Strong Ion Pairing Research Articles

Stability of DNA in Hydrated Ionic Liquids: Effect of Solvent Nanostructuring

Recently, hydrated ionic liquids (ILs) have been identified as novel solvent media in conferring greater stability on proteins and DNA. In an earlier study with a conventional imidazolium-based IL, we established intrusion of the IL cation into the solvation shell of DNA, particularly the minor groove, as a key factor in long-term stability. Nonetheless, there is a continuous surge to find and develop nontoxic ILs for biological applications. Thus, we extended our investigations to DNA stability in biocompatible hydrated choline-based ILs with a range of anions. Results from molecular dynamics simulations and CD spectroscopic data suggest that the native B-conformation of DNA is retained in these ILs. DNA stability is majorly attributable to IL cation-DNA interactions, and for a given anion, the imidazolium cation shows stronger binding to DNA than cholinium. The extent of DNA dehydration by these diverse classes of ILs is, however, comparable. The solvent microstructure revealed strong ion pairing in ILs composed of the cholinium cation and carboxylate anions. ILs of such kind, by virtue of their larger volume compared to ILs with solvent-separated ion-pairs, can dehydrate DNA effectively, despite being weakly binding ligands. Conversely, the imidazolium cations dehydrate by integrating into the DNA grooves through strong electrostatic and hydrogen bonding interactions, closely resembling groove binding drugs. The implications of this differential mode of dehydration, and consequent stability, of DNA by the two classes of ILs are expected to find relevance in different areas of DNA-IL-based applications.

Thermodynamic Studies of Aqueous Solutions of 2,2,2-Cryptand at 298.15 K: Enthalpy–Entropy Compensation, Partial Entropies, and Complexation with K+ Ions

The osmotic coefficient measurements for binary aqueous solutions of 2,2,2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8] hexacosane) in the concentration range of ~0.009 to ~0.24 mol·kg(-1) and in ternary aqueous solutions containing a fixed concentration of 2,2,2-cryptand of ~0.1 mol·kg(-1) with varying concentration of KBr (~0.06 to ~0.16 mol·kg(-1)) have been reported at 298.15 K. The diamine gets hydrolyzed in aqueous solutions and needs proper approach to obtain meaningful thermodynamic properties. The measured osmotic coefficient values are corrected for hydrolysis and are used to determine the solvent activity and mean ionic activity coefficients of solute as a function of concentration. Strong ion-pair formation is observed, and the ion-pair dissociation constant for the species [CrptH](+)[OH(-)] is reported. The excess and mixing thermodynamic properties (Gibbs free energy, enthalpy, and entropy changes) have been obtained using the activity data from this study and the heat data reported in the literature. Further, the data are utilized to compute the partial molal entropies of solvent and solute at finite as well as infinite dilution of 2,2,2-cryptand in water. The concentration dependent non-linear enthalpy-entropy compensation effect has been observed for the studied system, and the compensation temperature along with entropic parameter are reported. Using solute activity coefficient data in ternary solutions, the transfer Gibbs free energies for transfer of the cryptand from water to aqueous KBr as well as transfer of KBr from water to aqueous cryptand were obtained and utilized to obtain the salting constant (ks) and thermodynamic equilibrium constant (log K) values for the complex (2,2,2-cryptand:K(+)) at 298.15 K. The value of log K = 5.8 ± 0.1 obtained in this work is found to be in good agreement with that reported by Lehn and Sauvage. The standard molar entropy for complexation is also estimated for the 2,2,2-cryptand-KBr complex in aqueous medium.

Synergistic effects of mutations in cytochrome P450cam designed to mimic CYP101D1.

