Anion Recognition Sites Research Articles

Highly concentrated solution of lithium chloride in organic solvents with heteroditopic receptors

Abstract The solubilization and selective extraction of lithium salts, in particular lithium chloride, in organic solvents have recently attracted attention due to the application in industry. A heteroditopic receptor 1b, which has ether oxygens and 2 urea moieties as cation and anion recognition sites, respectively, cooperatively associates with LiCl in organic solvents. The selective solid–liquid extraction of LiCl and LiBr was observed by 1H NMR in CDCl3 and MeCN-d3. It is possible to prepare highly concentrated solutions of 1b and LiCl in MeCN up to a concentration of 9.6 M, and these solutions exhibit properties similar to those of ionic liquids.

Design of molecular sensors and switches based on luminescent ruthenium-terpyridine complexes bearing active methylene and triphenylphosphonium motifs as anion recognition sites: experimental and DFT/TD-DFT investigation.

Synthesis, characterization and thorough investigation of the photophysical and electrochemical properties of a new category of emissive homo- and heteroleptic Ru(II)-complexes derived from the [4'-(p-triphenylphosphonium methyl phenyl)-2,2':6',2''-terpyridine]bromide (tpy-PhCH2PPh3Br) ligand have been executed in this work. Incorporation of the PhCH2PPh3+Br- group at the terpyridine motif appropriately adjusts the triplet metal-to-ligand charge transfer (3MLCT) and metal-centered (3MC) excited states so that the complexes luminesce at room temperature (RT) having lifetimes within the range of 6.82-9.63 ns. The RT emission characteristics of the complexes get further enhanced via aggregation phenomena through the use of different solvent/non-solvent mixtures (DMSO/H2O and DMSO/PhCH3 mixtures). Temperature dependent emission spectral measurements indicate that the emission intensity, quantum yield and lifetime increase upon dropping down the temperature, thereby designated as the on-state, while the increase of temperature causes a reduction of the said properties, indicating the off-state and the process is fully reversible. Taking advantage of the active methylene group coupled with a phosphonium motif, anion sensing characteristics of the complexes are investigated systematically in DMSO through the use of various optical channels and spectroscopic tools. The complexes are very much sensitive to fluoride and to a lesser extent acetate and dihydrogen phosphate among the studied anions. In essence, the complexes function as sensors for temperature and fluoride ion. Computational investigations were also executed via density functional theory (DFT) and time-dependent (TD)-DFT to obtain a clear understanding of the electronic structures of the metalloreceptors, appropriate assignment of the spectral bands and their mode of interaction with selected anions.

Alkyl modified cationic COFs for preferential trapping of charge dispersed perrhenate: Synergistic hydrophobicity and anion-recognition effects

Charge dispersed oxoanionic pollutants (such as TcO4− and ReO4−) with low hydrophilicity are typically difficult to be preferentially extracted. Recently, cationic covalent organic frameworks (COFs) have received considerable attention for anions trapping. Two cationic COFs, denoted as Tp-S and Tp-D, were synthesized by incorporating ethyl and cyclic alkylated diquats into 2,2′-bipyridine-based COF. A synergistic effect of hydrophobic channel and anion-recognition sites were achieved by branched chains, which effectively surmounted the Hofmeister bias. Both Tp-S and Tp-D exhibited raising removal performance for surrogate ReO4− at high acidity with adsorption capacities of 435.6 and 291.4 mg g−1, respectively. Obvious variations caused by side chains were displayed in microstructures and adsorption performance. Specially, compared with Tp-D, Tp-S demonstrated desirable priority in uptake capacity and selectivity. In a real-scenario experiment, Tp-S could remove 72.8 % of ReO4− in a simulated Hanford LAW stream, which was attributed to the spatial effects and charge distribution arising from the open and flexible side chains of Tp-S. Otherwise, the rigid cyclic chains endowed pyridine-base Tp-D material an unprecedented alkaline stability. Spectra and theoretical calculations revealed a mechanism of preferential capture based on electrostatic interaction and hydrogen bonding between charge dispersed ReO4−/TcO4− and Tp-S/Tp-D. This work provides an innovative perspective to tailored materials for the treatment of oxoanionic contaminants.

Imidazolinium-based Multiblock Amphiphile as Transmembrane Anion Transporter.

