pH switchable anion transport by an oxothiosquaramide

A photocaged, pH-sensitive anion transporter with AND logic dual-stimuli activation

A pH-switchable anion transporter 1 was photocaged with two photolabile groups to enhance spatiotemporal control over its chloride transport activity. Simultaneous application of light irradiation and acidic pH restores the activity of 1, while either stimulus alone results in no or very low activity. The double activation strategy described herein has potential to yield more selectively cytotoxic anionophores for future medical applications.

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.

Length dependent reversible off-on activation of photo-switchable relay anion transporters

A homologous series of azobenzene-derived photo-switchable ion relay transporters is reported. We reveal that both the length and geometry of the relay strongly affect transport rate, allowing the relative activity of the E and Z isomers to be reversed and hence the wavelengths of light used for on and off switching to be exchanged.

Squaramide-based receptors in anion supramolecular chemistry: insights into anion binding, sensing, transport and extraction

Over the last 15 years, squaramide-based receptors have attracted the attention of supramolecular chemists working in the field of anion recognition. Herein, we highlight examples of squaramide-based receptors that are able to bind, sense, extract and transport anions.

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.

Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour

The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO−, SO42− HPO42−, and F−. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl−; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2.

Amidosquaramides - a new anion binding motif with pH sensitive anion transport properties

Stimuli responsive anion transport is becoming an important aspect of supramolecular anion recognition chemistry. Herein, we report the synthesis of a family of anion receptors that incorporate a new anion binding motif, amidosquaramides. We show using experimental and computational methods that these receptors have pK_a values close to physiological pH but also display intramolecular H-bonding interactions that affect anion recognition. Moreover, moderate activity in a Cl^-/NO₃^- exchange assay is observed at physiological pH that can be effectively 'switched on' when repeated under acidic conditions. The reported findings provide synthetic methods that can be used for the construction of more complex squaramide based anion receptors and also provide insight into the importance of conformational analysis when considering receptor design.

Structure-antitumor activity relationships of tripodal imidazolium-amino acid based salts. Effect of the nature of the amino acid, amide substitution and anion

The antitumor activity of imidazolium salts is highly dependent upon their lipophilicity that can be tuned by the introduction of different hydrophobic substituents on the nitrogen atoms of the imidazolium ring of the molecule. Taking this into consideration, we have synthesized and characterized a series of tripodal imidazolium salts derived from L-valine and L-phenylalanine containing different hydrophobic groups and tested them against four cancer cell lines at physiological and acidic pH. At acidic pH (6.2) the anticancer activity of some of the tripodal compounds changes dramatically, and this parameter is crucial to control their cytotoxicity and selectivity. Moreover, several of these compounds displayed selectivity against the control healthy cell line higher than four. The transmembrane anion transport studies revealed moderate transport abilities suggesting that the observed biological activity is likely not the result of just their transport activity. The observed trends in biological activity at acidic pH agree well with the results for the CF leakage assay. These results strongly suggest that this class of compounds can serve as potential chemotherapeutic agents.

Squaramide-Based Self-Associating Amphiphiles for Anion Recognition

The synthesis and characterisation of two novel self-assembled amphiphiles (SSAs) SQS-1 and SQS-2 are reported. Both compounds, based on the squaramide motif, were fully soluble in a range of solvents and were shown to undergo self-assembly through a range of physical techniques. Self-assembly was shown to favour the formation of crystalline domains on the nanoscale but also fibrillar film formation, as suggested by SEM analysis. Moreover, both SQS-1 and SQS-2 were capable of anion recognition in DMSO solution as demonstrated using 1 H NMR and UV/Vis absorption spectroscopy, but displayed lower binding affinities for various anions when compared against other squaramide based receptors. In more competitive solvent mixtures SQS-1 gave rise to a colourimetric response in the presence of HPO42- that was clearly visible to the naked eye. We anticipate that the observed response is due to the basic nature of the HPO42- anion when compared against other biologically relevant anions.