A close orthologue to cytochrome P450cam (CYP101A1) that catalyzes the same hydroxylation of camphor to 5-exo-hydroxycamphor is CYP101D1. There are potentially important differences in and around the active site that could contribute to subtle functional differences. Adjacent to the heme iron ligand, Cys357, is Leu358 in P450cam, whereas this residue is Ala in CYP101D1. Leu358 plays a role in binding of the P450cam redox partner, putidaredoxin (Pdx). On the opposite side of the heme, about 15-20 Å away, Asp251 in P450cam plays a critical role in a proton relay network required for O2 activation but forms strong ion pairs with Arg186 and Lys178. In CYP101D1 Gly replaces Lys178. Thus, the local electrostatic environment and ion pairing are substantially different in CYP101D1. These sites have been systematically mutated in P450cam to the corresponding residues in CYP101D1 and the mutants analyzed by crystallography, kinetics, and UV-vis spectroscopy. Individually, the mutants have little effect on activity or structure, but in combination there is a major drop in enzyme activity. This loss in activity is due to the mutants being locked in the low-spin state, which prevents electron transfer from the P450cam redox partner, Pdx. These studies illustrate the strong synergistic effects on well-separated parts of the structure in controlling the equilibrium between the open (low-spin) and closed (high-spin) conformational states.

Homogenous polyelectrolyte complex membranes incorporated with strong ion-pairs with high pervaporation performance for dehydration of ethanol

Solution-processable polyelectrolyte complexes (PECs) incorporated with both sulfate (OSO3) groups and carboxylic acid groups (COOH) were prepared from sulfated sodium carboxymethyl cellulose (SCMC) and poly(diallydimethylammonium chloride) (PDDA). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were utilized to characterize the chemical structures of SCMC and the compositions of SCMC/PDDA PECs. Membranes were made from PECs (SPECMs), and their surface properties were studied by field emission scanning electron microscopy and contact angle characterizations. Moreover, SPECMs were subjected to pervaporation dehydration of aqueous ethanol solution. Effects of SPECMs’ chemical compositions and operation conditions on pervaporation performance were studied and discussed in terms of membrane structures. It was found that OSO3 groups were incorporated into SPECMs in the form of sulfate–amine ion-pairs, and they improved both the selectivity and flux in the dehydration of aqueous ethanol solution. For instance, the flux and water content in permeate for SPECM-0.34 in dehydration of 10wt% water–ethanol mixture at 70°C were 1760gm−2h−1 and 98.7wt%, respectively, both of which were much higher than those for pristine PEC membranes with no sulfate groups. Moreover, the high separation performance of SPECMs was stable and both the tensile strength and elongation at break increased with increasing strong ion-pairs content. For instance, the SPECM-0.34 tensile strength and elongation at break were 64.5MPa and 9.33%.

Conformational transition of Poly (vinylbenzyltrimethylammonium chloride) (PVBTMAC) brush in the presence of Hofmeister anions

This study pertains to the conformational changes of Poly (vinylbenzyltrimethylammonium chloride) (PVBTMAC) brush onto SiO2-coated resonator; which was further incorporated by using quartz crystal microbalance with dissipation (QCM-D) in the presence of Hofmeister series of anions as a function of ionic strength. Gradual shrinkage of highly extended PVBTMAC followed by loose inhomogeneous layer was on account of change in frequency (∆f) and dissipation (ΔD) at low ionic strength. On the contrary, the increase in ionic strength collapses chains and dense homogenous layer is formed. The chaotropic anions comparativley bring drastic change in frequency unlike kosmotropic anions. This is because weakly hydrated chaotropic anions form strong ion pair with weakly hydrated trimethyl ammonium group on the PVBTMAC brush and as a result strong counter ion condensation was observed on polymer chain. Thus, the apparent mass associated with PVBTMAC chains increases that accounts for large change in ∆f.

Acid–base chemistry enables reversible colloid-to-solution transition of asphaltenes in non-polar systems

The conjugated π-bonding in asphaltenes, a naturally occurring member of the polyaromatic hydrocarbon family, provides a unique platform for investigating electrostatics and electronics in non-polar systems, but at the same time causes asphaltenes to be insoluble in all except aromatic liquids. Asphaltenes precipitate from petroleum fluids under a variety of conditions, including depressurization and compositional changes, plaguing both recovery operations and remediation in the case of equipment failure. Aromatic solvents like toluene dissolve asphaltenes, but only at very high concentrations, nearly 50% by weight. Polymeric dispersants can stabilize asphaltene colloids, and in some cases can inhibit asphaltene precipitation entirely. Strong organic acids such as dodecyl benzene sulfonic acid (DBSA) can dissolve precipitated asphaltenes when introduced in concentrations as little as 1 percent by weight. Here we demonstrate for the first time that DBSA enables a reversible transition from unstable to stable colloidal-scale asphaltene suspensions to molecularly stable solutions. A continuum of acid–base reactions explains the apparent dual-action of DBSA. The suspension–solution transition occurs through the protonation of heteroatomic asphaltene components and subsequent strong ion pairing with DBSA sulfonate ions, effectively forming DBSA-doped asphaltene complexes with a solvation shell.

