Improving the Enzyme Catalytic Efficiency Using Ionic Liquids with Kosmotropic Anions

The kosmotropicity of cations and anions in ionic liquids has a strong influence on the enzyme catalytic efficiency in aqueous environments. The kosmotropic anion CF3COO^- seemed to activate the protease, and the chaotropic anions tended to destabilize the enzyme.     

Chaotropic Monovalent Anion‐Induced Rectification Inversion at Nanopipettes Modified by Polyimidazolium Brushes

A nonintuitive observation of monovalent anion‐induced ion current rectification inversion at polyimidazolium brush (PimB)‐modified nanopipettes is presented. The rectification inversion degree is strongly dependent on the concentration and species of monovalent anions. For chaotropic anions (for example, ClO₄^?), the rectification inversion is easily observed at a low concentration (5 mm ), while there is no rectification inversion observed for kosmotropic anions (Cl^?) even at a high concentration (1 m ). Moreover, at the specific concentration (for example, 10 mm ), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl^?≥NO₃^?>BF₄^?>ClO₄^?>PF₆^?>Tf₂N^?). Estimation of the electrokinetic charge density (σ^ek) demonstrates that rectification inversion originates from the charge inversion owing to the over‐adsorption of chaotropic monovalent anion. To qualitatively understand this phenomenon, a concentration‐dependent adsorption mechanism is proposed.     

Sulfonium and phosphonium, new ion-pairing agents with unique selectivity towards polarizable anions

Ion-pair chromatography (IPC) almost universally relies upon ammonium-based ion-pairing agents (IPAs) for anion separations. This work compares tetrabutylammonium (TBA) with tetrabutylphosphonium (TBP) and tributylsulfonium (TBS). To best understand the retention behavior analytes used for characterization of the IPAs spanned the Hofmeister series; from kosmotropic monoanions (iodate, chloride, nitrite) and intermediate anions (nitrate, bromide) to chaotropic ions (perchlorate, thiocyanate, iodide). The studies demonstrate that tetrabutylphosphonium is the most chaotropic IPA, followed by tetrabutylammonium and finally tributylsulfonium is the least chaotropic. In the case of the chaotropic anions, the retention of perchlorate was least with tributylsulfonium, and greatest for tetrabutylphosphonium, with tetrabutylammonium being intermediate. The multivalent kosmotropic anions (sulfate, chromate, thiosulfate) demonstrated unique selectivity changes depending on the kosmotropic/chaotropic nature of the IPA. Demonstrating increases in retention with increasing IPA concentration only with tributylsulfonium, whereas the more chaotropic IPAs universally decreased the retention of the multivalent anions.     

Cation–Cation Pairing by NCH⋅⋅⋅O Hydrogen Bonds

The pairing of ions of opposite charge is a fundamental principle in chemistry, and is widely applied in synthesis and catalysis. In contrast, cation–cation association remains an elusive concept, lacking in supporting experimental evidence. While studying the structure and properties of 4‐oxopiperidinium salts [OC₅H₈NH₂]X for a series of anions X^? of decreasing basicity, we observed a gradual self‐association of the cations, concluding in the formation of an isolated dicationic pair. In 4‐oxopiperidinium bis(trifluoromethylsulfonyl)amide, the cations are linked by N? H???O?C hydrogen bonds to form chains, flanked by hydrogen bonds to the anions. In the tetra(perfluoro‐tert‐butoxy)aluminate salt, the anions are fully separated from the cations, and the cations associate pairwise by N? C? H???O?C hydrogen bonds. The compounds represent the first genuine examples of self‐association of simple organic cations based merely on hydrogen bonding as evidenced by X‐ray structure analysis, and provide a paradigm for an extension of this class of compounds.     