Transmembrane anion transport is an important biological process in maintaining cellular functions. Thus, synthetic anion transporters are widely developed for their biological applications. Imidazolinium was introduced as anion recognition site to a multiblock amphiphilic structure that consists of octa(ethylene glycol) and aromatic units. Ion transport assay using halide-sensitive lucigenin and pH-sensitive 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) revealed that imidazolinium-based multiblock amphiphile (IMA) transports anions and showed high selectivity for nitrate, which plays crucial roles in many biological events. Temperature-dependent ion transport assay using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) indicated that IMA works as a mobile carrier. 1 H NMR titration experiments indicated that the C2 proton of the imidazolinium ring recognizes anions via a (C-H)+ ⋅⋅⋅X- hydrogen bond. Furthermore, all-atom molecular dynamics simulations revealed a dynamic feature of IMA within the membranes during ion transportation.

Molecular Recognition Under Interfacial Conditions: Calix[4]pyrrole-Based Cross-linkable Micelles for Ion Pair Extraction.

An anthracene-functionalized, long-tailed calix[4]pyrrole 1, containing both an anion-recognition site and cation-recognition functionality, has been synthesized and fully characterized. Upon ion pair complexation with FeF2, receptor 1 self-assembles into multimicelles in aqueous media. This aggregation process is ascribed to a change in polarity from nonpolar to amphiphilic induced upon concurrent anion and cation complexation and permits molecular recognition-based control over chemical morphology under interfacial conditions. Photoirradiation of the micelles serves to cross-link the anthracene units thus stabilizing the aggregates. The combination of ion pair recognition, micelle formation, and cross-linking can be used to extract FeF2 ion pairs from bulk aqueous solutions. The present work helps illustrate how molecular recognition and self-assembly may be used to control the chemistry of extractants at interfaces.

Fluoride ion activated CO2 sensing using sol-gel system

CO2 detection has drawn a lot of attention due to its important role in the environment and in medical diagnosis. Two amino substituted naphthalimides are reported as new anion-induced CO2 selective chemosensors. The new naphthalimides contain either a hydrazinecarbothiourea or a hydrazinecarboxylate group which served as an anion recognition site. The addition of F− induced a distinct color change and a fluorescent quenching effect, which was revived by subsequent addition of CO2. The naphthalimide bearing a hydrazinecarboxylate group also served as an organo gelator, which showed a unique sol-gel transition as well as color/fluorescent changes with F− and CO2.

Selective Nitrate Recognition by a Halogen‐Bonding Four‐Station [3]Rotaxane Molecular Shuttle

The synthesis of the first halogen bonding [3]rotaxane host system containing a bis‐iodo triazolium‐bis‐naphthalene diimide four station axle component is reported. Proton NMR anion binding titration experiments revealed the halogen bonding rotaxane is selective for nitrate over the more basic acetate, hydrogen carbonate and dihydrogen phosphate oxoanions and chloride, and exhibits enhanced recognition of anions relative to a hydrogen bonding analogue. This elaborate interlocked anion receptor functions via a novel dynamic pincer mechanism where upon nitrate anion binding, both macrocycles shuttle from the naphthalene diimide stations at the periphery of the axle to the central halogen bonding iodo‐triazolium station anion recognition sites to form a unique 1:1 stoichiometric nitrate anion–rotaxane sandwich complex. Molecular dynamics simulations carried out on the nitrate and chloride halogen bonding [3]rotaxane complexes corroborate the 1H NMR anion binding results.

Superior anion induced shuttling behaviour exhibited by a halogen bonding two station rotaxane

Two bistable halogen and hydrogen bonding-naphthalene diimide [2]rotaxanes have been prepared and the system incorporating a halogen bond donor anion recognition site is demonstrated to exhibit superior anion induced translational motion of the macrocyclic wheel component relative to the hydrogen bonding analogue. Proton NMR spectroscopy is used to estimate the percentage occupancies of the macrocycle at the respective station and importantly indicates that the halogen bonding rotaxane displays superior positional integrity in competitive protic solvent media as a consequence of strong halogen bond-halide anion binding interactions.

Hopping-mediated anion transport through a mannitol-based rosette ion channel.

Artificial anion selective ion channels with single-file multiple anion-recognition sites are rare. Here, we have designed, by hypothesis, a small molecule that self-organizes to form a barrel rosette ion channel in the lipid membrane environment. Being amphiphilic in nature, this molecule forms nanotubes through intermolecular hydrogen bond formation, while its hydrophobic counterpart is stabilized by hydrophobic interactions in the membrane. The anion selectivity of the channel was investigated by fluorescence-based vesicle assay and planar bilayer conductance measurements. The ion transport by a modified hopping mechanism was demonstrated by molecular dynamics simulation studies.

Cooperativity and complexity in the binding of anions and cations to a tetratopic ion-pair host.