Ionic Association and Solvation of the Ionic Liquid 1-Hexyl-3-methylimidazolium Chloride in Molecular Solvents Revealed by Vapor Pressure Osmometry, Conductometry, Volumetry, and Acoustic Measurements

A systematic study of osmotic coefficient, conductivity, volumetric and acoustic properties of solutions of ionic liquid 1-hexyl-3-methylimidazolium chloride ([C(6)mim][Cl]) in various molecular solvents has been made at different temperatures in order to study of ionic association and solvation behavior of [C(6)mim][Cl] in different solutions. Precise measurements on electrical conductances of solutions of [C(6)mim][Cl] in water, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and acetonitrile at 293.15, 298.15, and 303.15 K are reported and analyzed with Barthel's low-concentration chemical model (lcCM) to obtain the limiting molar conductivities and association constants of this ionic liquid in the investigated solvents. Strong ion pairing was found for the ionic liquid in 2-propanol, 1-butanol, and 1-propanol, whereas ion association in acetonitrile, methanol and ethanol is rather weak and in water the ionic liquid is fully dissociated. In the second part of this work, the apparent molar volumes and isentropic compressibilities of [C(6)mim][Cl] in water, methanol, ethanol, acetonitrile, 1-propanol, 2-propanol, and 1-butanol are obtained at the 288.15-313.15 K temperature range at 5 K intervals at atmospheric pressure from the precise measurements of density and sound velocity. The infinite dilution apparent molar volume and isentropic compressibility values of the free ions and ion pairs of [C(6)mim][Cl] in the investigated solvents as well as the excess molar volume of the investigated solutions are determined and their variations with temperature and type of solvents are also studied. Finally, the experimental measurements of osmotic coefficient at 318.15 K for binary solutions of [C(6)mim][Cl] in water, methanol, ethanol, 2-propanol, and acetonitrile are taken using the vapor pressure osmometry (VPO) method and from which the values of the solvent activity, vapor pressure, activity coefficients, and Gibbs free energies are calculated. The results are interpreted in terms of ion association, ion-dipole interactions, and structural factors of the ionic liquid and investigated organic solvents. The ionic liquid is solvated to a different extent by the molecular solvents, and ionic association is affected significantly by ionic solvation.

Oligonucleotide Bioanalysis: Sensitivity Versus Specificity

Oligonucleotide Bioanalysis: Sensitivity Versus Specificity

1-Ethyl-3-methylimidazolium Ethylsulfate in Water, Acetonitrile, and Dichloromethane: Molar Conductivities and Association Constants

Molar conductivities, Λ, of dilute solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate in water, acetonitrile (AN), and dichloromethane (DCM) were determined as a function of temperature (AN: T = (273.15 to 313.15) K; DCM: T = (273.15 to 308.15) K; water: T = (278.15 to 313.15) K) in the concentration range c = (≈0.25 to ≈5)·10−3 mol·dm−3. The data were analyzed with Barthel's low-concentration chemical (lcCM) model to obtain the limiting molar conductivities, Λ∞(T), and association constants, KA°(T), of this electrolyte in the investigated solvents. From Λ∞(T) the Eyring activation enthalpy of charge transport was determined and found to be slightly larger than the corresponding value for viscous flow of the solvent. Strong ion pairing was found for the electrolyte in DCM (KA° ≈ 6·104 mol−1·dm3), whereas ion association in AN is rather weak (KA° ≈ 40 mol−1·dm3). From the temperature dependence of KA°(T) the enthalpy and entropy of the ion-pairing process were calculated, and ion associ...

