Ion-pair Interactions Research Articles

Chiral Sensor for Enantiodiscrimination of Varied Acids.

A chiral thiophosphoroamide 4 derived from (1R,2R)-1,2-diaminocyclohexane is used as a highly effective chiral sensor for the chiral recognition of varied acids via ion-pairing and hydrogen-bonding interactions using (1)H, (19)F and (31)P NMR.

Ion-pairing and aggregation of ionic liquid-neutralized polyoxometalate salts in aqueous solutions

The ion-pairing interaction for the ionic liquids (ILs)-based polyoxometalate (POM) salts in the aqueous solution is critical to the chemistry of the ILs-POM hybrid. Herein, for the first time, molecular dynamics simulations have been performed to investigate the aqueous solutions of the representative α-PW12O403− Keggin anion (PW3−), neutralized by three different types of alkyl-functionalized imidazole-typed IL cations (1-alkyl-3-methylimidazolium cations, alkyl: n-ethyl, n-butyl, and n-octyl). The simulations provide a direct visual observation on the aggregate morphology for the POM anions in aqueous solution. Meanwhile, distinctive ion-pairing interaction mechanism and interfacial solvation behavior have been revealed. Our simulation results indicate that the alkyl-functionalized imidazole cations can affect the effective interaction between the charged POM anions, that is, the alkyl chain length in imidazolium cation could be applied to regulate the aggregation behavior among the negative POM anions. The unique ion-pairing phenomena can be ascribed to the variable modulating effects from neutralizing IL-typed cations, which not only acts as the screening counterions of electrostatic repulsion, but also induces the alkyl chain-anion interaction and overcomes the anion-anion repulsion.

Water-Mediated Ion Pairing: Occurrence and Relevance

We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li(+) and Na(+) and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems.

Experimental Binding Energies in Supramolecular Complexes.

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity.

The deployment of transformational nonaqueous CO2-capture solvent systems is encumbered by high viscosities even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we present key molecular features that control bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of internal hydrogen bonds between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed here are readily extensible to other CO2 capture technologies.

Experimental Investigation of Wettability Alteration in Oil-Wet Reservoirs Containing Heavy Oil

Summary Solvent injection is an effective way to lower the viscosity of heavy oil and is considered when thermal techniques are not practically applicable. However, for the economic success of the method, large fractions of the injected solvent must be recovered. This requires further treatments of the reservoir, including water injection with chemicals to penetrate into the oil-wet matrix by changing the wettability. In this paper, we investigate the effect of wettability alteration of the oil-wet rock on solvent as well as oil recovery. Different wettability-alteration agents were tested, including cationic and anionic surfactants, ionic liquids, nanofluids, high-pH solutions, and low-salinity water. The potential of these materials to modify the wettability of aged sandstone and limestone samples was evaluated by use of imbibition tests. After conducting a total of 35 experiments, the most-promising wettability-alteration agents were identified to be anionic surfactants and high-pH solutions in addition to low-salinity water for the sandstone cases. Ion-pair interaction in sandstone and the dissolution of mixed-wet clay particles in carbonate are the main mechanisms of wettability alteration by those chemicals. Cationic surfactants and high-pH solutions were identified as the best wettability modifiers for the limestone samples. Although cationic surfactant changes the wettability by the ion-pair-interaction mechanism, the pH solution is believed to restore the water-wetness by decreasing the attraction forces between calcite and organic components. Ionic liquid at low concentration is able to alter the wettability of carbonate better than other conventional wettability modifiers. One important finding of this work is that solvent injection in heavy-oil-containing reservoirs is essential to condition the reservoir (i.e., to dilute the heavy oil before any wettability-alteration treatment can take place).

Two fold modified chitosan for enhanced adsorption of hexavalent chromium from simulated wastewater and industrial effluents

Ionic solid (Ethylhexadecyldimethylammoniumbromide) impregnated phosphated chitosan (ISPC) was synthesized and applied for enhanced adsorption of hexavalent chromium from industrial effluent. The compound obtained was extensively characterized using instrumental techniques like FT-IR, TGA-DTA, XRD, SEM, BET and EDX. ISPC showed high adsorption capacity of 266.67mg/g in accordance with Langmuir isotherm model at pH 3.0 due to the presence of multiple sites which contribute for ion pair and electrostatic interactions with Cr(VI) species. The sorption kinetics and thermodynamic studies revealed that adsorption of Cr(VI) followed pseudo-second-order kinetics with exothermic and spontaneous behaviour. Applicability of ISPC for higher sample volumes was discerned through column studies. The real chrome plating industry effluent was effectively treated with total chromium recovery of 94%. The used ISPC was regenerated simply by dilute ammonium hydroxide treatment and tested for ten adsorption-desorption cycles with marginal decrease in adsorption efficiency.