Fluidity modulation of phospholipid bilayers by electrolyte ions: insights from fluorescence microscopy and microslit electrokinetic experiments

Fluidity and charging of supported bilayer lipid membranes (sBLMs) prepared from 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) were studied by fluorescence recovery after photobleaching (FRAP) and microslit electrokinetic measurements at varying pH and ionic composition of the electrolyte. Measurements in neutral electrolytes (KCl, NaCl) revealed a strong correlation between the membrane fluidity and the membrane charging due to unsymmetrical water ion adsorption (OH(-) ? H(3)O(+)). The membrane fluidity significantly decreased below the isoelectric point of 3.9, suggesting a phase transition in the bilayer. The interactions of both chaotropic anions and strongly kosmotropic cations with the zwitterionic lipids were found to be related with nearly unhindered lipid mobility in the acidic pH range. While for the chaotropic anions the observed effect correlates with the increased negative net charge at low pH, no correlation was found between the changes in the membrane fluidity and charge in the presence of kosmotropic cations. We discuss the observed phenomena with respect to the interaction of the electrolyte ions with the lipid headgroup and the influence of this process on the headgroup orientation and hydration as well as on the lipid packaging.     

Correlation between Dynamic Heterogeneity and Local Structure in a Room‐Temperature Ionic Liquid: A Molecular Dynamics Study of [bmim][PF6]

The complex dynamics of a room‐temperature ionic liquid, 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim][PF₆]), is studied using equilibrium classical molecular dynamics simulations in the temperature range of 250–450 K. The activation energies for the self‐diffusion of ions are around 30–34 kJ mol^?1, with that of the anion a little higher than that for the cation. The electrical conductivity of the liquid is calculated and good agreement with experiments is obtained. Structural relaxation is studied through the decay of coherent (total density–density correlation) and incoherent (self part of density–density correlation) intermediate scattering functions over a range of temperatures and wave vectors relevant to the system. The relaxation data are used to identify and characterize two processes, α and β. The dependence of the two relaxation times on temperature and wave vector is obtained. The dynamical heterogeneity of the ions determined through the non‐Gaussian parameter indicates the motion of the cation to be more heterogeneous than that of the anion. The faster ones among the cations are coordinated to faster anions, while slower cations are surrounded predominantly by slower anions. Thus, the dynamical heterogeneity in this ionic liquid is shown to have structural signatures.     

Chaotropic‐Anion‐Induced Supramolecular Self‐Assembly of Ionic Polymeric Micelles

Traditional micelle self‐assembly is driven by the association of hydrophobic segments of amphiphilic molecules forming distinctive core–shell nanostructures in water. Here we report a surprising chaotropic‐anion‐induced micellization of cationic ammonium‐containing block copolymers. The resulting micelle nanoparticle consists of a large number of ion pairs (≈60 000) in each hydrophobic core. Unlike chaotropic anions (e.g. ClO₄^?), kosmotropic anions (e.g. SO₄^2?) were not able to induce micelle formation. A positive cooperativity was observed during micellization, for which only a three‐fold increase in ClO₄^? concentration was necessary for micelle formation, similar to our previously reported ultra‐pH‐responsive behavior. This unique ion‐pair‐containing micelle provides a useful model system to study the complex interplay of noncovalent interactions (e.g. electrostatic, van der Waals, and hydrophobic forces) during micelle self‐assembly.     

A comparative study of two polymorphs of L‐aspartic acid hydrochloride

Two polymorphs of L‐aspartic acid hydrochloride, C₄H₈NO₄⁺·Cl⁻, were obtained from the same aqueous solution. Their crystal structures have been determined from single‐crystal data collected at 100 K. The crystal structures revealed three‐ and two‐dimensional hydrogen‐bonding networks for the triclinic and orthorhombic polymorphs, respectively. The cations and anions are connected to one another via N—H...Cl and O—H...Cl interactions and form alternating cation–anion layer‐like structures. The two polymorphs share common structural features; however, the conformations of the L‐aspartate cations and the crystal packings are different. Furthermore, the molecular packing of the orthorhombic polymorph contains more interesting interactions which seems to be a favourable factor for more efficient charge transfer within the crystal.     