Sodium Fluoride at the Air/Water Interface

The zeta potentials of a single N2 bubble in near neutral aqueous solutions of 10–5 to 10–2 M NaF have been measured with a rotating cell zetameter. The zeta potentials in the NaF solutions are slightly more negative than those of comparable NaI concentrations. A simple interpretation of this observation is that the small size of the fluoride ion leads to its stronger adsorption at the interface, despite its smaller dielectric decrement. This would not be consistent with independent electrospray results from 100 µM solutions that bromide ion is less strongly adsorbed than iodide ion, i.e. that surface affinity increases with ion size. An alternative interpretation of the fluoride experiment is that the fluoride surface affinity is the consequence of stronger ion pairing with the NaOH that spontaneously forms at the interface.

The influence of water on the structural and transport properties of model ionic liquids

Molecular dynamics simulations are used to investigate the influence of water on model ionic liquids. Several models, where the ions vary in size, and in the location of the charge with respect to the center of mass, are considered. Particular attention is focused on the variation in transport properties (diffusion coefficients, shear viscosity, and electrical conductivity) with water concentration. An effort is made to identify the underlying physical reasons for water's influence. The results for our model ionic liquids fall loosely into two categories, depending on the molecular characteristics of the constituent ions. If the ion size disparity is not too large (cation:anion diameter ratio < or approximately 2:1), and if the ion charge location is such that directional ion pair bonds are relatively weak, then we find that the ionic diffusion coefficients and the electrical conductivity increase, and the viscosity decreases with increasing water concentration. This agrees with what is commonly observed experimentally for room temperature ionic liquids (RTILs). For these systems, we do not find changes in the equilibrium structure that can account for the strong influence of water on the transport properties. Rather, by varying the molecular mass of water in our simulations, we demonstrate that the dominant effect of water can be dynamical in origin. In RTIL-water mixtures, the molecular mass of water is generally much less than that of the ions it replaces. These lighter water molecules tend to displace much heavier counterions from the ion coordination shells. This reduces caging and increases the diffusivity, which leads to higher conductivities and lower viscosities. For models with a larger ion size disparity (3:1), or in charge-off-center systems, where strong directional ion pairs are important in the pure ionic liquid, the behavior can be quite different. In these systems, the diffusion coefficients and electrical conductivity can still display conventional behavior and increase when water is added even though the reasons for this can be more complex than in the simpler cases noted above. However, in these systems the viscosity can increase, sometimes quite steeply, with increasing water concentration. We trace this unusual behavior to the formation of associated structures, extended anion-water chains that can weave among the cations in the size disparate case, and strongly bound cation-water-anion clusters in the charge-off-center systems.

Ion Pairing and Salt Structure in Organic Salts through Diffusion, Overhauser, DFT and X‐ray Methods

Pulsed gradient spin-echo (PGSE) diffusion characteristics for a) the new [brucinium][X] salts 6 a-f [a: X = BF(4)(-); b: X = PF(6)(-); c: X = MeSO(3)(-), d: X = CF(3)SO(3)(-); e: X = BArF(-); f: X = PtCl(3)(C(2)H(4))(-)], b) 4-tert-butyl-N-benzyl analogue, 7 and c) the aryl carbocations (p-R-C(6)H(4))(2)CH 9 a (R = CH(3)O) and 9 b (R = (CH(3))(2)N), (p-CH(3)O-C(6)H(4))(x)CPh(3-x)(+) 10 a-c (x = 1-3, respectively) and (p-R-C(6)H(4))(3)C(+) 11 (R = (CH(3))(2)N) and 12 (R = H) all in several different solvents, are reported. The solvent dependence suggests strong ion pairing in CDCl(3), intermediate ion pairing in CD(2)Cl(2) and little ion pairing in [D(6)]acetone. (1)H, (19)F HOESY NMR spectra (HOESY: heteronuclear Overhauser effect spectroscopy) for 6 and 7 reveal a specific approach of the anion with respect to the brucinium cation plus subtle changes, which are related to the anion itself. Further, for carbocations 9-12, (all as BF(4)(-) salts) based on the NOE results, one finds marked changes in the relative positions of the BF(4)(-) anion. In these aryl cationic species the anion can be located either a) very close to the carbonium ion carbon b) in an intermediate position or c) proximate to the N or O atom of the p-substituent and remote from the formally positive C atom. This represents the first example of such a positional dependence of an anion on the structure of the carbocation. DFT calculations support the experimental HOESY results. The solid-state structures for 6 c and the novel Zeise's salt derivative, [brucinium][PtCl(3)(C(2)H(4))], 6 f, are reported. Analysis of (195)Pt NMR and other NMR measurements suggest that the eta(2)-C(2)H(4) bonding to the platinum centre in 6 f is very similar to that found in K[PtCl(3)(C(2)H(4))]. Field dependent T(1) measurements on [brucinium][PtCl(3)(C(2)H(4))] and K[PtCl(3)(C(2)H(4))], are reported and suggested to be useful in recognizing aggregation effects.

