Supramolecular methods: the 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) transport assay

N,N'-Dimethylsquaramide as a central scaffold for anionophore design

The N,N'-dimethylation of a diphenylsquaramide induces a conformational change in the orientation of the phenyl rings. This has been exploited to create a series of bis-urea, -thiourea and -squaramide anionophores. The compounds were shown to bind to Cl- using proton NMR titration techniques and to transport H+/Cl- through the lipid bilayers, whereas a non-methylated analogue displayed limited transport activity. Despite their potency in transport studies, the series had a negligible impact on cancer cell viability.

Tunable Pnictogen Bonding at the Service of Hydroxide Transport across Phospholipid Bilayers

Our growing interest in the design of pnictogen-based strategies for anion transport has prompted an investigation into the properties of three simple triarylcatecholatostiboranes (1-3) of the general formula (o-C6Cl4O2)SbAr3 with Ar = Ph (1), o-tolyl (2), and o-xylyl (3) for the complexation and transport of hydroxide across phospholipid bilayers. A modified hydroxypyrene-1,3,6-trisulfonic acid (HPTS) assay carried out in artificial liposomes shows that 1 and 2 are potent hydroxide transporters while 3 is inactive. These results indicate that the steric hindrance imposed by the three o-xylyl groups prevents access by the hydroxide anion to the antimony center. Supporting this interpretation, 1 and 2 quickly react with TBAOH·30 H2O ([TBA]+ = [nBu4N]+) to form the corresponding hydroxoantimonate salts [nBu4N][1-OH] and [nBu4N][2-OH], whereas 3 resists hydroxide coordination and remains unperturbed. Moreover, the hydroxide transport activities of 1 and 2 are correlated to the +V oxidation state of the antimony atom as the parent trivalent stibines show no hydroxide transport activity.

Artificial DNA Framework Channel Modulates Antiapoptotic Behavior in Ischemia-Stressed Cells via Destabilizing Promoter G-Quadruplex

Regulating folding/unfolding of gene promoter G-quadruplexes (G4s) is important for understanding the topological changes in genomic DNAs and the biological effects of such changes on important cellular events. Although many G4-stabilizing ligands have been screened out, effective G4-destabilizing ligands are extremely rare, posing a great challenge for illustrating how G4 destabilization affects gene function in living cells under stress, a long-standing question in neuroscience. Herein, we report a distinct methodology able to destabilize gene promoter G4s in ischemia-stressed neural cells by mitigating the ischemia-induced accumulation of intracellular K+ with an artificial membrane-spanning DNA framework channel (DFC). We also show that ischemia-triggered K+ influx is positively correlated to anomalous stabilization of promoter G4s and downregulation of Bcl-2, an antiapoptotic gene with neuroprotective effects against ischemic injury. Intriguingly, the DFC enables rapid transmembrane transport of excessive K+ mediated by the internal G4 filter, leading to the destabilization of endogenous promoter G4 in Bcl-2 and subsequent turnover of gene expression at both transcription and translation levels under ischemia. Consequently, this work enriches our understanding of the biological roles of endogenous G4s and may offer important clues to study the cellular behaviors in response to stress.

Molecular clips with spatially proximal urea residues for efficient transmembrane co-transport of H+/Cl- ions

We report the design of bis(urea) functionalized amphiphilic molecular clips viz. 1a-1e to achieve efficient transmembrane co-transport of H+/Cl- ions. The most promising molecule 1a demonstrated a low nanomolar EC50 value (6.96 nM) to co-transport H+/Cl- ions via a carrier-mediated pathway and showed selective toxicity against cancerous HeLa cells as a result.

Molecular Machines For The Control Of Transmembrane Transport

Nature embeds some of its molecular machinery, including ion pumps, within lipid bilayer membranes. This has inspired chemists to attempt to develop synthetic analogues to exploit membrane confinement and transmembrane potential gradients, much like their biological cousins. In this perspective, we outline the various strategies by which molecular machines─molecular systems in which a nanomechanical motion is exploited for function─have been designed to be incorporated within lipid membranes and utilized to mediate transmembrane ion transport. We survey molecular machines spanning both switches and motors, those that act as mobile carriers or that are anchored within the membrane, mechanically interlocked molecules, and examples that are activated in response to external stimuli.

