Anion binding of a neutral bis(cyclopeptide) in water-methanol mixtures containing up to 95% water

Anion-exchange facilitated selective extraction of sulfate and phosphate by overcoming the Hofmeister bias

Selective recognition and removal of sulfate and phosphates from aqueous media in the presence of highly competing anions is very demanding because of their biological and environmental implications. In this paper, we present the anion recognition approach for the selective and efficient extraction of sulfate by nitrophenyl-functionalized tris-urea receptors (L₁-L₂) from highly competitive aqueous media with an equivalent concentration of nitrate and other anions. Tetrabutylammonium hydroxide has been used for the first time as a phase transfer anionic extractant for sulfate-exchange from the aqueous phase to the organic phase (dichloromethane) containing a tris-urea receptor (L₁-L₃). The sulfate extraction efficacy of L₂ (≈84-90%) was observed to be higher than those of L₁ (≈76-82%) and L₃ (≈68-75%) in competitive extraction experiments. In contrast, an analogous nitrophenyl-functionalized tris-thiourea receptor (L₄) has been recognized for the selective and efficient extraction of phosphates from aqueous media in the presence of several competing anions including sulfate and nitrate, with ≈85-92% extraction efficiency. In this case, tetrabutylammonium acetate has been used as a phase transfer anionic extractant for phosphate exchange between the two immiscible phases. Due to the higher acidity of tris-thiourea -NH groups in comparison to the analogous tris-urea, tetrabutylammonium hydroxide could deprotonate a hydrogen bond donating -NH group of the thiourea receptor and phosphate extraction was observed to be inefficient in such a case. Several liquid-liquid extraction (LLE) experiments have been carried out to establish the selective removal of sulfate and phosphates by the tripodal receptors from competitive aqueous media having different combinations of two or more anions. The LLE products obtained from organic phases were characterized by NMR (¹H, ¹³C, ³¹P, and ¹⁹F) spectroscopy to affirm the oxoanion selectivity of the receptors and purity of the complexes. The tripodal receptors can easily be recycled for successive extraction cycles by simply washing the LLE products (oxoanion complexes) with a methanol-water (1 : 1, v/v) solvent system followed by filtration.

Engineered Binding Microenvironments in Halogen Bonding Polymers for Enhanced Anion Sensing

Mimicking Nature's polymeric protein architectures by designing hosts with binding cavities screened from bulk solvent is a promising approach to achieving anion recognition in competitive media. Accomplishing this, however, can be synthetically demanding. Herein we present a synthetically tractable approach, by directly incorporating potent supramolecular anion-receptive motifs into a polymeric scaffold, tuneable through a judicious selection of the co-monomer. A comprehensive analysis of anion recognition and sensing is demonstrated with redox-active, halogen bonding polymeric hosts. Notably, the polymeric hosts consistently outperform their monomeric analogues, with especially large halide binding enhancements of ca. 50-fold observed in aqueous-organic solvent mixtures. These binding enhancements are rationalised by the generation and presentation of low dielectric constant binding microenvironments from which there is appreciable solvent exclusion.

Characterizing the Properties of Anion-Binding Bis(cyclopeptides) with Solvent-Independent Energy Increments

The binding energies of 121 complexes between anions and bis(cyclopeptides) differing in the structure and the number of linking units between the two cyclopeptide rings were analyzed. These Gibbs free energies were obtained in earlier work for different anions, under different conditions, and with different methods. The multiparametric analysis of a subset of 42 binding energies afforded linear relationships that allowed the relatively reliable estimation of the iodide and sulfate affinity of three structurally related bis(cyclopeptides) in water/methanol and water/acetonitrile mixtures at different solvent compositions. Three parameters were required to achieve a satisfactory correlation, namely, the Gibbs free energy of transferring the respective anion from water into the solvent mixture in which complex stability was determined, and the Kamlet–Taft parameters α and β. Based on these relationships, the anion affinities of the other bis(cyclopeptides) were evaluated, giving rise to a set of energy increments that allow quantifying the effects of the linker structure or the nature of the anion on binding affinity relative to the reference system.