In-situ fabrication of polyacrylate–silver nanocomposite through photoinduced tandem reactions involving eosin dye

A silver–acrylate nanocomposite was prepared using a novel one-pot strategy involving eosin dye as visible sensitizer and an amine derivative as radicals' source. The mechanism highlighted by steady state photolysis and time-resolved absorption spectroscopy lies on the initial formation of a strong ion-pair complex between eosin and Ag +. Upon visible irradiation, the excited triplet state of the metal complex efficiently abstracts a hydrogen atom from the amine and produces an α-aminoalkyl radical. In acrylate monomer matrix, such a strong reactive species initiates a free radical photopolymerization and also provides the reduction of the silver cation. Through this ‘in-situ’ fabrication method, the kinetics formation of the nanocomposite and its detailed structural analysis are characterized by UV–visible, real-time FTIR absorption spectroscopy and by transmission electron microscopy.

Arsenic speciation in cattail (Typha latifolia) using chromatography and mass spectrometry

Typha latifolia, commonly known as cattail, is widely used as traditional food and medicinal ingredients by indigenous people. There have been concerns over the high levels of total arsenic in cattail plants, but the chemical species of arsenic in cattail have not been characterized. We describe here the determination of arsenic species in the various compartments of cattail. Average concentrations of total arsenic from 9 to 19 cattail plants were 1120 microg/kg (range 68-2600 microg/kg) in the fine (hairy) roots, 575 microg/kg (range 16-1400) in the skin of tuber, 26 microg/kg (range 2-82) in the core of the tuber, 6 microg/kg (range 5-12) in the stem, and 420 microg/kg (range 4-1970) in the whole tuber. Speciation analysis using strong anion exchange, ion pairing, and strong cation exchange chromatography separation with MS detection revealed the presence of inorganic arsenite, arsenate, dimethylarsinic acid, and monomethylarsonic acid. The two inorganic arsenic species accounted for >80% of the total arsenic. Further analyses of arsenic and iron concentrations showed a strong correlation between arsenic and iron in the fine roots and skin. These results suggest that arsenic and iron are colocalized (codeposited) in the skin of the cattail plants, consistent with the previous findings. The level of exposure to arsenic from the use of cattail as food and medicine can be substantially reduced by removing the skin of cattail.

Specificity of Ion−Protein Interactions: Complementary and Competitive Effects of Tetrapropylammonium, Guanidinium, Sulfate, and Chloride Ions

The interactions of ions with a model peptide (a single melittin alpha-helix) in solutions of tetrapropylammonium sulfate or guanidinium chloride were examined by molecular dynamics simulations. The tetrapropylammonium cation shares the geometrical property of essentially flat faces with the previously examined guanidinium cation, and it was found that that this geometry leads to a strong preference for tetrapropylammonium to interact in a similar stacking-type fashion with flat nonpolar groups such as the indole side chain of tryptophan. In contrast to guanidinium, however, tetrapropylammonium does not exhibit strong ion pairing or clustering with sulfate counterions in the solution. Sulfate was found to interact almost exclusively and strongly with the cationic groups of the peptide, such that, already in a 0.1 m solution of tetrapropylammonium sulfate, the 6+ charge of the peptide is effectively locally neutralized. In combination with previous simulations, neutron scattering studies, and experiments on the conformational stability of model peptides, the present results suggest that the Hofmeister series can be explained in higher detail by splitting ions according to the effect they have on hydrogen bonding, salt bridges, and hydrophobic interactions in the protein and how these effects are altered by the counterion.