Carbazole-Based Colorimetric Anion Sensors

Owing to their strong carbazole chromophore and fluorophore, as well as to their powerful and convergent hydrogen bond donors, 1,8-diaminocarbazoles are amongst the most attractive and synthetically versatile building blocks for the construction of anion receptors, sensors, and transporters. Aiming to develop carbazole-based colorimetric anion sensors, herein we describe the synthesis of 1,8-diaminocarbazoles substituted with strongly electron-withdrawing substituents, i.e., 3,6-dicyano and 3,6-dinitro. Both of these precursors were subsequently converted into model diamide receptors. Anion binding studies revealed that the new receptors exhibited significantly enhanced anion affinities, but also significantly increased acidities. We also found that rear substitution of 1,8-diamidocarbazole with two nitro groups shifted its absorption spectrum into the visible region and converted the receptor into a colorimetric anion sensor. The new sensor displayed vivid color and fluorescence changes upon addition of basic anions in wet dimethyl sulfoxide, but it was poorly selective; because of its enhanced acidity, the dominant receptor-anion interaction for most anions was proton transfer and, accordingly, similar changes in color were observed for all basic anions. The highly acidic and strongly binding receptors developed in this study may be applicable in organocatalysis or in pH-switchable anion transport through lipophilic membranes.

Boosting Anion Transport Activity of Diamidocarbazoles by Electron Withdrawing Substituents

Artificial chloride transporters have been intensely investigated in view of their potential medicinal applications. Recently, we have established 1,8-diamidocarbazoles as a versatile platform for the development of active chloride carriers. In the present contribution, we investigate the influence of various electron-withdrawing substituents in positions 3 and 6 of the carbazole core on the chloride transport activity of these anionophores. Using lucigenin assay and large unilamellar vesicles as models, the 3,6-dicyano- and 3,6-dinitro- substituted receptors were found to be highly active and perfectly deliverable chloride transporters, with EC50,270s value as low as 22 nM for the Cl-/NO3 - exchange. Mechanistic studies revealed that diamidocarbazoles form 1:1 complexes with chloride in lipid bilayers and facilitate chloride/nitrate exchange by carrier mechanism. Furthermore, owing to its increased acidity, the 3,6-dinitro- substituted receptor acts as a pH-switchable transporter, with physiologically relevant apparent pKa of 6.4.

Synthesis and biological activity of squaramido-tethered bisbenzimidazoles as synthetic anion transporters

A series of squaramido-tethered bisbenzimidazoles were synthesized from the reaction of diethyl squarate with substituted 2-aminomethylbenzimidazoles. These conjugates exhibit moderate binding affinity toward chloride anions. They are able to facilitate the transmembrane transport of chloride anions most probably via an anion exchange process, and tend to be more active at acidic pH than at physiological pH. The viability of these conjugates toward four selected solid tumor cell lines was evaluated using an MTT assay and the results suggest that some of these conjugates exhibit moderate cytotoxicity probably in an apoptotic fashion.

A series of squaramido-tethered bisbenzimidazoles were synthesized and found to exhibit moderate anion transport and cytotoxicity.

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.

Synthesis and biological effect of lysosome-targeting fluorescent anion transporters with enhanced anionophoric activity

Two lysosome-targeting fluorescent anion transporters derived from coumarins, trifluoromethylated arylsquaramides and morpholines were synthesized, and their specificity and efficiency to target and alkalize lysosomes were investigated. They are able to target lysosomes specifically. Compared with the previous analogue without trifluoromethyl substituents, these two conjugates, in particular the one having a 3,5-bis(trifluoromethyl) substituent, exhibit significantly higher ability to facilitate the transport of chloride anions, alkalize lysosomes and reduce the activity of lysosomal Cathepsin B enzyme. The present finding suggests that improving the anionophoric activity of lysosome-targeting fluorescent anion transporters is favorable to the efficiency to alkalize lysosomes and deactivate lysosomal Cathepsin B enzyme.

Anion Binding Affinity: Acidity versus Conformational Effects

High-level quantum chemical calculations were used to elucidate the gas- and solution-phase conformational equilibria for a series of symmetrically substituted (thio)ureas, (thio)squaramides, and croconamides. Gas-phase calculations predict that the thermodynamic conformer of many of these anion receptors is not the dual-hydrogen-bond-facilitating anti-anti conformer as is commonly assumed. For N,N'-diaryl thiosquaramides and croconamides, the syn-syn conformer is typically the predominant conformer. Solution-phase calculations show that the anti-anti conformer is increasingly stabilized as the polarity of the solvent increases. However, the syn-syn conformer remains the lowest energy conformation for croconamides. These predictions are used to explain the acidity versus chloride binding affinity correlations recently reported for some of these compounds. The chloride binding constants for thioureas and croconamides are significantly lower than expected on the basis of their pK_a values, and this may be due in part to the need for these receptors to reorganize into the anti-anti conformer. Experimental NMR nuclear Overhauser effect (NOE) measurements of an asymmetrically substituted squaramide and its thio analogue are consistent with the syn-syn conformation being predominant at 298 K. The conformational equilibria should therefore be an important consideration for the design and development of future anion receptors and organocatalysts.