Synthetic Receptors Based on Abiotic Cyclo(pseudo)peptides

Work on the use of cyclic peptides or pseudopeptides as synthetic receptors started even before the field of supramolecular chemistry was firmly established. Research initially focused on the development of synthetic ionophores and involved the use of macrocycles with a repeating sequence of subunits along the ring to facilitate the correlation between structure, conformation, and binding properties. Later, nonnatural amino acids as building blocks were also considered. With growing research in this area, cyclopeptides and related macrocycles developed into an important and structurally diverse receptor family. This review provides an overview of these developments, starting from the early years. The presented systems are classified according to characteristic structural elements present along the ring. Wherever possible, structural aspects are correlated with binding properties to illustrate how natural or nonnatural amino acids affect binding properties.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Revealing the Hidden Costs of Organization in Host-Guest Chemistry Using Chloride-Binding Foldamers and Their Solvent Dependence

Preorganization is a key concept in supramolecular chemistry. Preorganized receptors enhance binding by minimizing the organization costs associated with adopting the conformation needed to orient the binding sites toward the guest. Conversely, poorly organized receptors show affinities below what is possible based on the potential of their specific binding interactions. Despite the fact that the organization energy is paid each time like a tax, its value has never been measured directly, though many compounds have been developed to measure its effects. We present a method to quantify the hidden costs of receptor organization by independently measuring the contribution it makes to chloride complexation by a flexible foldameric receptor. This method uses folding energy to approximate organization energy and relies on measurement of the coil-helix equilibrium as a function of solvent. We also rely on the finding, established with rigid receptors, that affinity is inversely related to the solvent dielectric and expect the same for the foldamer's helically organized state. Increasing solvent polarity across nine dichloromethane-acetonitrile mixtures we see an unusual V-shape in affinity (decrease then increase). Quantitatively, this shape arises from weakened hydrogen-bonding interactions with solvent polarity followed by solvent-driven folding into an organized helix. We confirm that dielectric screening impacts the stability of host-guest complexes of flexible foldamers just like rigid receptors. These results experimentally verify the canonical model of binding (affinity depends on the sum of organization and noncovalent interactions). The picture of how solvent impacts complex stability and conformational organization thereby helps lay the groundwork for de novo receptor design.

Structure directing roles of weak noncovalent interactions and charge-assisted hydrogen bonds in the self-assembly of solvated podands: Example of an anion-assisted dimeric water capsule

Crystal structures of two new podand molecules (1 and 2) synthesized from 1,3,5-tris(bromomethyl)mesitylene and two bromide salts of tris(4-amino-N-ethylbenzamide)amine (3) were elucidated to witness the structure directing roles of weak...

Urea-Based Imprinted Polymer Hosts with Switchable Anion Preference

The design of artificial oxyanion receptors with switchable ion preference is a challenging goal in host–guest chemistry. We here report on molecularly imprinted polymers (MIPs) with an external phospho-sulpho switch driven by small molecule modifiers. The polymers were prepared by hydrogen bond-mediated imprinting of the mono- or dianions of phenyl phosphonic acid (PPA), phenyl sulfonic acid (PSA), and benzoic acid (BA) using N-3,5-bis-(trifluoromethyl)-phenyl-Ń-4-vinylphenyl urea (1) as the functional host monomer. The interaction mode between the functional monomer and the monoanions was elucidated by ¹H NMR titrations and ¹H–¹H NMR NOESY supported by molecular dynamic simulation, which confirmed the presence of high-order complexes. PPA imprinted polymers bound PPA with an equilibrium constant K_eq = 1.8 × 10⁵ M^–1 in acetonitrile (0.1% 1,2,2,6,6-pentamethylpiperidine) and inorganic HPO₄^2– and SO₄^2– with K_eq = 2.9 × 10³ M^–1 and 4.5 × 10³ M^–1, respectively, in aqueous buffer. Moreover, the chromatographic retentivity of phosphonate versus sulfonate was shown to be completely switched on this polymer when changing from a basic to an acidic modifier. Mechanistic insights into this system were obtained from kinetic investigations and DSC-, MALDI-TOF-MS-, ¹H NMR-studies of linear polymers prepared in the presence of template. The results suggest the formation of template induced 1–1 diad repeats in the polymer main chain shedding unique light on the relative contributions of configurational and conformational imprinting.