Electrochemical and Theoretical Investigation of Corannulene Reduction Processes

The voltammetric generation of corannulene anions was investigated over a large range of experimental conditions comprising either "traditional" electrochemical solvents, such as dimethylformamide, acetonitrile, and tetrahydrofuran, or "unconventional" solvents, such as liquid ammonia, liquid methylamine, or liquid dimethylamine, and several different supporting electrolytes. Strong ion pairing effects were found to dominate the electrochemical generation of corannulene higher anions, and through the suitable choice of the solvent/electrolyte system, we observed, for the first time, the reversible electrochemical generation of up to the triply reduced corannulene. The standard potentials obtained experimentally compared rather well with the theoretical values calculated by ab initio and density functional methods, in which solvation and ion pairing effect were explicitly taken into account. In particular, the calculations considered the effect of the electrolyte cation size on ion pairing in order to rationalize the occurrence of the third reduction within the experimental potential window.

Quantitative voltammetry of the reduction of methyl benzoate in THF reveals strong ion pairing of the radical anion with tetra‐n‐butyl cations

AbstractThe reduction of methyl benzoate was studied in tetrahydrofuran (THF), 0.1 M TBAP, both at platinum and glassy carbon macroelectrodes and at platinum microelectrodes. While cyclic voltammograms obtained at macroelectrodes show extensive distortion due to ohmic drop, this distortion is negligible for cyclic voltammograms obtained at microelectrodes. The system methyl benzoate/methyl benzoate radical anion was found to be chemically reversible. The fitting of chronoamperometric measurements using the Shoup and Szabo's expression obtained at microdisc electrodes allowed us to estimate the diffusion coefficient of methyl benzoate. Cyclic voltammetry was modelled allowing the determination of the formal potential, $E_f^0$, of the transfer coefficient, α, and of the standard heterogeneous kinetic constant for the electron transfer, k 0. Furthermore, it was found that the diffusion coefficient for the radical anion produced at the electrode surface has an apparent value of 2.1 × 10−6 ± 0.5 cm2 s−1 (25 °C) which is ca. 10 times smaller than the value obtained for the diffusion coefficient of methyl benzoate (2.1 × 10−5 ± 0.1 cm2 s−1). Taking into account the species present in solution, this result could only be explained by a strong ion pairing between the methyl benzoate anion radical and the tetra‐n‐butylammonium cation used as an electrolyte ion in the solution. The ion pair is found to be stable on the time scale of cyclic voltammetry and no evidence of any following homogeneous chemical reaction was found. Further studies involved the performance of cyclic voltammetry and chronoamperometry at low temperatures (down up to −81 °C) and the analysis of this data allowed the determination of the activation energy, E A, through an Arrhenius plot, both for the diffusion coefficient and for the electron transfer reaction. Copyright © 2008 John Wiley &amp; Sons, Ltd.

An 18-member dinucleating unsymmetrical [N10] macrocycle and its bimetallic complexes M2LCl2(ClO4) (M=Co, Ni, Cu, Zn, Cd or Hg)

