Anion Recognition Research Articles

Recognition of halide anions by N,N’-bis(4-nitrophenyl)urea in the gas-phase and solution: a computational study

ABSTRACT This study investigates the recognition of halide anions by N,N’-bis(4-nitrophenyl)urea using computational methods in both gas-phase and solution. It examines interaction, stabilization, and deformation energies, revealing a gas-phase selectivity trend of [L⋯F]−>[L⋯Cl]−>[L⋯Br]−>[L⋯I]−. NBO, QTAIM, and EDA-NOCV analyses highlight N-H∙∙∙X hydrogen bonds, with electrostatic forces contributing around 60% to the total interaction. Despite unfavorable solvation energy changes, the intrinsic affinity remains the main factor for selectivity, which aligns with experimental data. The strong correlation between theoretical and experimental formation constants supports the reliability of the computational findings.

Adsorption of Polycyclic Aromatics on Crystal Surface of Coordination Cages: Photophysical Properties and Förster Resonance Energy Transfer.

Self-assembly reactions of PdX2 (X- = NO3-, BF4-, ClO4-, ReO4-, PF6-, and CF3SO3-) with 9,10-bis((isoquinolin-5-yloxy)methyl)anthracene (L) in Me2SO give rise to single crystals of coordination cages, [X@Pd2L4]X3, irrespective of X- anions, in high yields. The intracage Pd···Pd distance is significantly sensitive to the nestled X- anion, which can serve as a gauge for recognition of ubiquitous polyatomic anions. One interesting feature is that, via π-π interactions, various polycyclic aromatics (PAs) are characteristically adsorbed on the crystal surface of designed coordination cages with a wall of anthranyl moiety. This unprecedented ensemble system shows an efficient Förster resonance energy transfer (FRET) process from the anthracene (ANT) to pyrene (PYR) via a large degree of spectral overlap between the ANT emission and PYR absorption bands, in contrast to a simple mixture of ANT and PYR.

Polyanionic Receptors for Carboxylates in Water.

Receptors for carboxylate anions have many possible biomedical applications, including mimicry of the vancomycin group of antibiotics. However, binding carboxylates in water, the biological solvent, is highly challenging due to the hydrophilicity of these polar anions. Here we report, for the first time, the recognition of simple carboxylates such as acetate and formate in water by synthetic receptors with charge-neutral binding sites. The receptors are solubilised by polyanionic side-chains which, remarkably, do not preclude anion binding. The tricyclic structures feature two identical binding sites linked by polyaromatic bridges, capable of folding into closed, twisted conformations. This folding is hypothesised to preorganise the structures for anion recognition, mimicking the process which generates many protein binding sites. The architecture is suitable for elaboration into enclosed structures with potential for selective recognition of biologically relevant carboxylates.

Polyanionic Receptors for Carboxylates in Water

AbstractReceptors for carboxylate anions have many possible biomedical applications, including mimicry of the vancomycin group of antibiotics. However, binding carboxylates in water, the biological solvent, is highly challenging due to the hydrophilicity of these polar anions. Here we report, for the first time, the recognition of simple carboxylates such as acetate and formate in water by synthetic receptors with charge‐neutral binding sites. The receptors are solubilised by polyanionic side‐chains which, remarkably, do not preclude anion binding. The tricyclic structures feature two identical binding sites linked by polyaromatic bridges, capable of folding into closed, twisted conformations. This folding is hypothesised to preorganise the structures for anion recognition, mimicking the process which generates many protein binding sites. The architecture is suitable for elaboration into enclosed structures with potential for selective recognition of biologically relevant carboxylates.

Organocatalytic Applications of Sulfonyl Squaramides in Anion Recognition Strategies

A modular, 3‐steps protocol for the synthesis of N‐sulfonyl squaramides has been developed. The strategic installation of a tetrahedral, electron‐withdrawing sulfonyl group into the squaramido core allowed the prevention of undesired self‐aggregations, therefore upgrading the solubility in common organic solvents, and moreover, enhancing their H‐bond donor abilities for molecular recognition. These unique features have been efficiently exploited in two different ion‐pairing reactions: (i) the challenging C4‐selective dearomatization of 2‐picoline with silyl ketene acetals and (ii) the tritylation of Nmethylindole. Furthermore, their catalytic activities have been directly compared with other common and well‐established (thio)urea analogues and related H‐bond donors, revealing that highly acidic designs are essential to reach optimal catalytic performances.

