carrier-mediated chloride-anion antiport mechanism, and the supramolecular interactions involved in chloride recognition within the sandwich complex were revealed from theoretical studies. Reversible trans-cis photoisomerization of the azobenzene was achieved without any significant contribution from the thermal cis→trans isomerization at room temperature. Photoregulatory transport activity across the lipid bilayer membrane inferred an outstanding off-on response of the azobenzene photoswitch.

Gas-Phase Fragmentation of Cyclic Oligosaccharides in Tandem Mass Spectrometry

Modern mass spectrometry, including electrospray and MALDI, is applied for analysis and structure elucidation of carbohydrates. Cyclic oligosaccharides isolated from different sources (bacteria and plants) have been known for decades and some of them (cyclodextrins and their derivatives) are widely used in drug design, as food additives, in the construction of nanomaterials, etc. The peculiarities of the first- and second-order mass spectra of cyclic oligosaccharides (natural, synthetic and their derivatives and modifications: cyclodextrins, cycloglucans, cyclofructans, cyclooligoglucosamines, etc.) are discussed in this minireview.

Self-assembly of small-molecule fumaramides allows transmembrane chloride channel formation

Intermolecular hydrogen bonding of N¹,N⁴-dicyclohexylfumaramide favors the formation of self-assembled ion channels, which facilitates selective Cl⁻ transport across a lipid bilayer membrane.

Bis(sulfonamide) transmembrane carriers allow pH-gated inversion of ion selectivity

Bis(sulfonamide) based synthetic carriers are reported for inversion of ion selectivity upon deviation of pH within a narrow window. A liposomal membrane potential is also generated when potassium ions are passively transported by these carriers.