Receptors for sulfate that function across a wide pH range in mixed aqueous-DMSO media

Analytical methods for quantitating sulfate in plasma and serum

Circulating sulfate needs to be maintained at sufficiently high levels for healthy growth and development. Animal studies have shown the adverse physiological consequences of low circulating sulfate level on the skeletal, neurological and reproductive systems. However, sulfate is not routinely measured in clinical investigations, despite the importance of sulfate being documented over the past several decades. Several methods have been developed for measuring serum and plasma sulfate level in animals and humans, including a range of barium sulfate precipitation techniques that have been a major focus of sulfate analytics since the 1960s. Evaluation of an ion chromatography method demonstrated its utility for investigation of sulfate levels in human health. More recently, liquid chromatography-tandem mass spectrometry has been used to show hyposulfatemia in a human case of mild skeletal dysplasia. This article provides an overview of analytical methods for measuring sulfate in serum and plasma, highlighting the strengths and limitations of each method.

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.

A charge-neutral organic cage selectively binds strongly hydrated sulfate anions in water

In biological systems, enzymes and transport proteins can bind anions in aqueous media solely by forming hydrogen bonds with charge-neutral motifs. Reproducing this functionality in synthetic systems presents challenges and incurs high costs, particularly when targeting strongly hydrated anions such as sulfate. Here we report a [2.2.2]urea cryptand (cage), synthesized in one pot, that selectively binds sulfate in a mixture of dimethyl sulfoxide and water and in water with affinities in the micromolar to millimolar range. The neutral cage bearing six urea groups donates 12 strong hydrogen bonds to encapsulate a sulfate anion, showing favourable enthalpy even in pure water. Sulfate binding can be further enhanced by using micelles to provide a low-polarity microenvironment. The cage finds utility in analysing divalent anions in water and beverage samples or in removing sulfate. The work demonstrates the achievability of robust and selective anion binding in water with minimal synthetic efforts, by using neutral NH hydrogen bonds akin to those found in biology.

Anion-templated synthesis of a switchable fluorescent [2]catenane with sulfate sensing capability

Anion templation strategies have facilitated the synthesis of various catenane and rotaxane hosts capable of strong and selective binding of anions in competitive solvents. However, this approach has primarily relied on positively charged precursors, limiting the structural diversity and the range of potential applications of the anion-templated mechanically interlocked molecules. Here we demonstrate the synthesis of a rare electroneutral [2]catenane using a powerful, doubly charged sulfate template and a complementary diamidocarbazole-based hydrogen bonding precursor. Owing to the unique three-dimensional hydrogen bonding cavity and the embedded carbazole fluorophores, the resulting catenane receptor functions as a sensitive fluorescent turn-ON sensor for the highly hydrophilic sulfate, even in the presence of a large excess of water. Importantly, the [2]catenane exhibits enhanced binding affinity and selectivity for sulfate over its parent macrocycle and other acyclic diamidocarbazole-based receptors. We demonstrate also, for the first time, that the co-conformation of the catenane may be controlled by reversible acid/base induced protonation and deprotonation of the anionic template, SO42-. This approach pioneers a new strategy to induce molecular motion of interlocked components using switchable anionic templates.

A Cationic Catechol Derivative Binds Anions in Competitive Aqueous Media

A simple dihydroxy isoquinolinium molecule (3+ ) was prepared by a modification of a literature procedure. Interestingly, during optimisation of the synthesis a small amount of the natural product pseudopalmatine was isolated, and characterised for the first time by X-ray crystallography. Compound 3+ contains a catechol motif and positive charge on the same scaffold and was found to be a potent anion receptor, binding sulfate strongly in 8 : 2 d6 -acetone:D2 O and 7 : 3 d6 -acetone:D2 O (Ka >104 and 2,100 M-1 , respectively). Unsurprisingly, chloride binding was much weaker, even in the less polar solvent mixture 9 : 1 d6 -acetone:D2 O. The sulfate binding is remarkably strong for such a simple molecule, however anion binding studies were complicated by the tendency of the molecule to react with BPh4 - or BF4 - species during anion metathesis reactions. This gave two unusual zwitterions containing tetrahedral boronate centres, which were both characterised by X-ray crystallography.