pH-Regulated anion transport activities of bis(iminourea) derivatives across the cell and vesicle membrane

Recently, synthetic anion transporters have gained considerable attention because of their ability to disrupt cellular anion homeostasis and promote cell death. Herein, we report the development of bis(iminourea) derivatives as a new class of selective Cl- ion carrier. The bis(iminourea) derivatives were synthesized via a one-pot approach under mild reaction conditions. The presence of iminourea moieties suggests that the bis(iminourea) derivatives can be considered as unique guanidine mimics, indicating that the protonated framework could have much stronger anion recognition properties. The cooperative interactions of H+ and Cl- ions with these iminourea moieties results in the efficient transport of HCl across the lipid bilayer in an acidic environment. Under physiological conditions these compounds weakly transport Cl- ions via an antiport exchange mechanism. This pH-dependent gating/switching behavior (9-fold) within a narrow window could be due to the apparent pKa values (6.2-6.7) of the compounds within the lipid bilayer. The disruption of ionic homeostasis by the potent compounds was found to induce cell death.

Squaramide-Naphthalimide Conjugates as "Turn-On" Fluorescent Sensors for Bromide Through an Aggregation-Disaggregation Approach

The syntheses of two new squaramide-naphthalimide conjugates (SQ1 and SQ2) are reported where both compounds have been shown to act as selective fluorescence "turn on" probes for bromide in aqueous DMSO solution through a disaggregation induced response. SQ1 and SQ2 displayed a large degree of self-aggregation in aqueous solution that is disrupted at increased temperature as studied by ¹H NMR and Scanning Electron Microscopy (SEM). Moreover, the fluorescence behavior of both receptors was shown to be highly dependent upon the aggregation state and increasing temperature gave rise to a significant increase in fluorescence intensity. Moreover, this disaggregation induced emission (DIE) response was exploited for the selective recognition of certain halides, where the receptors gave rise to distinct responses related to the interaction of the various halide anions with the receptors. Addition of F^- rendered both compounds non-emissive; thought to be due to a deprotonation event while, surprisingly, Br^- resulted in a dramatic 500-600% fluorescence enhancement thought to be due to a disruption of compound aggregation and allowing the monomeric receptors to dominate in solution. Furthermore, optical sensing parameters such as limits of detection and binding constant of probes were also measured toward the various halides (F^-, Cl^-, Br^-, and I^-) where both SQ1 and SQ2 were found to sense halides with adequate sensitivity to measure μM levels of halide contamination. Finally, initial studies in a human cell line were also conducted where it was observed that both compounds are capable of being taken up by HeLa cells, exhibiting intracellular fluorescence as measured by both confocal microscopy and flow cytometry. Finally, using flow cytometry we were also able to show that cells treated with NaBr exhibited a demonstrable spectroscopic response when treated with either SQ1 or SQ2.

Synthesis and biological evaluation of aza-crown ether–squaramide conjugates as anion/cation symporters

Aim: Anion/cation symport across cellular membranes may lead to cell apoptosis and be developed as a strategy for new anticancer drug discovery. Methodology: Four aza-crown ether–squaramide conjugates were synthesized and characterized. Their anion recognition, anion/cation symport, cytotoxicity and probable mechanism of action were investigated in details. Conclusion: These conjugates are able to form ion-pairing complexes with chloride anions and facilitate the transmembrane transport of anions via an anion/cation symport process. They can disrupt the cellular homeostasis of chloride anions and sodium cations and induce the basification of acidic organelles in live cells. These conjugates exhibit moderate cytotoxicity toward the tested cancer cells and trigger cell apoptosis by mediating the influx of chloride anions and sodium cations into live cells.

Inhibition of autophagic flux by cyclometalated iridium(iii) complexes through anion transportation

We report two phosphorescent cyclometalated iridium(iii) complexes that can inhibit autophagic flux through anion transportation.

Synthetic anion transporters that can interfere with the intracellular pH homeostasis are gaining increasing attention for tumor therapy, however, the biological mechanism of anion transporters remains to be explored. In this work, two phosphorescent cyclometalated Ir(iii) complexes containing 2-phenylpyridine (ppy) as the cyclometalated ligand, and 2,2′-biimidazole (H₂biim, Ir1) or 2-(1H-imidazol-2-yl)pyridine (Hpyim, Ir2) as the ancillary ligands have been synthesized and characterized. Due to the protonation and deprotonation process of the N–H groups on H₂biim and Hpyim, Ir1 and Ir2 display pH-dependent phosphorescence and can specifically image lysosomes. Both Ir1 and Ir2 can act as anion transporters mainly through the anion exchange mechanism with higher potency observed for Ir1. Mechanism investigation shows that Ir1 and Ir2 can induce caspase-independent cell death through reactive oxygen species (ROS) elevation. As Ir1 and Ir2 can alkalinize lysosomes through anion disturbance, they can inhibit autophagic flux. Our work provides a novel anticancer mechanism of metal complexes, which gives insights into the innovative structure-based design of new metallo-anticancer agents.

