High-affinity anion binding by steroidal squaramide receptors

Anthracene Bisureas as Powerful and Accessible Anion Carriers

Synthetic anion carriers (anionophores) have potential as biomedical research tools and as treatments for conditions arising from defective natural transport systems (notably cystic fibrosis). Highly active anionophores that are readily accessible and easily deliverable are especially valuable. Previous work has resulted in steroid and trans-decalin based anionophores with exceptional activity for chloride/nitrate exchange in vesicles, but poor accessibility and deliverability. This work shows that anthracene 1,8-bisureas can fulfil all three criteria. In particular, a bis-nitrophenyl derivative is prepared in two steps from commercial starting materials, yet shows comparable transport activity to the best currently known. Moreover, unlike earlier highly active systems, it does not need to be preincorporated in test vesicles but can be introduced subsequent to vesicle formation. This transporter also shows the ability to transfer between vesicles, and is therefore uniquely effective for anion transport at low transporter loadings. The results suggest that anthracene bisureas are promising candidates for application in biological research and medicine.

Croconamides: a new dual hydrogen bond donating motif for anion recognition and organocatalysis

We introduce bis-aryl croconamides as a new member in the family of dual hydrogen bonding anion receptors. In this study a series of croconamides are synthesised, and the selectivity for anion binding is investigated (Cl^- > Br^- > I^- in CH₂Cl₂). The croconamides exhibit different structures in the crystal phase depending on the substituents on the aromatic rings, and furthermore, the crystal structure revealed the presence of tautomers. DFT calculations elucidated the complex structures formed upon addition of anion to the croconamides, confirming the order of association constants towards the halogen anions. The use of croconamides as organocatalysts in a proof-of-concept study is demonstrated in the formation of THP ethers. In addition to this, construction of a Hammet plot further elucidates the mechanism in action on formation of THP ethers.

H-Bonded anion–anion complex trapped in a squaramido-based receptor

We report the experimental observation (X-ray characterization) of an anion–anion complex (anion = hydrogen fumarate) stabilized by H-bonds that is trapped in a secondary squaramide receptor.

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.

An ion-pair receptor comprising urea groups and N-benzyl-aza-18-crown-6: effective recognition and liquid–liquid extraction of KCl salt

Heteroditopic ion-pair receptor that contains two urea groups and N-benzyl-18-crown-6 is shown to effectively recognizes and extracts KCl salt.

Miranda M, Busschaert N, Howe ENW, Clarke HJ, Haynes CJE, Kirby IL, Rodilla AM, Pérez-Tomás R, Gale PA, Félix V. Full elucidation of the transmembrane anion transport mechanism of squaramides using in silico investigations

The anion carrier mechanism promoted by squaramide-based molecules has been elucidated by molecular dynamics and chloride efflux studies.

Anion transport by ortho-phenylene bis-ureas across cell and vesicle membranes

These simple bis-ureas are found to be powerful anionophores in synthetic vesicles, and also in a live cell assay employing yellow fluorescent protein.

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.

Cyclo[4]carbazole, an Iodide Anion Macrocyclic Receptor

A novel preorganized and rigid iodide anion macrocyclic receptor, cyclo[4]carbazole (Cy[4]C), is reported here. The structure of Cy[4]C was confirmed by single-crystal X-ray analysis. The binding affinity of Cy[4]C for iodide anion was investigated by UV-vis and ¹H NMR spectroscopic techniques. The crystal structure of the complex between Cy[4]C and chloroform also provided evidence for the recognition ability of Cy[4]C toward iodide anion. Furthermore, the 1:1 complexation stoichiometry between Cy[4]C and iodide anion was confirmed by high-resolution mass spectrometry and molecular modeling.

Dipeptide-Based Carbohydrate Receptors and Polymers for Glycopeptide Enrichment and Glycan Discrimination

Glycoproteomics identifies and catalogs protein glycosylation and explores its impact on protein conformations and biofunctions. However, these studies are restricted by the bottleneck to enrich low-abundance glycopeptides from complex biosamples and the difficulties in analyzing glycan structures by mass spectrometry. Here, we report dipeptide as a simple but promising carbohydrate binding platform to tackle these problems. We build a hydropathy-index-based strategy for sequence optimization and screen out three optimal dipeptide sequences from 54 types of dipeptides. The optimized dipeptide-based homopolymers display excellent performance (e.g., selectivity up to ∼70% for real biosamples and strong anti-interference capacity capable of resisting 1000-fold bovine serum albumin interference) in glycopeptide enrichment. Meanwhile, our polymers exhibit high-efficiency chromatographic separation toward oligosaccharides with different compositions, polymerization degrees and even their linkage isomers. This brings another attractive feature that our materials can discriminate subtly variable glycan structures of glycopeptides, especially, isomeric glycosidic linkages. These features provide a solid foundation to analyze the complex glycan structures and glycosites simultaneously, which will benefit future development of glycoproteomics and glycobiology.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Tilting and Tumbling in Transmembrane Anion Carriers: Activity Tuning through n-Alkyl Substitution

Anion transport by synthetic carriers (anionophores) holds promise for medical applications, especially the treatment of cystic fibrosis. Among the factors which determine carrier activity, the size and disposition of alkyl groups is proving remarkably important. Herein we describe a series of dithioureidodecalin anionophores, in which alkyl substituents on one face are varied from C0 to C10 in two-carbon steps. Activities increase then decrease as the chain length grows, peaking quite sharply at C6 . Molecular dynamics simulations showed the transporter chloride complexes releasing chloride as they approach the membrane-aqueous interface. The free transporter then stays at the interface, adopting an orientation that depends on the alkyl substituent. If chloride release is prevented, the complex is positioned similarly. Longer chains tilt the binding site away from the interface, potentially freeing the transporter or complex to move through the membrane. However, chains which are too long can also slow transport by inhibiting movement, and especially reorientation, within the phospholipid bilayer.