Cooperative interactions play a very important role in both natural and synthetic supramolecular systems. We report here on the cooperative binding properties of a tetratopic ion-pair host 1. This host combines two isophthalamide anion recognition sites with two unusual "half-crown/two carbonyl" cation recognition sites as revealed by the combination of single-crystal X-ray analysis of the free host and the 1:2 host:calcium cation complex, together with two-dimensional NMR and computational studies. By systematically comparing all of the binding data to several possible binding models and focusing on four different variants of the 1:2 binding model, it was in most cases possible to quantify these complex cooperative interactions. The data showed strong negative cooperativity (α = 0.01-0.05) of 1 toward chloride and acetate anions, while for cations the results were more variable. Interestingly, in the competitive (CDCl3/CD3OD (9:1, v/v)) solvent, the addition of calcium cations to the tetratopic ion-pair host 1 allosterically switched "on" chloride binding that is otherwise not present in this solvent system. The insight into the complexity of cooperative interactions revealed in this study of the tetratopic ion-pair host 1 can be used to design better cooperative supramolecular systems for information transfer and catalysis.

Ion pair recognition of ditopic receptor bearing silanol groups as anion recognition sites and a 2,2′-bipyridine moiety as a metal-coordination site

A ditopic receptor 1 bearing silanol groups as anion recognition sites and a 2,2′-bipyridine moiety as a metal-coordination site was synthesized. The X-ray crystallographic analysis revealed that the receptor 1 simultaneously associated Ag+ at the bipyridine site and NO3- at the anion recognition site surrounded by two silanol groups and one C–H proton of the bipyridine. The titration experiments suggested the large affinity of the receptor with AgNO3, caused by the cooperative cation and anion binding. The cooperative effect of metal cation could enhance anion recognition property of silanol-based receptor.

Conjugate‐Base‐Stabilized Brønsted Acids as Asymmetric Catalysts: Enantioselective Povarov Reactions with Secondary Aromatic Amines

Caught in the act: A new concept for asymmetric Brønsted acid catalysis is presented. Compounds containing an acidic functionality in addition to an anion recognition site act as powerful conjugate-base-stabilized Brønsted acid catalysts. This strategy was applied to the first catalytic enantioselective three-component Povarov reaction of indoline and other secondary aromatic amines (see scheme; M.S.=molecular sieves).

Colorimetric macrocyclic anion probes bearing nitrophenylurea and nitrophenylthiourea binding groups

Two novel molecular probes bearing two urea (sensor 1) or thiourea (sensor 2) groups (as anion recognition site) coupled with a nitrophenyl group (chromogenic unit) were synthesized and evaluated according to the binding site-signaling subunit approach.The behavior of these different compounds toward metal ions (Co2+, Ni2+, Cu2+, Zn2+, and Cd2+) and anions (F−, Cl−, Br−, I−, ClO4−, NO3−, CN−, OH−, CH3COO−, and H2PO4−) was investigated by UV–vis spectroscopy in DMSO.

Synthesis of a Pillar[5]arene‐Based Heteroditopic Host and Its Complexation with n‐Octyltriethylammonium Salts

AbstractA new pillar[5]arene‐based heteroditopic receptor containing a urea anion recognition site was designed and made. It was demonstrated that the introduction of the urea group remarkably promoted the binding affinity to n‐octyltriethylammonium salts with different counterions in chloroform, as the corresponding monotopic host, 1,4‐dimethoxypillar[5]arene, showed weak complexation with these guests. The important role of the urea unit was investigated and it was shown that the urea group and the macrocyclic unit not only provided positioned binding sites for the guests but also acted cooperatively in binding these guests.

Highly selective recognition of acetate and bicarbonate by thiourea-functionalised inverse opal hydrogel in aqueous solution

We have synthesised an inverse opal hydrogel functionalised with a thiourea moiety as an anion recognition site. The resulting hydrogel showed remarkable changes of its structural colour with respect to acetate and bicarbonate ions selectively in aqueous solution, indicating a potential as colorimetric sensing materials for hydrophilic anions.

Efficient gene delivery into cells by a surprisingly small three-armed peptide ligand

The development of new non-viral transfection vectors for gene transport into cells is of current interest. A small, three-armed peptide ligand 1 is derived from the cationic dipeptide Lys-Phe with an additional anion recognition site at its N-terminus, binds to DNA with high affinity (K ca. 107 M−1) and efficiently delivers a GFP plasmid into cells. Compared to the cationic polymer polyethyleneimine (PEI), routinely used as a standard vector for transfection, 1 is significantly more efficient and also less cytotoxic. As DLS and AFM studies show, ligand 1 condenses DNA into tightly packed cationic aggregates which are then taken up by the cells. In contrast, the analogous divalent peptide ligand 2 of identical amino acid sequence and the highly charged divalent DNA-binder 3 (Lys-Lys-Arg) do not enable gene delivery though they also bind with high affinity (2: K ca. 106 M−1; 3: K ca. 107 M−1). All three ligands are able to transfer genetic material into cells but only the trivalent gene carrier is able to escape from the endosome due to its superior buffering capacity.