Phase-transfer extraction for the fast quantification of perchlorate anions in water

Supramolecular approaches for the quantitative anion analysis in water remain scarce due to the lack of receptors that effectively bind anions in this medium. Herein, we present a novel, fast and easy, supramolecular approach for a selective and quantitative analysis of perchlorate anions in water, coupling the UV-Vis spectroscopic method and phase-transfer extraction of anions by a water-insoluble anion receptor.

A fluoride selective water-soluble anion receptor based on a 1,2-phenylenediacetic acid and calcium ion dimer

The dimeric receptor 1 from 1,2-phenylenediacetic acid and calcium ions recognized fluoride ions almost exclusively in 100% water.

Selective turn-on fluorescence sensing of sulfate in aqueous-organic mixtures by an uncharged bis(diamidocarbazole) receptor

A linear, uncharged, hydrogen bonding receptor A with two carbazole-based binding domains was synthesised and evaluated for its anion binding properties in DMSO/H₂O mixtures. ¹H NMR titrations revealed that, in DMSO/H₂O 0.5%, A forms both 1 : 1 and 1 : 2 complexes with SO₄^2-, H₂PO₄^-, PhCOO^- and Cl^-. In 1 : 1 complexes the receptor encloses the tetrahedral anions tightly, forming a helical structure, while Cl^- binds with a single carbazole unit only. In the presence of 10% of water the 1 : 2 complexes with SO₄^2- and PhCOO^- disappear, and the respective 1 : 1 binding constants decrease sufficiently to be quantified by UV-Vis titration. In this highly competitive medium, A binds sulfate with K_1:1 = 10^5.47 M^-1, i.e., it binds approx. 30, 360 and >1000 times more strongly than H₂PO₄^-, PhCOO^- and Cl^-, respectively. Furthermore, the association with sulfate is over 50 times stronger than that for a model diamidocarbazole 1 under identical conditions, suggesting a very strong chelating effect due to the diglycoyl linker. Increasing the amount of water to 25% (the solubility limit of A) lowers the 1 : 1 binding constant with SO₄^2- to 10^3.73 M^-1. Receptor A was shown to act as a selective turn-on fluorescent sensor for sulfate, able to sense sulfate in sulfate-rich mineral water.

Straightforward Design of Fluorescent Receptors for Sulfate: Study of Non-Covalent Interactions Contributing to Host-Guest Formation

A straightforward design of receptors for binding and sensing of sulfate in aqueous medium was developed. The design involves the connection of two naphthalimide-based pH probes through a hydrogen-bonding motif. The structure of the receptor-sulfate complex, predicted by DFT calculations, was unambiguously confirmed by NMR measurements. There are three major interactions stabilizing the host-guest complex: electrostatic interactions, hydrogen bonding, and stacking interactions of the dyes. Study of two control receptors containing either one dye or methyl amide groups instead of amides, revealed that electrostatic and hydrogen bonding interactions contribute the most to affinity and selectivity of receptors. The receptors can detect sulfate in a 1:1 THF-buffer mixture in pH window 3.6-4.5 demonstrating up to 7-fold fluorescence enhancement. To the best of our knowledge, the reported PET (photoinduced electron transfer) anion probes possess the largest response for sulfate in aqueous solution yet described.

Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors

Anion receptors often rely on coordinative or multiple ionic interactions to be active in water. In the absence of such strong interactions, anion binding in water can also be efficient, however, as demonstrated by a number of anion receptors developed in recent years. The cyclopeptide-derived receptors comprising an alternating sequence of l-proline and 6-aminopicolinic acid subunits are an example. These cyclopeptides are neutral and, at first sight, can only engage in hydrogen-bond formation with an anionic substrate. Nevertheless, they even interact with strongly solvated sulfate anions in water. The intrinsic anion affinity of these cyclopeptides can be related to structural aspects of their highly preorganized concave binding site, which comprises a wall of hydrophobic proline units arranged around the peptide NH groups at the cavity base. When anions are incorporated into this cavity they can engage in hydrogen-bonding interactions to the NH groups, and complex formation also benefits from cavity dehydration. Formation of 1:1 complexes, in which an anion binds to a single cyclopeptide ring, is associated with only small stability constants, however, whereas significantly more stable complexes are formed if the anion is buried between two cyclopeptide molecules. A major contribution to the formation of these sandwich complexes derives from the addition of the second ring to the initially formed 1:1 cyclopeptide-anion complex. This step brings the apolar proline residues of both cyclopeptides in close proximity, which causes the resulting structure to be stabilized to a large extent by hydrophobic effects. Solvent dependent binding studies provided an estimate to which degree these solvent effects contribute to the overall complex stability. In these studies, bis(cyclopeptides) were used, featuring two cyclopeptide rings covalently connected via linkers that enable both rings to simultaneously interact with the anion. Bis(cyclopeptides) with additional solubilizing groups allowed binding studies in a wide range of solvents, including in water. The systematic analysis of the solvent dependence of anion affinity yielded a quantitative correlation between complex stability and parameters relating to the solvation of the anions and solvent properties, confirming that solvent effects contribute to anion binding. Interestingly, the thermodynamic signature of complex formation in water mirrors that of sulfate binding to a protein complex but is opposite to that of other recently described anion receptors, which also do not engage in ionic or coordinative interactions with the substrate. These receptors not only differ in terms of the thermodynamics of binding from the cyclopeptides but also possess a characteristically different anion selectivity in that they prefer to bind weakly coordinating anions but fail to bind sulfate. Solvent effects likely control the anion binding of both receptors types but their impact on complex formation and anion selectivity seems to be profoundly different. Future work in the area of anion coordination chemistry will benefit from the deeper understanding of these effects and how they can be controlled.

Effects of Solvent Properties on the Anion Binding of Neutral Water-Soluble Bis(cyclopeptides) in Water and Aqueous Solvent Mixtures

In this study, the anion-binding bis(cyclopeptide) 2 is introduced, which dissolves freely in water, affording up to 10 mM concentrations, thanks to triethylene glycol-derived substituents in the cyclopeptide subunits and the linker connecting them. Binding studies provided evidence that the anion affinity previously demonstrated for less-soluble analogs of this compound is retained under highly competitive aqueous conditions. The highest affinity in water was observed for iodide, closely followed by sulfate anions, whereas binding of soft and weakly coordinating anions could not be observed. The anion selectivity of 2 thus differs from that of other recently described receptors, which also do not require electrostatic or coordinative interactions for anion binding in water but typically fail to bind strongly coordinating sulfate anions. The ability of 2 to overcome sulfate hydration is attributed to the special mode of binding, combining direct N-H···A- interactions with the release of water molecules from the receptor cavity. The characterization of the anion binding of 2 and a related bis(cyclopeptide) in a variety of different solvents and aqueous solvent mixtures furthermore allowed the correlation of the binding properties with solvent parameters. These analyses provided qualitative and even quantitative insights into the solvent properties and solvation phenomena that mainly affect anion complexation.

