Membrane-active calixarenes: toward ‘gating’ transmembrane anion transport

Substituent effects of fluorinated bambusurils on their anion transport

Anionophores are molecules that can transport ions across membranes. Several structural design criteria must be met for anionophores to be highly active. Fluorinated anionophores are usually more potent than their non-fluorinated analogues due to their higher lipophilicity and increased affinity for anions. Clear structure-activity relationships have been described for small and relatively simple anionophores. However, such studies are more challenging for large and macrocyclic anionophores, as their preparation is usually more complicated, limiting the number of compounds tested in anion transport studies. Here we present a series of twelve macrocyclic bambusuril anion transporters to investigate how variations in fluorinated substituents affect their transport properties. Measurements of Cl-/HCO3- antiport activities in liposomes revealed links between parameters such as lipophilicity or substituent polarity and transport activity. For some bambusurils, an unusually large effect of the presence of cholesterol in the membrane on transport activity was found. Further studies showed that for very potent anion receptors, such as the bambusurils described here, the binding selectivity towards anions becomes more important than the absolute binding affinity to anions when considering anion exchange across the membrane.

Enhancing Photothermal Energy Transduction through Inter- and Intramolecular Interactions of Multiple Two-Photon Dyes Appended onto Calix[4]arene

Organic dyes-based photothermal agents (OPTAs) have received increasing attention as alternative to inorganic materials due to their higher biocompatibility and extensive diversification. Maximizing nonradiative deexcitation channels is crucial to improve the photothermal conversion efficiency (PCE) of OPTAs. This is typically achieved through individual molecular design or collective enhancement using supramolecular strategies. Furthermore, photothermal therapy (PTT) generally relies on linear one-photon absorption of the light source by the OPTA, with less consideration given to nonlinear two-photon absorption (2PA) strategies, despite their potential benefits. Here, a synergistic strategy, which combines intramolecular and intermolecular quenching, is employed to maximize the photothermal efficiency of diphenylamino-substituted distyryl dicyanobenzene (DSB), an outstanding two-photon-absorbing chromophore. One to three DSB units have been introduced on the conic p-tert-butyl-calix[4]arene (CX), serving as a preorganizing platform to allow aggregate formation and promote intramolecular quenching within the multichromophoric systems. Importantly, the multichromophoric molecules had very high two-photon absorption capabilities with cross sections (δ2PA) reaching maximal values of 3290 GM at 810 nm. Experimental data accompanied by large-scale molecular dynamics simulations and time-dependent density functional theory calculations shed light onto the interaction mechanism in those multiple DSB-appended CX compounds to rationalize their optical properties. Then, the formulation with Pluronic F127 amphiphile yields water-dispersible nanoprecipitates (Nps), in which the PCE is further maximized and the photobleaching is reduced due to the combination of intra- and intermolecular quenching. The high two-photon absorption in the near-infrared (NIR) window associated with the high PCE of these nanosized OPTAs could serve as a basis to future in vivo 2P-PTT applications.

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.

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.

Protonophoric and mitochondrial uncoupling activity of aryl-carbamate substituted fatty acids

Aryl-urea substituted fatty acids are protonophores and mitochondrial uncouplers that utilise a urea-based synthetic anion transport moiety to carry out the protonophoric cycle. Herein we show that replacement of the urea group with carbamate, a functional group not previously reported to possess anion transport activity, produces analogues that retain the activity of their urea counterparts. Thus, the aryl-carbamate substituted fatty acids uncouple oxidative phosphorylation and inhibit ATP production by collapsing the mitochondrial proton gradient. Proton transport proceeds via self-assembly of the deprotonated aryl-carbamates into membrane permeable dimeric species, formed by intermolecular binding of the carboxylate group to the carbamate moiety. These results highlight the anion transport capacity of the carbamate functional group.

Calixarene-based artificial ionophores for chloride transport across natural liposomal bilayer: Synthesis, structure-function relationships, and computational study

An amphiphilic calix[6]arene, alone or complexed with an axle to form a pseudo-rotaxane, has been embedded into liposomes prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and the permeability of the membrane-doped liposomes towards Cl^- ions has been evaluated by using lucigenin as the fluorescent probe. The pseudo-rotaxane promotes transmembrane transport of Cl^- ions more than calix[6]arene does. Surprisingly, the quenching of lucigenin was very fast for liposomes doped with the positively charged axle alone. Molecular dynamics (MD) simulations and quantum-chemical calculations were also carried out for providing a semi-quantitative support to the experimental results.

Triamide macrocyclic chloride receptors via a one-pot tandem reduction–condensation–cyclization reaction

A chloride binding triamide macrocycle has been developed in one pot from the corresponding monomer via tandem reduction–condensation–cyclization reactions.

Oxacalix[2]arene[2]triazine based ion-pair transporters

Heteracalixaromatics are a new generation of macrocyclic hosts showing a unique structure and versatile recognition properties towards various guests.

Stereoelectronic effects: a simple yet powerful tool to manipulate anion affinity

Stereoelectronic effects on anion binding were examined, IR spectroscopy was used to probe structures, and a well aligned non-interacting group can be more significant than a hydrogen bond donor.