Anion-Exchange Properties of Trifluoroacetate and Triflate Salts of N-Alkylammonium Resorcinarenes

The synthesis of N-benzyl- and N-cyclohexylammonium resorcinarene trifluoroacetate (TFA) and triflate (OTf) salt receptors was investigated. Solid-state analysis by single-crystal X-ray diffraction revealed that the N-alkylammonium resorcinarene salts (NARSs) with different upper substituents had different cavity sizes and different affinities for anions. Anion-exchange experiments by mixing equimolar amounts of N-benzylammonium resorcinarene trifluoroacetate and N-cyclohexylammonium resorcinarene triflate, as well as N-benzylammonium resorcinarene triflate and N-cyclohexylammonium resorcinarene trifluoroacetate showed that the NARS with flexible benzyl groups preferred the larger OTf anion, whereas the rigid cyclohexyl groups preferred the smaller TFA anions. The anion-exchange processes were confirmed in the solid state by single-crystal and powder X-ray diffraction experiments and in the gas phase by electrospray ionization mass spectrometry.

Highly effective ion-pair receptors based on 2,2-bis(aminomethyl)-propionic acid

Working in concert cation and anion binding domains of ditopic receptors based on 2,2-bis-(aminomethyl)-propionic acid allow for highly effective complexation of sodium salts of chloride, bromide and nitrate.

Reversible modulation of the activity of thiourea catalysts with anions: a simple approach to switchable asymmetric catalysis

A simple and straightforward approach to switchable asymmetric catalysis is presented, based on the interactions of thiourea catalysts with anions.

A hybrid macrocyclic anion receptor exploiting the pyrrole-2,5-diacetamide unit

Macrocyclic compound built on pyrrole-2,5-diacetic bisamide was found to be an efficient receptor for H₂PO₄⁻ and PhCOO⁻.

Tetrahedral cage complex with planar vertices: selective synthesis of Pt4L6 cage complexes involving hydrogen bonds driven by halide binding

The first synthesis of tetrahedral cage complexes with four square-planar Pt complexes at the vertex positions was achieved.

AuICl-bound N-heterocyclic carbene ligands form MII4(LAuCl)6 integrally gilded cages

The incorporation of an N-heterocyclic carbene (NHC) moiety into a self-assembled MII4L6 cage framework required the NHC first to be metallated with gold(i). Bimetallic cages could then be constructed using zinc(ii) and cadmium(ii) templates, showing weak luminescence. The cages were destroyed by the addition of further gold(i) in the form of AuI(2,4,6-trimethoxybenzonitrile)2SbF6, which caused the reversibly-formed cages to disassemble and controllably release the AuI-NHC subcomponent into solution. This release in turn induced the growth of gold nanoparticles. The rate of dianiline release could be tuned by capsule design or through the addition of chemical stimuli, with different release profiles giving rise to different nanoparticle morphologies.

Efficient, non-toxic anion transport by synthetic carriers in cells and epithelia

Transmembrane anion transporters (anionophores) have potential for new modes of biological activity, including therapeutic applications. In particular they might replace the activity of defective anion channels in conditions such as cystic fibrosis. However, data on the biological effects of anionophores are scarce, and it remains uncertain whether such molecules are fundamentally toxic. Here, we report a biological study of an extensive series of powerful anion carriers. Fifteen anionophores were assayed in single cells by monitoring anion transport in real time through fluorescence emission from halide-sensitive yellow fluorescent protein. A bis-(p-nitrophenyl)ureidodecalin shows especially promising activity, including deliverability, potency and persistence. Electrophysiological tests show strong effects in epithelia, close to those of natural anion channels. Toxicity assays yield negative results in three cell lines, suggesting that promotion of anion transport may not be deleterious to cells. We therefore conclude that synthetic anion carriers are realistic candidates for further investigation as treatments for cystic fibrosis.

pH switchable anion transport by an oxothiosquaramide

An oxothiosquaramide was shown to bind to chloride through hydrogen bonding interactions in DMSO and found to exhibit pH switchable choride transport across phospholipid bilayers via an antiport transport mechanism.

Dimeric resorcinarene salt capsules with very tight encapsulation of anions and guest molecules

The N-cyclohexyl ammonium resorcinarene triflate forms two structurally different capsules with cavity volumes of 341 and 679 ų encapsulating either four or eight guest molecules, with very high packing coefficients of 0.74 and 0.67.

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.