Nanostructuration Effect on the Thermal Behavior of Ionic Liquids.

This work shows how the nanostructuration of ionic liquids (ILs) governs the glass and melting transitions of the bistriflimide imidazolium-based [Cn C1 im][NTf2 ] and [Cn Cn im][NTf2 ] series, which highlights the trend shift that occurs at the critical alkyl size (CAS) of n=6. An initial increase in the glass temperature (Tg ) with an increase in the alkyl side chain was observed due to the intensification of the dispersive interactions (van der Waals). Above the CAS, the -CH2 - increment has the same effect in both glass and liquid states, which leads to a plateau in the glass transition after nanostructuration. The melting temperature (Tm ) of the [Cn C1 im][NTf2 ] and [Cn Cn im][NTf2 ] series presents a V-shaped profile. For the short-alkyl ILs, the -CH2 - increment affects the electrostatic ion pair interactions, which leads to an increase in the conformational entropy. The -CH2 - increment disturbs the packing ability of the ILs and leads to a higher entropy value (ΔslSm○ ) and consequently a decrease in Tm . Above the CAS, the -CH2 - contribution to the melting temperature becomes more regular, as a consequence of the nanostructuration of the IL into polar and nonpolar domains. The dependence of the alkyl chain on the temperature, enthalpy, and entropy of melting in the ILs above the CAS is very similar to the one observed for the alkane series, which highlights the importance of the nonpolar alkyl domains on the ILs thermal behavior.

Bipyridinium Polymers That Dock Tetrathiafulvalene Guests in Water Driven by Donor-Acceptor and Ion Pair Interactions.

Two water-soluble para-xylylene-connected 4,4'-bipyridinium (BIPY(2+) ) polymers have been prepared. UV-Vis absorption, (1) H NMR spectroscopy, and cyclic voltammetry experiments support that in water the BIPY(2+) units in the polymers form stable 1:1 charge-transfer complexes with tetrathiafulvalene (TTF) guests that bear two or four carboxylate groups. These charge-transfer complexes are stabilized by the donor-acceptor interaction between electron-rich TTF and electron-deficient BIPY(2+) units and electrostatic attraction between the dicationic BIPY(2+) units and the anionic carboxylate groups attached to the TTF core. On the basis of UV-Vis experiments, a lower limit to the apparent association constant of the TTF⋅BIPY(2+) complexes of the mixtures, 1.8×10(6) m(-1) , has been estimated in water. Control experiments reveal substantially reduced binding ability of the neutral TTF di- and tetracarboxylic acids to the BIPY(2+) molecules and polymers. Moreover, the stability of the charge-transfer complexes formed by the BIPY(2+) units of the polymers are considerably higher than that of the complexes formed between two monomeric BIPY(2+) controls and the dicarboxylate-TTF donor; this has been attributed to the mutually strengthened electron-deficient nature of the BIPY(2+) units of the polymers due to the electron-withdrawing effect of the BIPY(2+) units.

Acylated-naproxen as the surface-active template in the preparation of micro- and nanospherical imprinted xerogels by emulsion techniques

A strategy based on water-in-oil emulsion for the dispersion of a sol–gel mixture into small droplets was employed with the view of the production of naproxen-imprinted micro- and nanospheres. The procedure, aiming at a surface imprinting process, comprised the synthesis of a naproxen-derived surfactant. The imprinting process occurred at the interface of the emulsions or microemulsions, by the migration of the NAP-surfactant head into the sol–gel drops to leave surficial imprints due mainly to ion-pair interaction with a cationic group contained within the growing sol–gel network. The surface-imprinted microspheric particles exhibited a log-normal size distribution with geometric mean diameter of 3.1μm. A mesoporous texture was found from measurements of the specific surface area (206m2/g) and pore diameter (Dp 2nm). Evaluation of the microspheres as packed HPLC stationary phases resulted in the determination of the selectivity factor against ibuprofen (α=2.1), demonstrating the successful imprinting. Chromatographic efficiency, evaluated by the number of theoretical plates (222platescm−3), emerged as an outstanding feature among the set of all relatable formats produced before, an advantage intrinsic to the location of the imprinted sites on the surface. The material presented a capacity of 3.2μmolg−1. Additionally, exploratory work conducted on their nanoscale counterparts resulted in the production of nanospheres in the size order of 10nm providing good indications of a successful imprinting process.