Computational Justification Towards Detection of Dual Anions on a Single Molecular Platform: The Role of Solvent in Decoration of Dual Channels

ABSTRACTRatiometric optical detection of analytes is a convenient strategy as the technique is devoid of relative error and background correction. Herein, solvent‐guided ratiometric optical recognition of fluoride and bisulfate anions by a low‐cost, “off‐the‐shelf” bioactive molecule, harmane (HRH) is thoroughly explored. Interestingly, solvent plays a dynamic role in the selective recognition of the dual anions via the dual channels of HRH in an intelligent manner. The probe displays high‐fidelity recognition behavior towards fluoride ion in an aprotic solvent (acetonitrile) and towards bisulfate ion in a protic environment (acetonitrile/water; 5:1; v/v). Both the channels of HRH are very selective for a particular anion (F−/HSO4−) in a specific solvent. Organized and comprehensive theoretical calculation denotes that hydrogen bonding between the acidic pyrrolic proton of HRH and fluoride for the first channel and the acidic proton of bisulfate and the pyridinic nitrogen for the second channel of HRH led to the formation of a hydrogen‐bonded ion pair (HBIP). Consequently, significant optical changes are observed in the visible region, which is convenient for real‐life detection of F− and HSO4− independently. The essential role of solvent in tuning the dual channels of HRH is an important artifact in the literature of fundamental photochemistry.

Synthesis, X-ray structures, and anion binding properties of 1,8-disulfonamidocarbazole-di(thio)amide hybrid macrocycles

An easy-to-prepare 1,8-disulfonamidocarbazole-diamide hybrid macrocycle 1 and its thioamide analogue 2 were reported, along with their anion recognition properties as determined by UV-vis and 1H NMR spectroscopic titrations as well as single crystal X-ray diffraction analyses. The conversion of amides into thioamides in the macrocyclic structures generated notable effects on the anion-binding affinity and selectivity. For example, amide-bearing macrocycle 1 was found to bind H2PO4- over one order of magnitude more strongly than thioamide-containing macrocycle 2, despite having more acidic NH groups for the latter. Additionally, macrocycle 1 displayed the highest affinity for PhCOO- in DMSO (Ka >105 M-1 for the formed 1:1 complex) followed by H2PO4- and AcO- ions, whereas a different binding order AcO- > PhCOO- >> H2PO4- was observed for macrocycle 2. More importantly, the single crystal X-ray diffraction analysis clearly revealed that macrocycle 1 indeed formed a 1:1 complex with PhCOO- ion in the solid state.

Synthesis and Anion Recognition of Triangular Prism Quinoline Cage with Reduced Symmetry

AbstractOrganic molecular cages with low symmetry are promising to generate complex chemical environments for molecular recognition and catalysis. Herein, a new triangular prism cage with three rigid 8‐aminoquinoline‐2‐carboxylic acid derivatives as edges were synthesized. This molecular cage possesses reduced symmetry and contains both secondary amine and amide as hydrogen donating groups for molecular recognition. It could bind anions, such as NO3−, AcO−, and F− with moderate affinity.

Sulphide and iodide anion recognition by a selective copper hydrogel of a chloro substituted terpyridine ligand

2,2′:6′,2″-Terpyridines (tpy) are well known for their ability to form a gel in the presence of transition metals. Due to the strong binding affinities towards transition metal ions aided by non-covalent interactions, tpy tends to form hydrogel with more than one metal ion. Interestingly, a simple chloro substituted 2,2′:6′,2″-terpyridine ligand L selectively forms a gel with only Cu(II) ion in a mild acidic medium. The hydrogel has a fibrous structure and is sensitive to S2− and I− anions, which is confirmed by both morphological analysis and rheological study. The copper complex was characterized using a variety of techniques, including HRMS, ESR, FTIR, elemental analysis, and DFT calculations. Further, DFT calculations were used to study the non-covalent interactions within the complex and to understand why the hydrogel responds to specific anions. The free energies of the CuI and CuS precipitate formation were calculated to get an idea of the feasibility of the process. Notably, UV–visible absorption spectroscopic study shows that the copper(II) complex CuCl2L selectively detected S2− and I− anions in aqueous acetonitrile with LOD of 2.63 and 2.08 μM, respectively.