A 16‐connected three‐dimensional hydrogen‐bonding network in tetrakis(4‐aminopyridinium) perylene‐3,4,9,10‐tetracarboxylate octahydrate

The novel title organic salt, 4C₅H₇N₂⁺·C₂₄H₈O₈⁴⁻·8H₂O, was obtained from the reaction of perylene‐3,4,9,10‐tetracarboxylic acid (H₄ptca) with 4‐aminopyridine (4‐ap). The asymmetric unit contains half a perylene‐3,4,9,10‐tetracarboxylate (ptca⁴⁻) anion with twofold symmetry, two 4‐aminopyridinium (4‐Hap⁺) cations and four water molecules. Strong N—H...O hydrogen bonds connect each ptca⁴⁻ anion with four 4‐Hap⁺ cations to form a one‐dimensional linear chain along the [010] direction, decorated by additional 4‐Hap⁺ cations attached by weak N—H...O hydrogen bonds to the ptca⁴⁻ anions. Intermolecular O—H...O interactions of water molecules with ptca⁴⁻ and 4‐Hap⁺ ions complete the three‐dimensional hydrogen‐bonding network. From the viewpoint of topology, each ptca⁴⁻ anion acts as a 16‐connected node by hydrogen bonding to six 4‐Hap⁺ cations and ten water molecules to yield a highly connected hydrogen‐bonding framework. π–π interactions between 4‐Hap⁺ cations, and between 4‐Hap⁺ cations and ptca⁴⁻ anions, further stabilize the three‐dimensional hydrogen‐bonding network.     

Ion‐Pair Recognition by a Heteroditopic Triazole‐Containing Receptor

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C? H hydrogen‐bonding modes, respectively, of the triazole motif has been prepared. This ion‐pair receptor cooperatively binds halide/monovalent‐cation combinations in an aqueous mixture, with selectivity trends being established by ¹H NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor’s isophthalamide cavity enhance cation recognition. ¹H NMR investigations in solution suggest that the receptor’s triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid‐state X‐ray crystallographic structural analysis of a variety of receptor ion‐pair adducts further demonstrates the dual cation–anion binding role of the triazole group.     

The supramolecular architecture in 4,4′‐bipyridinium bis(hydrogen oxalate)

The asymmetric unit of the title compound, C₁₀H₁₀N₂²⁺·2C₂HO₄⁻, consists of one half of a 4,4′‐bipyridinium cation, which has inversion symmetry, and a hydrogen oxalate anion, in which an intramolecular hydrogen bond exists. The cations and anions are connected by O—H...O, N—H...O and C—H...O hydrogen bonds, forming a two‐dimensional network, whereas π–π stacking interactions involving the 4,4′‐bipyridinium cations lead to the formation of a three‐dimensional supramolecular structure. An unusual deca‐atomic ring is formed between two hydrogen oxalate anions, which are linked side‐to‐side via O—H...O hydrogen‐bonding interactions.     

Investigating the imidazolium–anion interaction through the anion-templated construction of interpenetrated and interlocked assemblies

The interaction between imidazolium cations and coordinating anions is investigated through the anion‐templated assembly of interpenetrated and interlocked structures. The orientation of the imidazolium motif with respect to anion binding, and hence the hydrogen bond donor arrangement, was varied in acyclic receptors, interpenetrated assemblies, and the first mono‐imidazolium interlocked systems. Their anion recognition properties and co‐conformations were studied by solution‐phase ¹H NMR investigations, solid‐state structures, molecular dynamics simulations, and density functional theory calculations. Our findings suggest that the imidazolium‐anion binding interaction is dominated by electrostatics with hydrogen‐bonding contributions having weak orientational dependence.     