Chloride transport activities of trans- and cis-amide-linked bisureas

A stimuli-responsive synthetic chloride transporter has been devised based on the different transport abilities of bisurea compounds linked by cis- and trans-amides.

Light-driven directed proton transport across the liposomal membrane

We have developed a simple artificial photoresponsive ion-gating device by inserting molecular switches in the membrane of liposomes. A controlled and directed proton transport across the bilayer membrane can lower the internal pH of the liposomes from neutral to around 4 under combined light and chemical stimulation.

Advances in anion supramolecular chemistry: from recognition to chemical applications

Since the start of this millennium, remarkable progress in the binding and sensing of anions has been taking place, driven in part by discoveries in the use of hydrogen bonding, as well as the previously under-exploited anion-π interactions and halogen bonding. However, anion supramolecular chemistry has developed substantially beyond anion recognition, and now encompasses a diverse range of disciplines. Dramatic advance has been made in the anion-templated synthesis of macrocycles and interlocked molecular architectures, while the study of transmembrane anion transporters has flourished from almost nothing into a rapidly maturing field of research. The supramolecular chemistry of anions has also found real practical use in a variety of applications such as catalysis, ion extraction, and the use of anions as stimuli for responsive chemical systems.

Azobenzene-based chloride transporters with light-controllable activities

Azobenzene-based chloride transporters exhibit photoresponsive transport activities across lipid and plasma membranes.

Thiosquaramides: pH switchable anion transporters

The transport of anions across cellular membranes is an important biological function governed by specialised proteins. In recent years, many small molecules have emerged that mimick the anion transport behaviour of these proteins, but only a few of these synthetic molecules also display the gating/switching behaviour seen in biological systems. A small series of thiosquaramides was synthesised and their pH-dependent chloride binding and anion transport behaviour was investigated using ¹H NMR titrations, single crystal X-ray diffraction and a variety of vesicle-based techniques. Spectrophotometric titrations and DFT calculations revealed that the thiosquaramides are significantly more acidic than their oxosquaramide analogues, with pK_a values between 4.0 and 9.0. This led to the observation that at pH 7.2 the anion transport ability of the thiosquaramides is fully switched OFF due to deprotonation of the receptor, but is completely switched ON at lower pH.

Structure-activity relationships in tripodal transmembrane anion transporters: the effect of fluorination

A series of easy-to-make fluorinated tripodal anion transporters containing urea and thiourea groups have been prepared and their anion transport properties studied. Vesicle anion transport assays using ion-selective electrodes show that this class of compound is capable of transporting chloride through a lipid bilayer via a variety of mechanisms, including chloride/H(+) cotransport and chloride/nitrate, chloride/bicarbonate, and to a lesser extent an unusual chloride/sulfate antiport process. Calculations indicate that increasing the degree of fluorination of the tripodal transmembrane transporters increases the lipophilicity of the transporter and this is shown to be the major contributing factor in the superior transport activity of the fluorinated compounds, with a maximum transport rate achieved for clog P = 8. The most active transporter 5 contained a urea functionality appended with a 3,5-bis(trifluoromethyl)phenyl group and was able to mediate transmembrane chloride transport at receptor to lipid ratios as low as 1:250000. Proton NMR titration and single crystal X-ray diffraction revealed the ability of the tripodal receptors to bind different anions with varying affinities in a 1:1 or 2:1 stoichiometry in solution and in the solid state. We also provide evidence that the most potent anion transporters are able to induce apoptosis in human cancer cells by using a selection of in vitro viability and fluorescence assays.

A new photo-switchable "on-off" host-guest system

A new photo-switchable "on-off" host-guest system comprising cucurbit[7]uril (CB[7]) and a photoresponsive cinnamamide derivative (trans-(3-phenyl-acryloylamino)-acetic acid, E-1) is studied. The cinnamamide derivative and CB[7] forms a stable 1 : 1 host-guest complex (CB[7]·E-1) with a high binding constant (K = 2.1 × 10(4) M(-1)). Irradiation of UV light (300 nm) to an aqueous solution of CB[7]·E-1 induces the E- to Z-conformational change of the cinnamamide derivative, which then leads to the dissociation of the complex as evidenced by UV-visible and (1)H-NMR spectroscopy. The reverse process, photo-induced host-guest complex formation between CB[7] and Z-1 is achieved by irradiation of UV light at 254 nm. The photo-switchable "on-off" host-guest system shows high reversibility and switching efficiency, which makes it potentially useful in designing photoresponsive gating systems.

Recent synthetic transport systems

This critical review covers progress with synthetic transport systems, particularly ion channels and pores, between January 2006 and December 2009 in a comprehensive manner. This is the third part of a series launched in the year 2000, covering a rich collection of structural and functional motifs that should appeal to a broad audience of non-specialists, including to organic, biological, supramolecular and polymer chemists. Impressive breakthroughs have been achieved over the past four years in part because of a fruitful expansion toward new types of interactions, including metal-organic, π-π, aromatic electron donor-acceptor, anion-π or anion-macrodipole interactions as well as dynamic covalent bonds (169 references).