A squaramide cage capable of binding and extracting H2PO4- and HP2O73- in highly polar protic media

A novel squaramide cage (2) binds H2PO4- and HP2O73- with high selectivity and affinity in a highly polar protic solvent system. Receptor 2 is also able to extract these hydrophilic anions into a chloroform phase from water. The X-ray crystal structure demonstrated that compound 2 forms a complex with H2PO4- with 1 : 1 stoichiometry in the solid state.

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.

Fluorescent sensing of non-steroidal anti-inflammatory drugs naproxen and ketoprofen by dansylated squaramide-based receptors

Bis-squaramide receptors L1-L4 bearing a dansyl moiety were synthesised and their potential applications as fluorescent probes towards non steroidal anti-inflammatory drugs naproxen and ketoprofen was investigated. A detailed photophysical characterization in CH₃CN/DMSO solution (9 : 1 v/v) was conducted and demonstrated that the two macrocyclic receptors L1 and L2 show good sensitivity towards ketoprofen with an ON-OFF fluorescent response, while the two open chain receptors L3 and L4 behave similarly with the three guests considered. DFT theoretical calculations carried out on L2 and L4 as model receptors allowed to propose a possible coordination mode towards the guests. Finally, ¹H-NMR spectroscopy in DMSO-d₆/0.5% water solution demonstrated that the four receptors interact with the considered guests via H-bonds.

Highly Selective Coextraction of H+/SO42- Using a Strapped Calix[4]pyrrole

We report on the synthesis of a cage-type calix[4]pyrrole (1) bearing an additional basic pyridinebisthiazolamine group on the strap. The receptor in its protonated form shows strong affinity and selectivity for sulfate over a wide range of inorganic anions. With receptor 1 as a liquid-liquid extractant, H⁺/SO₄^2- in the form of H₂SO₄ is almost quantitatively extracted from an aqueous solution containing HNO₃ at a high concentration to CH₂Cl₂ in a recyclable manner.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C_3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C₃ conformers of NH cryptands can be enlarged more remarkably than the C_3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C₃ conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F^- to I^- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

Selective Removal of Sulfate from Water by Precipitation with a Rigid Bis-amidinium Compound

A simple, readily prepared biphenyl bis-amidinium compound (1⋅Cl₂ ) is able to selectively precipitate sulfate from water. The precipitant is effective at concentrations as low as 1 mM and shows complete selectivity against monovalent anions, and high selectivity even against CO₃ ^2- and HPO₄ ^2- . It is highly effective (>90 % sulfate removed) in both seawater and highly acidic conditions relevant to mining waste-streams. X-ray crystallography reveals that 1⋅SO₄ forms a tightly packed, anhydrous, structure where each sulfate anion receives eight hydrogen bonds from amidinium N-H hydrogen bond donors.

Anchoring H-Bond Donating/Accepting Pyrrolic Derivatives on Preorganized Scaffolds: Conformationally Switchable Bipedal/Tripodal and Locked Molecular Cage Ionophores for Potentiometric Sensing of Phosphate and Fluoride

The ionophore properties of a myriad of conformationally switchable bipedal/tripodal receptors and locked molecular cages were evaluated here for the first time to fabricate potentiometric sensors for the determination of environmentally important phosphate and fluoride. Owing to the competent ionophore properties such as high binding selectivity and affinity, the developed ion-selective electrodes displayed response preference for phosphate and fluoride with a selectivity pattern that differs distinctly from traditional Hofmeister series. Binding constants of the ionophore-anion complexes are determined to underscore how modifications in the preorganization and H-bond donating/accepting ability of a given series of ionophores can be exploited to improve the performance for potentiometric sensing. While conformationally switchable bipedal/tripodal ionophores prefer tetrahedral oxyanions, locked molecular cages shift their preference to spherical halides gradually. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membrane electrodes in aqueous solutions within the concentration range of 10^-6.5-10^-2.0 M. Moreover, potentiometric determination of phosphate and fluoride in mineral water, soil, and tap water samples was achieved in a low μM concentration range with high accuracy, confirming their promising utility in real world applications.