Expedient Decagram-Scale Synthesis of Robust Organic Cages That Bind Sulfate Strongly and Selectively in Water.

Selective anion recognition remains a key challenge in supramolecular chemistry: only a very small number of systems that can function in water are known, and these nearly always preferentially bind hydrophobic anions. In this work, we report three robust hexa-cationic cages that can be prepared on scales up to 14 g in two simple and high-yielding steps from commercially available materials. One of these cages displays unusually strong sulfate binding in water (Ka = 12,000 M-1), and demonstrates high selectivity for this anion over H2PO4-/HPO42- in DMSO/buffer mixtures. These results demonstrate that relatively large, three-dimensional supramolecular hosts can be prepared in high yields and on large scales, and can be highly potent receptors.

Amino acid – Squaramide conjugates as anion binding receptors

Squaramides have garnered significant attention as anion binding motifs and have become a popular supramolecular synthon owing to their unique structural features and propensity to form strong hydrogen bond interactions particularly with anionic species. However, their conjugation to biomaterials to form biomimetic receptors remains underexplored even though such species would combine the biocompatibility of amino acids with the strong hydrogen bonding propensity of squaramides and may prove useful across broad applications in the chemical and biological sciences. In this study, we present the design and synthesis of a series of amino acid – squaramide conjugates (AASqs) 1 – 5, as well as studying their anion recognition properties. 1–5 were prepared using facile synthetic methods and their anion binding properties were investigated via 1H NMR titrations with a range of anions in 0.5 % H2O in DMSO‑d6. Our findings constitute a cautionary tale, where it was shown that the presence of the C-terminal carboxylic acid of the amino acids complicate the binding behaviour of the receptors towards anions. We show that the C-terminal carboxylates compete with other anionic species for the squaramide binding site leading to complex binding equilibria in solution. However, upon esterification of the C-termini to afford compound 5, a simplification of the binding profile was observed and was easily fitted to a 1:1 (Receptor:Anion) binding stoichiometry. This work sheds light on the potential of AASqs as anion binding receptors whilst underlining the influence of the C-terminus on anion recognition. It also provides a direct solution to mitigate the complex binding behaviour of 1–4 through esterification and will prove useful in the design of more complex squaramide bioconjugates.

A Synthetic Tetra‐Amide Macrocycle for Anion Recognition at Molecular Level

A Synthetic Tetra‐Amide Macrocycle for Anion Recognition at Molecular Level

Anion recognition by phosphoric triamide-based receptors

Anion recognition by phosphoric triamide-based receptors

Pyrrole-based acyclic hexapodal aldehyde for anion recognition

A novel acyclic hexapodal host (4a) featuring with multiple formyl groups, pyrrole cores, and ester linkages has been prepared and characterized through 1H NMR, 13C NMR, high-resolution mass spectrometry and single-crystal X-ray diffraction techniques. This versatile host molecule exhibits robust anion recognition for CN−, F−, SO42−, HSO4−, H2PO4−, and HP2O73− with the binding constants spanning 0.18 × 105 M−1 to 7.72 × 105 M−1. Interactions with these anions were elucidated through UV–visible and 1H NMR titrations, complemented by DFT calculations. At low guest concentrations (5- to 100-fold excess), a 1:1 complexation is predominant. However, at high concentrations (1000-fold excess), the host's flexibility promotes the assembly of other host-guest complexes. Comparative studies with related pyrrole-based hexapodal ester 4b and tetrapodal aldehyde 4c highlight the importance of peripheral formyl groups, pyrrolic NH groups, the number of pendant arms, and conformational adaptability in anion recognition. These insights underscore the potential of acyclic multipodal hosts in advancing supramolecular chemistry and anion recognition.

Recognition of anion-water clusters by peptide-based supramolecular capsules

The biological and technological importance of anion-mediated processes has made the development of improved methods for the selective recognition of anions one of the most relevant research topics today. The hydration sphere of anions plays an important role in the functions performed by anions by forming a variety of cluster complexes. Here we describe a supramolecular capsule that recognizes hydrated anion clusters. These clusters are most likely composed of three ions that form hydrated C3 symmetry complexes that are entrapped within the supramolecular capsule of the same symmetry. The capsule is made of self-assembled α,γ-cyclic peptide containing amino acid with by five-membered rings and equipped with a tris(triazolylethyl)amine cap. To recognise the hydrated anion clusters, the hexapeptide capsule must disassemble to entrap them between its two subunits.