Exploiting the Catenane Mechanical Bond Effect for Selective Halide Anion Transmembrane Transport

The first examples of [2]catenanes capable of selective anion transport across a lipid bilayer are reported. The neutral halogen bonding (XB) [2]catenanes were prepared via a chloride template-directed strategy in an unprecedented demonstration of using XB⋅⋅⋅anion interactions to direct catenane assembly from all-neutral components. Anion binding experiments in aqueous-organic solvent media revealed strong halide over oxoanion selectivity, and a marked enhancement in the chloride and bromide affinities of the catenanes relative to their constituent macrocycles. The catenanes additionally displayed an anti-Hofmeister binding preference for bromide over the larger iodide anion, illustrating the efficacy of employing sigma-hole interactions in conjunction with the mechanical bond effect to tune receptor selectivity. Transmembrane anion transport studies conducted in POPC LUVs revealed that the catenanes were more effective anion transporters than the constituent macrocycles, with high chloride over hydroxide selectivity, which is critical to potential therapeutic applications of anionophores. Remarkably these outperform existing acyclic halogen bonding anionophores with regards to this selectivity. Record chloride over nitrate anion transport selectivity was also observed. This represents a rare example of the direct translation of intrinsic anion binding affinities to anion transport behaviour, and demonstrates the key role of the catenane mechanical bond effect for enhanced anion transport selectivity.

Small molecule anion carriers facilitate lactate transport in model liposomes and cells

An excessive production of lactate by cancer cells fosters tumor growth and metastasis. Therefore, targeting lactate metabolism and transport offers a new therapeutic strategy against cancer, based on dependency of some cancer cells for lactate as energy fuel or as oncogenic signal. Herein we present a family of anionophores based on the structure of click-tambjamines that have proved to be extremely active lactate carriers across phospholipid membranes. Compound 1, the most potent lactate transmembrane carrier, was studied in HeLa cells. The use of a monocarboxylate transporters (MCTs) inhibitor proved that 1 is an active lactate transporter in living cells, confirming the results obtained in phospholipid vesicles. Moreover, an additive effect of compound 1 with cisplatin was observed in HeLa cells. Identification of active lactate anionophores working in living cells opens up ways to exploit this class of compounds as molecular tools and drugs addressing dysregulated lactate metabolism.

Formulation and evaluation of anion transporters in nanostructured lipid carriers

Six novel click-tambjamines (1-6) bearing an alkyl chain of varying length linked to the imine moiety have been formulated in nanostructured lipid carriers (NLCs) to evaluate their transmembrane anion transport activity both when free (i.e., not encapsulated) and nanoformulated. Nanostructured lipid carriers (NLCs) are an example of drug delivery systems (DDSs) that stand out because of their versatility. In this work we show that NLCs can be used to efficiently formulate highly lipophilic anionophores and experiments conducted in model liposomes reveal that these formulations are adequate to deliver anionophores without compromising their transport activity. This result paves the way to facilitate the study of highly lipophilic anionophores and their potential use as future drugs.

Gallium Triggers Ferroptosis through a Synergistic Mechanism

The gallium ion (Ga3+ ) has long been believed to disrupt ferric homeostasis in the body by competing with iron cofactors in metalloproteins, ultimately leading to cell death. This study revealed that through an indirect pathway, gallium can trigger ferroptosis, a type of non-apoptotic cell death regulated by iron. This is exemplified by the gallium complex of the salen ligand (Ga-1); we found that Ga-1 acts as an effective anion transporter that can affect the pH gradient and change membrane permeability, leading to mitochondrial dysfunction and the release of ferrous iron from the electron transfer chain (ETC). In addition, Ga-1 also targeted protein disulfide isomerases (PDIs) located in the endoplasmic reticulum (ER) membrane, preventing the repair of the antioxidant glutathione (GSH) system and thus enforcing ferroptosis. Finally, a combination treatment of Ga-1 and dietary polyunsaturated fatty acids (PUFAs), which enhances lipid peroxidation during ferroptosis, showed a synergistic therapeutic effect both in vitro and in vivo. This study provided us with a strategy to synergistically induce Ferroptosis in tumor cells, thereby enhancing the anti-neoplastic effect.

Transmembrane Transporter Constructed from PlatinumMetal-organic Cage

A perylene diimide-based metal-organic cage (MOC4c) was found to be an efficient transmembrane transporter for ions and small molecules through the internal cavity of the cage. MOC4c could selectively transport different anions, as evidenced by vesicle-based fluorescenceassays and planar lipid bilayer-based current recordings.Furthermore, MOC4c appears tofacilitate calcein transport across the lipid bilayer membrane of a livingcell, suggesting that MOC4c could be used as a biologicaltool for small molecule drugstransmembrane transportation.

Photoswitchable Bis(amidopyrroles): Modulating Anion Transport Activity Independent of Binding Affinity

Toward photocontrol of anion transport across the bilayer membrane, stiff-stilbene, which has dimethyl substituents in the five-membered rings, is functionalized with amidopyrrole units. UV-vis and 1H NMR studies show high photostability and photoconversion yields. Where the photoaddressable (E)- and (Z)-isomers exhibit comparable binding affinities, as determined by 1H NMR titrations, fluorescence-based transport assays reveal significantly higher transport activity for the (Z)-isomers. Changing the binding affinity is thus not a necessity for modulating transport. Additionally, transport can be triggered in situ by light.