Evaluating the Effects of Metal Adduction and Charge Isomerism on Ion-Mobility Measurements using m-Xylene Macrocycles as Models

Gas-phase ion-mobility spectrometry provides a unique platform to study the effect of mobile charge(s) or charge location on collisional cross section and ion separation. Here, we evaluate the effects of cation/anion adduction in a series of xylene and pyridyl macrocycles that contain ureas and thioureas. We explore how zinc binding led to unexpected deprotonation of the thiourea macrocyclic host in positive polarity ionization and subsequently how charge isomerism due to cation (zinc metal) and anion (chloride counterion) adduction or proton competition among acceptors can affect the measured collisional cross sections in helium and nitrogen buffer gases. Our approach uses synthetic chemistry to design macrocycle targets and a combination of ion-mobility spectrometry mass spectrometry experiments and quantum mechanics calculations to characterize their structural properties. We demonstrate that charge isomerism significantly improves ion-mobility resolution and allows for determination of the metal binding mechanism in metal-inclusion macrocyclic complexes. Additionally, charge isomers can be populated in molecules where individual protons are shared between acceptors. In these cases, interactions via drift gas collisions magnify the conformational differences. Finally, for the macrocyclic systems we report here, charge isomers are observed in both helium and nitrogen drift gases with similar resolution. The separation factor does not simply increase with increasing drift gas polarizability. Our study sheds light on important properties of charge isomerism and offers strategies to take advantage of this phenomenon in analytical separations.

Separation of Sulfate Anion From Aqueous Solution Governed by Recognition Chemistry: A Minireview

The sulfate anion (SO₄^2–) is known as an end metabolite of cysteine and methionine, and its proper concentration is associated with the expression of key functions in the physiological system. Thus, maintaining sulfate concentration at a precise level is of great significance for biology, environments, and industrial productions. Fundamental research for sulfate anion chemistry can help understand sulfate-associated physiological processes and related applications, for example, remediation. In this minireview, we summarized recent research progresses in sulfate recognition and separation using crystallization and liquid–liquid extraction. We focused on the studies wherein molecular recognition is the key element and is considered the driving force for selective sulfate separations from aqueous solution.

Bis[squaramido]ferrocenes as electrochemical sulfate receptors

The bis[squaramido]ferrocene scaffold is introduced and shown to selectively bind and electrochemically report sulfate in competitive water/DMSO mixtures.

Multiple Molecular Logic Gate Arrays in One System of (2-(2’-Pyridyl)imidazole)Ru(II) Complexes and Trimeric Cyclophanes in Water

Shape-switchable cyclophane hosts allow the controlled capture and release of reactive polypyridineRu(ii) complexes in water. This gives rise to a network of host–guest binding, acid–base reactions in ground and excited states, and chemical redox interconversions. In the case of (2-(2′-pyridyl)imidazole)Ru(ii) complexes, several molecular logic gate arrays of varying complexity emerge as a result. Cyclophane-induced ‘off–on’ switching of luminescence in neutral solution is found to originate from two features of these aromatic hosts: enhancement of radiative decay by the polarizable host and the suppression of nonradiative decay involving deprotonation by reducing the water content within the deep host cavity. These are examples of nanometric coordination chemistry/physics being controlled by inclusion in an open box. The aromatic units of the macrocycle are also responsible for the shape-switching mechanism of wall collapse/erection.

Transactions of a polypyridineRu(ii) complex with photons, protons and shape-switchable hosts show several types of molecular logic.

Novel Fluorescence Guanidine Molecules for Selective Sulfate Anion Detection in Water Complex Samples over a Wide pH Range