Chalcogen Bonding in Selective Recognition and Liquid-Liquid Extraction of Perrhenate.

Efficient recognition and extraction of hazardous anionic pollutants from water medium is of great significance for environmental concerns, representing a challenging area of research in supramolecular chemistry. In this study, we present, for the first time, a comprehensive demonstration of the ability of chalcogen bonding (ChB) to recognize and remove the ReO4 - from 100 % water medium. The anion recognition ability is well elucidated through solution phase NMR and ITC studies, which clearly reveal the selective binding of ReO4 - over other oxo-anions. Moreover, the selenoimidazolium scaffold effectively engages in Se•••O ChB interaction with ReO4 - as confirmed by X-ray crystal structure and XPS analysis. More importantly, the binding of ReO4 - with different prolongations of the σ-holes, along with Se•••Se chalcogen bonding interactions, lead to the formation of a 1D supramolecular assembly. Eventually, ChB receptor Se4Me-Br exhibits ~62 % ReO4 - extraction efficiency through precipitation as the extraction method. Furthermore, in efforts to enhance efficiency, a hydrophobic ChB receptor Se4Do-PF6 has been prepared, achieving an efficiency of up to ~93 % at a very low concentration (~5 ppm) by liquid-liquid extraction.

Atomistic characterization of β2-glycoprotein I domain V interaction with anionic membranes

BackgroundInteraction of β2-glycoprotein I (β2GPI) with anionic membranes is crucial in antiphospholipid syndrome (APS), implicating the role of its membrane-binding domain, domain V (DV). The mechanism of DV binding to anionic lipids is not fully understood. ObjectivesThis study aimed to elucidate the molecular details of β2GPI DV binding to anionic membranes. MethodsWe utilized molecular dynamics simulations to investigate the structural basis of anionic lipid recognition by DV. To corroborate the membrane-binding mode identified in the highly mobile membrane mimetic simulations, we conducted additional simulations using a full membrane model. ResultsThe study identified critical regions in DV, namely the lysine-rich loop and the hydrophobic loop, which are essential for membrane association via electrostatic and hydrophobic interactions, respectively. A novel lysine pair contributing to membrane binding was also discovered, providing new insights into β2GPI’s membrane interaction. Simulations revealed 2 distinct binding modes of DV to the membrane, with mode 1 characterized by the insertion of the hydrophobic loop into the lipid bilayer, suggesting a dominant mechanism for membrane association. This interaction is pivotal for the pathogenesis of APS, as it facilitates the recognition of β2GPI by antiphospholipid antibodies. ConclusionThe study advances our understanding of the molecular interactions between β2GPI’s DV and anionic membranes, which are crucial for APS pathogenesis. It highlights the importance of specific regions in DV for membrane binding and reveals a predominant binding mode. These findings have significant implications for APS diagnostics and therapeutics, offering a deeper insight into the molecular basis of the syndrome.

Molecular recognition on the surface of concentrated hydrochloric acid: Hydration configuration-driven preferential recognition of Rh (III) anions with inner-coordinated water molecules