New enantiopure binaphthyl-cinchona thiosquaramides: synthesis and application for enantioselective organocatalysis

Binaphthyl-cinchona squaramide and thiosquaramide were applied as organocatalysts in three types of asymmetric reactions with excellent yields and enantioselectivities.

Synthesis and properties of a lysosome-targeting fluorescent ionophore based on coumarins and squaramides

A squaramide derivative bearing a coumarin fluorophore and a morpholinyl group was synthesized and found to be able to target and efficiently deacidify lysosomes.

A ruthenium(iii) complex derived from N,N′-bis(salicylidene)ethylenediamine as a chemosensor for the selective recognition of acetate and its interaction with cells for bio-imaging: experimental and theoretical studies

A ruthenium(iii) complex ofN,N′-bis(salicylidene)ethylenediamine (L¹) was used as chemosensor for the recognition of acetate in cells for bio-imaging.

Synthesis of a dimeric 3α-hydroxy-7α,12α-diamino-5β-cholan-24-oate conjugate and its derivatives, and the effect of lipophilicity on their anion transport efficacy

A dimeric 3α-hydroxy-7α,12α-diamino-5β-cholan-24-oate conjugate and its derivatives were synthesized, and lipophilicity was found to significantly affect their anion transport efficacy.

Anion transport and supramolecular medicinal chemistry

New approaches to the transmembrane transport of anions are discussed in this review.

pH-Regulated Nonelectrogenic Anion Transport by Phenylthiosemicarbazones

Gated ion transport across biological membranes is an intrinsic process regulated by protein channels. Synthetic anion carriers (anionophores) have potential applications in biological research; however, previously reported examples are mostly nonspecific, capable of mediating both electrogenic and electroneutral (nonelectrogenic) transport processes. Here we show the transmembrane Cl(-) transport studies of synthetic phenylthiosemicarbazones mimicking the function of acid-sensing (proton-gated) ion channels. These anionophores have remarkable pH-switchable transport properties with up to 640-fold increase in transport efficacy on going from pH 7.2 to 4.0. This "gated" process is triggered by protonation of the imino nitrogen and concomitant conformational change of the anion-binding thiourea moiety from anti to syn. By using a combination of two cationophore-coupled transport assays, with either monensin or valinomycin, we have elucidated the fundamental transport mechanism of phenylthiosemicarbazones which is shown to be nonelectrogenic, inseparable H(+)/Cl(-) cotransport. This study demonstrates the first examples of pH-switchable nonelectrogenic anion transporters.

Membrane active cationic cholic acid-based molecular umbrellas

Herein, we report the synthesis of an umbrella thread and its covalent dimer and their transmembrane transport properties under physiological conditions.

Dual stimuli-induced formation of a μ-hydroxido bridged [Zn9L5(μ-OH)6]12+ half-pipe

Low-symmetry metal–organic architectures that feature unusual binding motifs are useful for exploring new modes of guest recognition.

Low-symmetry metal–organic architectures that feature unusual binding motifs are useful for exploring new modes of guest recognition. Such structures remain difficult to create using current rational design principles. One approach to constructing such architectures is to employ ligands with coordination vectors oriented to preclude the formation of simple, low nuclearity molecular assemblies upon complexation to metal ions. Here we report two new supramolecular assemblies generated from such a ligand: a simple metastable [Zn₃L₃]⁶⁺ assembly, which was observed to convert to a more complex [Zn₉L₅(μ-OH)₆]¹²⁺ twisted half-pipe architecture. Two chemically distinct stimuli—an anionic template and a base—must be applied for the conversion to occur. Perchlorate, perrhenate, trifluoromethanesulfonate and 2-naphthalenesulfonate were found to act as competent templates for the [Zn₉L₅(μ-OH)₆]¹²⁺ structure.

Does lipophilicity affect the effectiveness of a transmembrane anion transporter? Insight from squaramido-functionalized bis(choloyl) conjugates

Lipophilicity was found to have little effect on the effectiveness of squaramido-functionalized bis(choloyl) conjugates.