Investigating Possible Interactions between Ionizable Residues in Model Transmembrane Peptides

The study of charged residues in the hydrophobic interior of transmembrane proteins is crucial to the understanding of membrane proteins and their function. To this end, we have employed GWALP23 (acetyl-GGALW5LALALALALALALW19LAGA-amide), a constructive low-dynamic model peptide, for investigations of single-residue influence on protein-lipid interactions (JBC 285, 31723). We have substituted a single Leu residue at position 12, within the hydrophobic core of the GWALP23 sequence, with either Arg (R12) or Glu (E12). Specific 2H-labeled Ala residues have been incorporated within the -R12 sequence for detection by solid-state 2H NMR. GWALP23-R12 remains charged in DOPC bilayers, even under strongly alkaline pH conditions, and displays multi-state behavior (JACS 132, 5803). To determine if the presence of an adjacent peptide with Glu at position 12 would influence the behavior of the -R12 peptide, we incorporate a mixture of labeled GWALP23-R12 and unlabeled -E12 peptides in DOPC bilayers. Preliminary results for oriented samples with both peptides suggest that -R12 remains multi-state in the presence of the -E12 peptide in DOPC bilayers at pH 6. By contrast, oriented samples of -R12 in DLPC bilayers show clearly defined quadrupolar splittings. In the presence of -E12, we observe small changes in the quadrupolar splittings, indicative of changes in orientation of the -R12 peptide. It is possible that peptide crowding within the bilayer may influence ionic interactions between the -E12 and -R12 peptides; therefore we are currently investigating the effect of varying the peptide/peptide and peptide/lipid ratios. Similar experiments with GW3,21ALP23-R12, with the Trp residues moved to the opposite face of the helix, also suggest interaction between -E12 and -R12 in both DLPC and DOPC bilayers at pH 6. Additionally, we are studying possible ion-pair interactions in a peptide with two ionizable residues, GWALP23-R12,E16.

Characterization of oncogene-induced metabolic alterations in hepatic cells by using ultrahigh performance liquid chromatography-tandem mass spectrometry

Elucidation of altered metabolic pathways by using metabolomics may open new avenues for basic research on disease mechanisms and facilitate the development of novel therapeutic strategies. Here, we report the development of ultrahigh performance liquid chromatography-tandem mass spectrometry-based metabolomics platform with capability of measuring both cationic and anionic intermediates in cellular metabolism. The platform was established based on the hydrophobic ion-pairing interaction chromatography coupled with tandem mass spectrometry in multiple reaction monitoring (MRM) mode. The MRM transitions were created and optimized via energy-resolved collision-induced dissociation experiments, serving as an essential reference point for the quantification and identification. For chromatographic separation, application of hydrophobic ion-pairing interaction led to dramatic enhancement on retention of water-soluble metabolites and provision of good peak shapes. Two volatile ion-pairing reagents, namely heptafluorobutyric acid and tributylamine, were used with dedicated C18 columns as complementary separation systems coupled with the MRM analysis, allowing measurement of the metabolites of interest at nanomolar levels. The developed platform was successfully applied to investigate the altered metabolism in hepatic cells with over-expression of an oncogene, thus can provide important information on the rewired metabolism.

Tuning interionic interaction by rationally controlling solution pH for highly selective colorimetric sensing of arginine.