Comprehension of selective recognition of anions is confined to a phenomenological list of solvents, hardly surpassing Pedersen’s empirical observations of cationic-crown ether binding. In this work, a novel strategy employing thin-layer oil membrane extraction (TOMX) on the surface of concentrated hydrochloric acid is proposed for controllable recognition and separation of anions with different hydration configurations. Four typical chloric-complexing anions, RhCl63−, PdCl42−, PtCl62− and FeCl4−, usually coexisting in the concentrated hydrochloric acid leaching solutions of precious metal concentrates were focused. A surprising phenomenon was noticed that the octahedral RhCl5(H2O)2− anions with inner-coordinated water molecules were preferentially recognized by 18C6 macrocyclic molecule. However, the planar quadrilateral Pd (II) chloric-complexing anions with a hydration configuration via water molecules attacking the central Pd (II) atoms along the Z-axis direction can be separated subsequently. The Pt (IV) and Fe (III) chloric-complexing anions respectively with an octahedral and tetrahedral outer-coordinated hydration shell configuration were the last to be recognized. Such an order in extracting Rh (III) > Pd (II) > Pt (IV) > Fe (III) anions depends on their hydration capacity and the preferential affinity towards the interface, which is almost impossible during the conventional stirring and oil droplet dispersed extraction. Because of a lower concentration of dissociated hydrogen ions on the surface of concentrated hydrochloric acid compared to its bulk concentrations, the hydration of those chloric-complexing anions was enhanced. The Rh (III) anions with inner-coordinated water molecules exhibit a higher interfacial affinity than other anions. The water molecules entering the inner coordination layer of Rh (III) chloric-complexing anions, cooperating with its outer-coordinated hydration shell, act as a hydrogen bond “water bridge” to interact with 18C6 molecules, which makes it easier to feel the influence from the aqueous laminar shear flow under the thin-layer oil membrane, thereby driving the preferential recognition of easily hydrated Rh (III) anions at the interface. The controllable “water bridge” interactions driven by shear flow can be activated or deactivated on the surface of concentrated hydrochloric acid by adjusting the shear flow rate. This work lays a foundation for the future development of a controllable flexible molecular recognition technology at liquid–liquid interfaces.

Fluorescence chemosensor for anion recognition, solvatochromism and protein binding studies based on Schiff- base derivative

A new salicylidene-based chemosensor (E)-N1-(2-hydroxy-3-methoxy-5-nitrobenzylidene)-N2-phenylbenzene-1,2-diamine (MNP) has been developed by condensation of 3-methoxy-5-nitrosalicylaldehyde with N-phenyl-o-phenylenediamine and analysed using FT-IR, 1H-NMR and ESI-MS spectral techniques. The photophysical properties of MNP, including solvatochromism were investigated and the colour changed from bright yellow to red in the presence of water. This MNP specifically detects only Cu2+ with "turn-off" and a colour turned from light yellow to violet in CH3CN/H2O (9/1, v/v) mixture.The optimized geometry of the MNP-Cu complex was analyzed using DFT methods and the results is confirm the formation of the MNP-Cu complex. Besides, the F− and H2PO4−anions are preferentially sensed by the MNP in comparison to other anions, leading a "turn-off" and the colour change from light yellow to dark yellow in the THF medium. From the pH sensor data, the fluorescence intensity increased at pH 4.0 due to protonation of the -OH group. The interaction of the MNP with protein molecules (BSA, HSA) was investigated using absorption and emission spectral methods. The investigation computed values of the binding constant and the Stern-Volmer quenching constant demonstrated that the MNP had a far stronger affinity for BSA than for HSA. The interaction of the MNP with protein molecules was confirmed using docking studies.

Accelerated Activity-Based Sensing by Fluorogenic Reporter Engineering Enables to Rapidly Determine Unstable Analyte.

Accurate detection of labile analytes through activity based fluorogenic sensing is meaningful but remains a challenge because of nonrapid reaction kinetic. Herein, we present a signaling reporter engineering strategy to accelerate azoreduction reaction by positively charged fluorophore promoted unstable anion recognition for rapidly sensing sodium dithionite (Na2S2O4), a kind of widespread used but harmful inorganic reducing agent. Its quick decomposition often impedes application reliability of traditional fluorogenic probes in real samples because of their slow responses. In this work, four azo-based probes with different charged fluorophores (positive, zwitterionic, neutral, and negative) were synthesized and compared. Among of them, with sequestration effect of positively charged anthocyanin fluorophore for dithionite anion via electrostatic attraction, the cationic probe Azo-Pos displayed ultrafast fluorogenic response (∼2 s) with the fastest response kinetic (kpos' = 0.373 s-1) that is better than other charged ones (kzwi' = 0.031 s-1, kneu' = 0.013 s-1, kneg' = 0.003 s-1). Azo-Pos was demonstrated to be capable to directly detect labile Na2S2O4 in food samples and visualize the presence of Na2S2O4 in living systems in a timely fashion. This new probe has potential as a robust tool to fluorescently monitor excessive food additives and biological invasion of harmful Na2S2O4. Moreover, our proposed accelerating strategy would be versatile to develop more activity-based sensing probes for quickly detecting other unstable analytes of interest.