A study of salt effects on the complexation between β-cyclodextrins and bile salts based on the Hofmeister series

The Hofmeister series is the ranking of ions according to their ability to strengthen (kosmotropic ions) or weaken (chaotropic ions) hydrophobic interactions. Such ions are therefore expected to affect the strength of cyclodextrin (CD) inclusion complexes and may thereby affect the release of CD bound drug molecules. The influence of Hofmeister ions on the binding constants of complexes between CDs (β-CD and hydroxypropylated β-CD) and bile salts (glycocholate and glycochenodeoxycholate) were examined by isothermal titration calorimetry. The chaotropic anions tended to weaken these inclusion complexes. Conversely, kosmotropic ions increased the binding strength and this effect scaled with the buried hydrophobic surface area. Both effects are relatively weak at physiological ion concentrations and may be neglected for most pharmaceutical purposes.     

Rational Design of Heat‐Set and Specific‐Ion‐Responsive Supramolecular Hydrogels Based on the Hofmeister Effect

Smart supramolecular hydrogels have been prepared from a bolaamphiphilic L ‐valine derivative in aqueous solutions of different salts. The hydrogels respond selectively to different ions and are either reinforced or weakened. In one case, in contrast to conventional systems, the hydrogels are formed upon heating of the system. The use of the hydrogels in the controlled release of an entrapped dye is described as a proof of the potential applications of these systems. The responsive hydrogels were rationally designed by taking into account the noticeable effect of different ions from the Hofmeister series in the solubility of the hydrogelator, which was assessed by using NMR experiments. On the one hand, kosmotropic anions such as sulfate produce a remarkable solubility decrease in the gelator, which is associated with gel reinforcement, as measured by rheological experiments. On the other hand, chaotropic species such as perchlorate weaken the gel. A dramatic effect was observed in the presence of guanidinium chloride, which boosted the solubility of the gelator, in accordance with its chaotropic behaviour reported in protein science. In this case, a direct interaction of the guanidinium species with the carbonyl groups of the hydrogelator is detected by ¹³C NMR spectroscopy. The weakening of this interaction upon a temperature increase allows for the preparation of heat‐set hydrogelating systems.     

Tris(1,9‐diethyl­adeninium) triiodide diiodide

X‐ray analysis of the title compound reveals three crystallographically distinct cations of 1,9‐diethyl­adeninium, two iodide anions and one triiodide anion in the asymmetric unit, giving six residues and the formula 3C₉H₁₄N₅⁺·I₃⁻·2I⁻. Standard purine nomenclature is used to identify the atoms of each adenine moiety. Hydrogen bonding is observed between atoms N6 and N7 of a pair of cations [N⋯N = 2.885 (4)/2.902 (3) and 2.854 (3)/2.854 (3) Å], with additional hydrogen bonding to I⁻ anions via the other N6 H atom [N⋯I = 3.708 (3), 3.738 (3) and 3.638 (3) Å]. The triiodide anion is not involved in hydrogen bonding. The bond lengths and angles of the 1,9‐diethyl­adeninium cations are compared with literature values and confirm the formation of the imine tautomer.     

Multiple N—H...NC,C—H...NC and nitrile...π interactions in 4,4′‐bipyridine‐1,1′‐diium bis(1,1,3,3‐tetracyano‐2‐ethoxypropenide): structure determination and DFT calculations of anion...π cation interaction energies

Polynitrile anions are important in both coordination chemistry and molecular materials chemistry, and are interesting for their extensive electronic delocalization. The title compound crystallizes with two symmetry‐independent half 4,4′‐bipyridine‐1,1′‐diium (bpyH₂²⁺) cations and two symmetry‐independent 1,1,3,3‐tetracyano‐2‐ethoxypropenide (tcnoet⁻) anions in the asymmetric unit. One of the bpyH₂²⁺ ions is located on a crystallographic twofold rotation axis (canted pyridine rings) and the other is located on a crystallographic inversion center (coplanar pyridine rings). The ethyl group of one of the tcnoet⁻ anions is disordered over two sites with equal populations. The extended structure exhibits two separate N—H...NC hydrogen‐bonding motifs, which result in a sheet structure parallel to (010), and weak C—H...NC hydrogen bonds form joined rings. Two types of multicenter CN...π interactions are observed between the bpyH₂²⁺ rings and tcnoet⁻ anions. An additonal CN...π interaction between adjacent tcnoet⁻ anions is observed. Using density functional theory, the calculated attractive energy between cation and anion pairs in the tcnoet⁻...π(bipyridinediium) interactions were found to be 557 and 612 kJ mol⁻¹ for coplanar and canted bpyH₂²⁺ cations, respectively.     