Direct selective sensing of arginine in central nervous systems remains very essential to understanding of the molecular basis of some physiological events. This study presents the first demonstration on a simple yet effective method for arginine sensing with gold nanoparticles (Au-NPs) as the signal readout. The rationale for the method is based on the pH-dependent feature of the interionic interaction between cysteine and arginine. At pH 6.0, cysteine can only interact with arginine through the ion-pair interaction and such interaction can lead to the changes in both the solution color and UV-vis spectrum of the cysteine-protected Au-NPs upon the addition of arginine. These changes are further developed into an analytical strategy for effective sensing of arginine by rationally controlling the pH values of Au-NP dispersions with the ratio of the absorbance at 650 nm (A 650) to that at 520 nm (A 520) (A 650/A 520) as a parameter for analysis. The method is responsive to arginine without the interference from other species in the cerebral system; under the optimized conditions, the A 650/A 520 values are linear with the concentration of arginine within a concentration range from 0.80 to 64 μM, yet remain unchanged with the addition of other kinds of amino acids or the species in the central nervous system into the Au-NPs dispersion containing cysteine. The method demonstrated here is reliable and robust and could thus be used for detection of the increase of arginine in central nervous systems.

Does hydrohalic acid HX (X = F, Cl) form true N-protonated twisted amide salts? Effects of anions on the ion-pair interactions and on the amide moiety in N-protonated tricyclic twisted amide salts

The [BF4]− and [RSO3]− anions interact with N-protonated amide cations through N–H⋯F and N–H⋯O strong hydrogen bonds and hydrohalic acids form very weak N⋯H–X hydrogen bonds.

A novel graphene-based label-free fluorescence 'turn-on' nanosensor for selective and sensitive detection of phosphorylated species in biological samples and living cells.

A novel label-free fluorescence 'turn-on' nanosensor has been developed for highly selective and sensitive detection of phosphorylated species (Ps) in biological samples and living cells. The design strategy relies on the use of Ti(4+)-immobilized polydopamine (PDA) coated reduced graphene oxide (rGO@PDA-Ti(4+)) that serves as an attractive platform to bind riboflavin 5'-monophosphate molecules (FMNs) through ion-pair interactions between phosphate groups and Ti(4+). The as-prepared rGO@PDA-Ti(4+)-FMNs (nanosensor), fluoresce only weakly due to the ineffective Förster resonance energy transfer between the FMNs and rGO@PDA-Ti(4+). The experimental findings revealed that the microwave-assisted interaction of the nanosensor with α-, β-casein, ovalbumin, human serum, non-fat milk, egg white, and living cells (all containing Ps) releases FMNs (due to the high formation constant between phosphate groups and Ti(4+)), leading to an excellent fluorescence 'turn-on' response. The fluorescence spectroscopy, confocal microscopy, and MALDI-TOF MS spectrometry were used to detect Ps both qualitatively and quantitatively. Under the optimized conditions, the nanosensor showed a detection limit of ca. 118.5, 28.9, and 54.8 nM for the tryptic digests of α-, β-casein and ovalbumin, respectively. Furthermore, the standard addition method was used as a bench-mark proof for phosphopeptide quantification in egg white samples. We postulate that the present quantitative assay for Ps holds tremendous potential and may pave the way to disease diagnostics in the near future.

Metal Ions Analysis with Capillary Zone Electrophoresis.

Capillary electrophoresis has recently attracted considerable attention as a promising analytical technique for metal ion separations. Significant advances that open new application areas for capillary electrophoresis in the analysis of metal species occurred based on various auxiliary separation principles. These are mainly due to complexation, ion pairing, solvation, and micellization interactions between metal analytes and electrolyte additives, which alter the separation selectivity in a broad range. Likewise, many separation studies for metal ions have been concentrated on the use of preelectrophoresis derivatization methodology. Approaches suitable for manipulation of selectivity for different metal species including metal cations, metal complexes, metal oxoanions, and organometallic compounds, are discussed, with special attention paid to the related electrophoretic system variables using illustrative examples.

Encumbrance in desilylation triggered fluorogenic detection of the fluoride ion - a kinetic approach.