Structure and kinetic characterization of human sperm-specific glyceraldehyde-3-phosphate dehydrogenase, GAPDS

hGAPDS (human sperm-specific glyceraldehyde-3-phosphate dehydrogenase) is a glycolytic enzyme essential for the survival of spermatozoa, and constitutes a potential target for non-hormonal contraception. However, enzyme characterization of GAPDS has been hampered by the difficulty in producing soluble recombinant protein. In the present study, we have overexpressed in Escherichia coli a highly soluble form of hGAPDS truncated at the N-terminus (hGAPDSΔN), and crystallized the homotetrameric enzyme in two ligand complexes. The hGAPDSΔN-NAD+-phosphate structure maps the two anion-recognition sites within the catalytic pocket that correspond to the conserved Ps site and the newly recognized Pi site identified in other organisms. The hGAPDSΔN-NAD+-glycerol structure shows serendipitous binding of glycerol at the Ps and new Pi sites, demonstrating the propensity of these anion-recognition sites to bind non-physiologically relevant ligands. A comparison of kinetic profiles between hGAPDSΔN and its somatic equivalent reveals a 3-fold increase in catalytic efficiency for hGAPDSΔN. This may be attributable to subtle amino acid substitutions peripheral to the active centre that influence the charge properties and protonation states of catalytic residues. Our data therefore elucidate structural and kinetic features of hGAPDS that might provide insightful information towards inhibitor development.

A redox-active [3]rotaxane capable of binding and electrochemically sensing chloride and sulfate anions

A ferrocene functionalised redox-active [3]rotaxane which contains two interlocked anion recognition sites has been prepared by chloride anion templation. With chloride two equivalents of anion are bound, one in each of the interlocked cavities, while sulfate forms a 1:1 stoichimetric sandwich type complex; the rotaxane can also electrochemically sense the two anions in acetonitrile.

ChemInform Abstract: Ion Pair Receptors

AbstractReview: 110 refs.

A Calix[4]arene Strapped Calix[4]pyrrole: An Ion-Pair Receptor Displaying Three Different Cesium Cation Recognition Modes

An ion-pair receptor, the calix[4]pyrrole-calix[4]arene pseudodimer 2, bearing a strong anion-recognition site but not a weak cation-recognition site, has been synthesized and characterized by standard spectroscopic means and via single-crystal X-ray diffraction analysis. In 10% CD(3)OD in CDCl(3) (v/v), this new receptor binds neither the Cs(+) cation nor the F(-) anion when exposed to these species in the presence of other counterions; however, it forms a stable 1:1 solvent-separated CsF complex when exposed to these two ions in concert with one another in this same solvent mixture. In contrast to what is seen in the case of a previously reported crown ether "strapped" calixarene-calixpyrrole ion-pair receptor 1 (J. Am. Chem. Soc. 2008, 130, 13162-13166), where Cs(+) cation recognition takes place within the crown, in 2.CsF cation recognition takes place within the receptor cavity itself, as inferred from both single-crystal X-ray diffraction analyses and (1)H NMR spectroscopic studies. This binding mode is supported by calculations carried out using the MMFF94 force field model. In 10% CD(3)OD in CDCl(3) (v/v), receptor 2 shows selectivity for CsF over the Cs(+) salts of Cl(-), Br(-), and NO(3)(-) but will bind these other cesium salts in the absence of fluoride, both in solution and in the solid state. In the case of CsCl, an unprecedented 2:2 complex is observed in the solid state that is characterized by two different ion-pair binding modes. One of these consists of a contact ion pair with the cesium cation and chloride anion both being bound within the central binding pocket and in direct contact with one another. The other mode involves a chloride anion bound to the pyrrole NH protons of a calixpyrrole subunit and a cesium cation sandwiched between two cone shaped calix[4]pyrroles originating from separate receptor units. In contrast to what is seen for CsF and CsCl, single-crystal X-ray structural analyses and (1)H NMR spectroscopic studies reveal that receptor 2 forms a 1:1 complex with CsNO(3), with the ions bound in the form of a contact ion pair. Thus, depending on the counteranion, receptor 2 is able to stabilize three different ion-pair binding modes with Cs(+), namely solvent-bridged, contact, and host-separated.