Chiral hemicucurbit[8]uril as an anion receptor: selectivity to size, shape and charge distribution

A novel eight-membered macrocycle of the hemicucurbit[n]uril family, chiral (all-R)-cyclohexanohemicucurbit[8]uril (cycHC[8]) ‡The name cyclohexylhemicucurbituril, previously used for these macrocycles, is changed in accordance with the IUPAC nomenclature for fused cycles, as the cyclohexane substituents are fused with the parent hemicucurbituril. binds anions in a purely protic solvent with remarkable selectivity. The cycHC[8] portals open and close to fully encapsulate anions in a 1 : 1 ratio, resembling a molecular Pac-Man™. Comprehensive gas, solution and solid phase studies prove that the binding is governed by the size, shape and charge distribution of the bound anion. Gas phase studies show an order of SbF6- ≈ PF6- > ReO4- > ClO4- > SCN- > BF4- > HSO4- > CF3SO3- for anion complexation strength. An extensive crystallographic study reveals the preferred orientations of the anions within the octahedral cavity of cycHC[8] and highlights the importance of the size- and shape-matching between the anion and the receptor cavity. The solution studies show the strongest binding of the ideally fitting SbF6- anion, with an association constant of 2.5 × 105 M-1 in pure methanol. The symmetric, receptor cavity-matching charge distribution of the anions results in drastically stronger binding than in the case of anions with asymmetric charge distribution. Isothermal titration calorimetry (ITC) reveals the complexation to be exothermic and enthalpy-driven. The DFT calculations and VT-NMR studies confirmed that the complexation proceeds through a pre-complex formation while the exchange of methanol solvent with the anion is the rate-limiting step. The octameric cycHC[8] offers a unique example of template-controlled design of an electroneutral host for binding large anions in a competitive polar solvent.

Catching the chloride: searching for non-Hofmeister selectivity behavior in systematically varied polyamide macrocyclic receptors

Searching for regularities in the large set of structurally diverse macrocyclic probes allowed us to determine the structural requirements for the selective recognition of chloride over more basic anions such as H₂PO₄⁻ or RCO₂⁻ by a putative anion receptor.

Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies, Sulfate Extraction from Water, and Anion Exchange with Carbonate

A series of 9 homologous sulfate-incarcerating nanojars [SO₄⊂{Cu(OH)(pz)}_n]^2- (Cu_n; n = 27-33; pz = pyrazolate), based on combinations of three [Cu(OH)(pz)]_x rings (x = 6-14, except 11)-namely, 6 + 12 + 9 (Cu₂₇), 6 + 12 + 10 (Cu₂₈), 8 + 13 + 8 (Cu₂₉), 7 + 13 + 9 (Cu₂₉), 8 + 14 + 8 (Cu₃₀), 7 + 14 + 9 (Cu₃₀), 8 + 14 + 9 (Cu₃₁), 8 + 14 + 10 (Cu₃₂), and 9 + 14 + 10 (Cu₃₃)-has been obtained and characterized by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature ¹H NMR spectroscopy, and thermogravimetry. The X-ray crystal structure of Cu₂₉ (8 + 13 + 8) is described. Cu₃₂ and Cu₃₃, which are the largest nanojars in this series, are observed for the first time. Despite extensive overlap at a given temperature, monitoring the temperature-dependent variation of paramagnetically shifted pyrazole and OH proton signals in 60 different ¹H NMR spectra over a temperature range of 25-150 °C and a chemical shift range from 41 ppm to -59 ppm permits the assignment of individual protons in six different sulfate nanojars in a mixture. As opposed to ESI-MS, which only provides the size of nanojars, ¹H NMR offers additional information about their detailed composition. Thus, nanojars such as Cu₂₉ (8 + 13 + 8) and Cu₂₉ (7 + 13 + 9) can easily be differentiated in solution. High-temperature solution studies unveil a significant difference in the thermal stability of nanojars of different sizes obtained under kinetic control at ambient temperature, and aid in predicting the structure of the Cu₃₃ nanojar, as well as in explaining the absence of the Cu₁₁ ring from the Cu₆-Cu₁₄ series. Anion exchange studies using sulfate and carbonate reveal that, although each anion is thermodynamically preferred by a nanojar of a certain size, the exchange of an already incarcerated anion is hampered by a substantial kinetic barrier. The remarkably strong binding of anions by nanojars allows for the extraction of highly hydrophilic anions, such as sulfate and carbonate, from water into organic solvents, despite their very large hydration energies.

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.