Highly selective fluorogenic detection of the fluoride ion becomes viable due to its propensity towards cleaving Si-O and Si-C bonds, the key reactive elements in fluoride selective chemodosimeters. Herein, acridinedione derived, two novel fluorescent probes bearing tertiarybutyldiphenylsilyloxy (TBDPS) and tertiarybutyldimethylsilyloxy (TBDMS) groups were synthesized and their fluoride selective dosimetric action in organic solvents and in mixed aqueous medium was established through steady state and time resolved fluorescence techniques. Unusually, these molecular probes maintain their sensitivity down to 10 ppb in both organic and mixed aqueous medium; hence they can be considered as highly selective and sensitive fluorescent probes for the fluoride anion. By following the kinetics of the desilylation process it is established that the reaction follows second order kinetics with respect to fluoride ion concentration in acetonitrile whereas it becomes first order in mixed aqueous medium owing to its high degree of hydration. Also, the hydrophobic and sterically crowded substitution on the silyl receptor hampers the reaction kinetics only in organic solvents whereas its influence in mixed aqueous medium is relatively very less. However, common inorganic cations (Na+) effectively hinder the reaction kinetics through strong ion pair interaction and prolong the response time. Therefore, the indigenous influences of three different factors which encumber the desilylation process were quantitatively enumerated and the prospective application of these fluorescent probes in detecting and validating fluoride ions in various environmental samples is demonstrated.

Ferrocenyl chiral bisphosphorus ligands for highly enantioselective asymmetric hydrogenation via noncovalent ion pair interaction.

A new class of ferrocenyl chiral bisphosphorus ligand, Wudaphos, was developed, and exhibits excellent ee and activity (ee up to 99%, TON up to 20 000) for the asymmetric hydrogenation of both 2-aryl and 2-alkyl acrylic acids through ion pair noncovalent interaction under base free and mild reaction conditions. Well-known anti-inflammatory drugs such as naproxen and ibuprofen together with the intermediate for the preparation of Roche ester and some bioactive compounds were also efficiently obtained with excellent ee. Control experiments were conducted and revealed that the ion pair noncovalent interaction and chain length played important roles.

Analysis and modeling of alkali halide aqueous solutions

A new model is proposed for correlation and prediction of thermodynamic properties of electrolyte solutions. In the proposed model, terms of a second virial coefficient-type and of a KT-UNIFAC model are used to account for a contribution of binary interactions between ion and ion, and water and ion, respectively, with a Debye–Hückel term for electrostatic interactions. In a second approach of the model, additional parameters for interactions of ion pairs in the KT-UNIFAC are introduced as a correction to get better agreement with data. Structural parameters of ions used in the framework of UNIFAC or UNIQUAC are newly estimated using ionic radii for physically correct representation of the combinatorial part. Including temperature-dependent coefficients in the interaction parameters, significant improvements in accuracy are achieved for a wide range of temperatures. This work is focused on calculations for various electrolyte properties of alkali halide aqueous solutions such as mean ionic activity coefficients, osmotic coefficients, and salt solubilities. The model covers highly nonideal electrolyte systems such as lithium chloride, lithium bromide and lithium iodide, that is, systems that are very soluble in water, for example, up to more than 30 mol kg−1. Phase behaviors for the systems are analyzed at concentrations of salt up to the solubility in water at temperatures between 273 and 373 K by comparing calculated results with available experimental data and available models.

Diversification of β-Augmentation Interactions between CDI Toxin/Immunity Proteins

Contact-dependent growth inhibition (CDI) is a widespread mechanism of inter-bacterial competition mediated by the CdiB/CdiA family of two-partner secretion proteins. CdiA effectors carry diverse C-terminal toxin domains (CdiA-CT), which are delivered into neighboring target cells to inhibit growth. CDI+ bacteria also produce CdiI immunity proteins that bind specifically to cognate CdiA-CT toxins and protect the cell from auto-inhibition. Here, we compare the structures of homologous CdiA-CT/CdiI complexes from Escherichia coli EC869 and Yersinia pseudotuberculosis YPIII to explore the evolution of CDI toxin/immunity protein interactions. Both complexes share an unusual β-augmentation interaction, in which the toxin domain extends a β-hairpin into the immunity protein to complete a six-stranded anti-parallel sheet. However, the specific contacts differ substantially between the two complexes. The EC869 β-hairpin interacts mainly through direct H-bond and ion-pair interactions, whereas the YPIII β-hairpin pocket contains more hydrophobic contacts and a network of bridging water molecules. In accord with these differences, we find that each CdiI protein only protects target bacteria from its cognate CdiA-CT toxin. The compact β-hairpin binding pocket within the immunity protein represents a tractable system for the rationale design of small molecules to block CdiA-CT/CdiI complex formation. We synthesized a macrocyclic peptide mimic of the β-hairpin from EC869 toxin and solved its structure in complex with cognate immunity protein. These latter studies suggest that small molecules could potentially be used to disrupt CDI toxin/immunity complexes.