Hydrogen‐bonded sheets in 2‐(2‐aminophenyl)‐1H‐benzimidazol‐3‐ium dihydrogen phosphate

The title salt, C₁₃H₁₂N₃⁺·H₂PO₄⁻, contains a nonplanar 2‐(2‐aminophenyl)‐1H‐benzimidazol‐3‐ium cation and two different dihydrogen phosphate anions, both situated on twofold rotation axes in the space group C2. The anions are linked by O—H...O hydrogen bonds into chains of R₂²(8) rings. The anion chains are linked by the cations, via hydrogen‐bonding complementarities and electrostatic interactions, giving rise to a sheet structure with alternating rows of organic cations and inorganic anions. Comparison of this structure with that of the pure amine reveals that the two compounds generate characteristically different sheet structures. The anion–anion chain serves as a template for the assembly of the cations, suggesting a possible application in the design of solid‐state materials.     

Kosmotropic and chaotropic behavior of hydrated ions in aqueous solutions in terms of expansibility and compressibility parameters

Hydrated ions have fundamental applications in chemical and biological processes. Kosmotropic and chaotropic nature of hydrated ions affect the water structure in solutions depending upon their hydrophobicity or hydrophilicity nature. In present study Kosmotropic and chaotropic behavior of hydrated ions have been explained in terms of volumetric and acoustic parameters like apparent molar volume (V_ϕ), expansibility and compressibility factors for aqueous electrolytic solutions provide useful information about interactions among ions and water molecules. Results of V_ϕ showed that SO₄²⁻ ions due to stronger H-bonding with water molecules are termed as kosmotropes while Cl⁻ and HCO₃^– are chaotropes due to their weaker H-bonding with water molecules. More compressible structure of solutions in the presence of SO₄²⁻ ions indicated its kosmotropic behavior and comparatively less compressible structure of solutions in the presence of Cl⁻¹ and HCO₃^– ions renders them chaotropes. Results obtained from expansibility factor showed the dominance of electrostatic interactions over hydrophobic hydration of ions at higher temperatures. Greater values of expansibility factor for SO₄²⁻ ions as compared to Cl⁻¹ and HCO₃^– ions renders them kosmotropic ion while later are termed as chaotropes. Hence, thermo-acoustic parameters could be effectively used to describe the hydrogen bonding character of ionic solutions in terms of kosmotropic and chaotropic behavior of solutions.     

Effects of Hofmeister anions on the flocculation behavior of temperature-responsive poly(N-isopropylacrylamide) microgels

Effects of some sodium salts (NaCl, NaClO₃, and NaSCN) in the Hofmeister series on deswelling and temperature-induced aggregation behavior of microgels of poly(N-isopropylacrylamide) (PNIPAAM) and PNIPAAM-co-PAA with attached poly(acrylic acid) moieties were investigated with the aid of turbidimetry and dynamic light scattering. Addition of salt in the concentration range 0.1–0.5?M generated aggregation of the PNIPAAM microgel particles at elevated temperatures, but it was no distinct difference between chaotropic and kosmotropic anions. In contrast, the flocculation behavior at high temperatures for PNIPAAM-co-PAA revealed a prominent influence of salinity and type of anion on the formation of aggregates. The aggregation transition was shifted to the highest temperature for the most chaotropic anion (SCN^?), and the aggregation transition at the same salt concentration is consistent with the typical Hofmeister series. The turbidity results from the PNIPAAM-co-PAA microgels disclosed a two-step transition for the considered anions, and both a low and high temperature change in the turbidity data was observed. The high-temperature transition followed the Hofmeister series.