Fatty Acid-Activated Proton Transport by Bisaryl Anion Transporters Depolarises Mitochondria and Reduces the Viability of MDA-MB-231 Breast Cancer Cells

In respiring mitochondria, the proton gradient across the inner mitochondrial membrane is used to drive ATP production. Mitochondrial uncouplers, which are typically weak acid protonophores, can disrupt this process to induce mitochondrial dysfunction and apoptosis in cancer cells. We have shown that bisaryl urea-based anion transporters can also mediate mitochondrial uncoupling through a novel fatty acid-activated proton transport mechanism, where the bisaryl urea promotes the transbilayer movement of deprotonated fatty acids and proton transport. In this paper, we investigated the impact of replacing the urea group with squaramide, amide and diurea anion binding motifs. Bisaryl squaramides were found to depolarise mitochondria and reduce MDA-MB-231 breast cancer cell viability to similar extents as their urea counterpart. Bisaryl amides and diureas were less active and required higher concentrations to produce these effects. For all scaffolds, the substitution of the bisaryl rings with lipophilic electron-withdrawing groups was required for activity. An investigation of the proton transport mechanism in vesicles showed that active compounds participate in fatty acid-activated proton transport, except for a squaramide analogue, which was sufficiently acidic to act as a classical protonophore and transport protons in the absence of free fatty acids.

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.

Conformationally Flexible Cleft Receptor for Chloride Anion Transport

The design and synthesis of a cleft-shaped bis-diarylurea receptor for chloride anion transport is reported in this work. The receptor is based on the foldameric nature of N,N'-diphenylurea upon its dimethylation. The bis-diarylurea receptor exhibits a strong and selective affinity for chloride over bromide and iodide anions. A nanomolar quantity of the receptor efficiently transports the chloride across a lipid bilayer membrane as a 1:1 complex (EC₅₀ = 5.23 nm). The work demonstrates the utility of the N,N'-dimethyl-N,N'-diphenylurea scaffold in anion recognition and transport.

Fluorinated tetrapodal anion transporters

Synthetic anion transporters show potential in treating life-threatening diseases like cystic fibrosis and cancer. However, with increasingly complex transporter architectures designed to control anion binding and transport, it is important to consider solubility and deliverability during transporter design. The fluorination of synthetic anion transporters has been shown to tune the transporter lipophilicity, transport rates, and binding strength. In this work, we expand on our previously reported tetrapodal (thio)urea transporters with a series of fluorinated tetrapodal anion transporters. The effects of fluorination on tuning the lipophilicity, solubility, deliverability, and anion transport selectivity of the tetrapodal scaffold were investigated using anion-binding and transport assays. The primary mode of anion transport was H⁺/X^- cotransport, with the most fluorinated tetrathiourea (8) displaying the highest transport activity in the 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) assay. Intriguingly, inversion of the transmembrane Cl^- vs NO₃ ^- transport selectivity compared with previously reported tripodal (thio)urea transporters was observed under a modified HPTS assay.

Structure-Activity Relationship and Mechanistic Studies of Bisaryl Urea Anticancer Agents Indicate Mitochondrial Uncoupling by a Fatty Acid-Activated Mechanism

Targeting the cancer cell mitochondrion is a promising approach for developing novel anticancer agents. The experimental anticancer agent N,N'-bis(3,5-dichlorophenyl)urea (SR4) induces apoptotic cell death in several cancer cell lines by uncoupling mitochondrial oxidative phosphorylation (OxPhos) using a protein-free mechanism. However, the precise mechanism by which SR4 depolarizes mitochondria is unclear because SR4 lacks an acidic functional group typically found in protein-independent uncouplers. Recently, it was shown that structurally related thioureas can facilitate proton transport across lipid bilayers by a fatty acid-activated mechanism, in which the fatty acid acts as the site of protonation/deprotonation and the thiourea acts as an anion transporter that shuttles deprotonated fatty acids across the phospholipid bilayer to enable proton leak. In this paper, we show that SR4-mediated proton transport is enhanced by the presence of free fatty acids in the lipid bilayer, indicating that SR4 uncouples mitochondria through the fatty acid-activated mechanism. This mechanistic insight was used to develop a library of substituted bisaryl ureas for structure-activity relationship studies and subsequent cell testing. It was found that lipophilic electron-withdrawing groups on bisaryl ureas enhanced electrogenic proton transport via the fatty acid-activated mechanism and had the capacity to depolarize mitochondria and reduce the viability of MDA-MB-231 breast cancer cells. The most active compound in the series reduced cell viability with greater potency than SR4 and was more effective at inhibiting adenosine triphosphate production.

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.

A water-soluble membrane transporter for biologically relevant cations

Synthetic ionophores are promising therapeutic targets, yet poor water solubility limits their potential for translation into the clinic. Here we report a water soluble, supramolecular self-associating amphiphile (SSA) with cation transport function.