Quantitative analysis of sulfate anions in water still remains an important challenge for the society. Among all the methodologies, the most successful one is based on optical supramolecular receptors because the presence of small concentrations of sulfate anion modifies the photophysical properties of the receptor. In this case, fluorescence anion sensors have been designed by the incorporation of guanidine motifs into fluorenyl cores. The photophysical behaviors of the new mono- (M) and bis-guanidine (B) derivatives were studied through pH dependence, solvent effects, and ion sensing on steady-state spectra and time-resolved fluorescence spectroscopy. In more detail, the results demonstrate that M is a highly selective and sensitive sulfate ion receptor in real water samples and, even more importantly, its function remains unchanged at different ranges of pH. The reason behind this resides on the fluorescence quenching produced by an internal charge-transfer process when the sulfate anion is complexed with M. It is worth noting that the global and partial affinity constants (10¹⁰ M^-2 and 10⁵ M^-1, respectively) of complex formation are far above from the current sulfate sensors in water (10⁴ M^-1) which give an LOD of 0.10 μM in water with an analytical range of 2.5-10 μM. On the other hand, although it would seem, at first sight, that the B derivate will be the most promising one, the possibility of having two simultaneous protonation states reduces the complex formation and, therefore, its sensitivity to sulfate anions. The results presented here offer the possibility of using a new molecule in water environments, which opens the door to infinite applications such as the detection of trace amounts of sulfate ions in food or water.

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.

Tripodal, Squaramide-Based Ion Pair Receptor for Effective Extraction of Sulfate Salt

Combining three features-the high affinity of squaramides toward anions, cooperation in ion pair binding and preorganization of the binding domains in the tripodal platform-led to the effective receptor 2. The lack of at least one of these key elements in the structures of reference receptors 3 and 4 caused a lower affinity towards ion pairs. Receptor 2 was found to form an intramolecular network in wet chloroform, which changed into inorganic-organic associates after contact with ions and allowed salts to be extracted from an aqueous to an organic phase. The disparity in the binding mode of 2 with sulfates and with other monovalent anions led to the selective extraction of extremely hydrated sulfate anions in the presence of more lipophilic salts, thus overcoming the Hofmeister series.

Macrocyclic squaramides as ion pair receptors and fluorescent sensors selective towards sulfates

Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, an anion receptor R3, and a fluorescent sensor R4 using a combination of particular members of simple libraries consisting of synthesized diamines and methyl squarates, respectively. The receptors were investigated in terms of anion and ion pair binding using the ¹H NMR titration method in DMSO-d₆. We found that the major contribution to the anion binding comes from the interaction with the squaramide protons rather than with the amide functions of the receptors. The receptors demonstrated the highest affinity towards benzoates and sulfates over the anions tested, and in the case of sulfate binding more complex equilibria in solution were observed. Unlike the anion receptor R3, the ion pair receptor R1 was found to recognize anions in an enhanced manner with the assistance of sodium or potassium cations. Tethering of a simple fluorophore in close proximity to the amide function of receptor R4 resulted in an optical ion pair sensor selective towards sulfates. DFT calculations carried out for the 1 : 1 complexes of R3 with the anions helped clarify this selectivity, showing more effective participation of tetrahedral sulfate anions in binding with the amide function than in the case of benzoates or chlorides.

Hydrogen Bond-Based Macrocyclic and Tripodal Neutral Ionophores for Highly Selective Polymeric Membrane Sulfate-Selective Electrodes

Four hydrogen bond-based macrocyclic and tripodal neutral receptors with increasing conformational complementarity with sulfate were used for the first time as ionophores to develop polymeric membrane sulfate-selective electrodes. Optimizing the membrane composition such as ionophores, lipophilic additives, and plasticizers yielded ISEs which showed Nernstian response to sulfate with the best selectivity so far and improved detection limits (a slope of -29.8 mV/dec in the linear range of 1 × 10^-6-1 × 10^-1 M with a detection limit of 5 × 10^-7 M), which led to the success of the determination of sulfate in drinking water samples and neomycine tablets. The anion-ionophore complex constants in the membrane phase were determined and correlated with the selectivity sequence of the ISEs. Studies on the influence of pH of the sample solution demonstrated that the developed ISEs can be operated in a wide pH range of 3-8 with fast response and rapid (in 1 min) and long lifetime. The success of these ionophores represents a feasible strategy for overcoming the "Hofmeister series" by employing a combination of complementarity and hydrogen bonds.

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.

Extraction and transport of sulfate using macrocyclic squaramide receptors

Lipophilic macrocycles efficiently extract sulfate ions from water into chloroform and transport this ion across a bulk liquid membrane in the presence of competing anions (chloride, nitrate and dihydrogenphosphate).