The 18-member unsymmetrical dinucleating Shiff-base decaaza [N10] macrocyclic ligand, 2,8,11,17-tetramethyl-3,5,7,12,14,16-hexaphenyl-[1,3,5,7,9,10,12,14,16,18] decaazaoctadeca-2,8,11,17-tetraene dihydroperchlorate (L · 2HClO4) was obtained as a cream colored solid from reaction of N-acetylaniline and hydrazine in the presence of HClO4 followed by cyclization (capping) using aniline and excess HCHO in methanol at RT. Reactions of L · 2HClO4 with MCl2 and [M(Ph3P)2Cl2] afforded air stable complexes, M2LCl4(ClO4)2 (M = Co, Ni, Cu, Zn or Cd). The compounds have been characterized by physico-chemical and spectroscopic studies. FAB–Mass spectral data support bimetallic coordination by the macrocycle. IR studies confirm coordination/encapsulation of metal ions involving unsymmetrical aza sites. Magnetic susceptibility and UV–Visible data indicate a low-spin, five-coordinate geometry for Co(II) and Ni(II) complexes. The anisotropic EPR signals (g ∥ > g ⊥) for Cu2+ complex suggest an axial type tetragonality. Molecular model computations have been employed for a theoretical perspective of the ligand and complexes. The important bond lengths and bond angles have been estimated using CS Chem-3D software. The ionic association constant (K 1) and thermodynamic free energy change (ΔG) were evaluated from the conductometric data of the complexes, which indicate presence of a strong ion-pair even in solution. Cyclic voltametric studies on solutions of M2LCl4(ClO4)2 (M = Co, Ni or Cu) in DMSO indicate reversible and quasi-reversible redox couples.

Electrochemical carboxylation of fluorocontaining imines with preparation of fluorinated N-phenylphenylglycines

A possibility of obtaining fluorine-containing N-phenylphenylglycine derivatives at yields of up to 85% via the electrochemical carboxylation of corresponding benzalanilines was shown. The influence of imine's electron structure, the nature of supporting electrolyte and cathodic material on such processes is examined. It was found, that increasing electron accepting ability of the substituents in benzylidene and aniline fragments of the imine molecule lead to decrease of amino acid yields. The dependence of the N-phenyl- p-fluorophenylglycine yield on the cathode material (Zn, GC, Cu, Ag, Pt) and on the nature of the supporting electrolytes (Bu 4NBr, Et 4NBr, Et 4NClO 4, PhCH 2Me 3NClO 4, LiBF 4, LiClO 4, NaBF 4 and KBF 4) was investigated. The highest amino acid yields were obtained at cathodes (GC and Zn) that do not exhibit specific adsorption of fluorine-containing imines, as well as in the presence of background salts (Alk 4NBr) whose cations do not show tendency to strong ion pairing with anion radicals formed by the electrochemical activation of the imines.

Application of silica-based hyper-crosslinked sulfonate-modified reversed stationary phases for separating highly hydrophilic basic compounds

The separation and determination of hydrophilic basic compounds are of great importance in many fields including clinical and biological research, pharmaceutical development and forensic analysis. However, the most widely used analytical separation technique in these disciplines, reversed-phase liquid chromatography (RPLC), usually does not provide sufficient retention for several important classes of highly hydrophilic basic compounds including catecholamines, many drug metabolites and many drugs of abuse. Commonly eluents having little or no organic modifier and/or strong ion pairing agents must be used to achieve sufficient retention and separation. Use of highly aqueous eluents can lead to column failure by dewetting, resulting in poor retention, low selectivity and irreproducibility and slow recovery of performance. The use of a strong ion pairing agent to increase retention renders the separation incompatible with mass spectrometric detection and complicates preparative separations. This paper describes the successful applications of a novel type of silica-based, hyper-crosslinked, sulfonate-modified reversed stationary phase, denoted as −SO 3–HC–C 8–L, for the separation of highly hydrophilic cations and related compounds by a hydrophobically assisted cation-exchange mechanism. Compared to conventional reversed-phases, the −SO 3–HC–C 8–L phase showed significantly improved retention and separation selectivity for hydrophilic amines. Concurrently, due to the presence of both cation-exchange and reversed-phase retention mechanisms and the high acid stability of hyper-crosslinked phases, the separation can be optimized by changing the type or concentration of ionic additive or organic modifier, and by varying the column temperature. In addition, gradients generated by programming the concentration of either the ionic additive or the organic modifier can be applied to reduce the analysis time without compromising resolution. Furthermore, remarkably different chromatographic selectivities, especially toward cationic solutes, were observed upon comparing the −SO 3–HC–C 8–L phase with conventional reversed-phases. We believe that the combination of these two types of stationary phases will be very useful in two-dimensional liquid chromatography.