Macrocyclic squaramides: anion receptors with high sulfate binding affinity and selectivity in aqueous media

A number of macrocyclic squaramide-containing receptors (MSQs) have been designed and synthesised and their interaction with a range of inorganic anions was studied in solution by ¹H NMR spectroscopy and ESI-HRMS. The binding data revealed remarkable binding of sulfate in aqueous mixtures from 0.5 to 50% v/v H₂O/DMSO-d₆. The larger [3]-MSQs were found to better match the size and shape of the sulfate ion than the [2]-MSQs, providing high affinity and selectivity for sulfate while other tetrahedral divalent anions such as selenate, phosphate species and chromate have substantially lower binding affinities. In mixtures of anions mimicking the composition of either nuclear waste or plasma, the [3]-MSQs were still able to bind sulfate ions with high affinity.

Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective

The recognition of anions in water remains a key challenge in modern supramolecular chemistry, and is essential if proposed applications in biological, medical, and environmental arenas that typically require aqueous conditions are to be achieved. However, synthetic anion receptors that operate in water have, in general, been the exception rather than the norm to date. Nevertheless, a significant step change towards routinely conducting anion recognition in water has been achieved in the past few years, and this Review highlights these approaches, with particular focus on controlling and using the hydrophobic effect, as well as more exotic interactions such as C-H hydrogen bonding and halogen bonding. We also look beyond the field of small-molecule recognition into the macromolecular domain, covering recent advances in anion recognition based on biomolecules, polymers, and nanoparticles.

Iodide Recognition and Sensing in Water by a Halogen-Bonding Ruthenium(II)-Based Rotaxane

The synthesis and anion-recognition properties of the first halogen-bonding rotaxane host to sense anions in water is described. The rotaxane features a halogen-bonding axle component, which is stoppered with water-solubilizing permethylated β-cyclodextrin motifs, and a luminescent tris(bipyridine)ruthenium(II)-based macrocycle component. (1) H NMR anion-binding titrations in D2 O reveal the halogen-bonding rotaxane to bind iodide with high affinity and with selectively over the smaller halide anions and sulfate. The binding affinity trend was explained through molecular dynamics simulations and free-energy calculations. Photo-physical investigations demonstrate the ability of the interlocked halogen-bonding host to sense iodide in water, through enhancement of the macrocycle component's Ru(II) metal-ligand charge transfer (MLCT) emission.

Co-solvation effect on the binding mode of the α-mangostin/β-cyclodextrin inclusion complex

Cyclodextrins (CDs) have been extensively utilized as host molecules to enhance the solubility, stability and bioavailability of hydrophobic drug molecules through the formation of inclusion complexes. It was previously reported that the use of co-solvents in such studies may result in ternary (host:guest:co-solvent) complex formation. The objective of this work was to investigate the effect of ethanol as a co-solvent on the inclusion complex formation between α-mangostin (α-MGS) and β-CD, using both experimental and theoretical studies. Experimental phase-solubility studies were carried out in order to assess complex formation, with the mechanism of association being probed using a mathematical model. It was found that α-MGS was poorly soluble at low ethanol concentrations (0–10% v/v), but higher concentrations (10–40% v/v) resulted in better α-MGS solubility at all β-CD concentrations studied (0–10 mM). From the equilibrium constant calculation, the inclusion complex is still a binary complex (1:1), even in the presence of ethanol. The results from our theoretical study confirm that the binding mode is binary complex and the presence of ethanol as co-solvent enhances the solubility of α-MGS with some effects on the binding affinity with β-CD, depending on the concentration employed.

Anion recognition in water by a rotaxane containing a secondary rim functionalised cyclodextrin stoppered axle

The synthesis of a water soluble [2]rotaxane is reported using hydrophilic secondary rim functionalised permethylated β-cyclodextrin derivatives as the axle stopper groups. The rotaxane recognises halide anions in pure water with impressive selectivity over sulfate.

Real-time analysis of multiple anion mixtures in aqueous media using a single receptor

Bambusuril macrocycles allow qualitative and quantitative sensing of anion mixtures in water using ¹H NMR spectroscopy.