Counterion selectivity in cationic polyelectrolytes

Chloride ion activity coefficients in aqueous solutions of several cationic copolymers have been determined using ion-selective electrodes in both the absence and the presence of simple univalent and divalent salts. Without added salt, the activity coefficient depends on the polymer concentration. It increases with increasing concentration of the added salt. The extent of interaction between counterions and polyions at a given polymer concentration, as estimated form chloride anion activity, is greater for bivalent than for univalent anions. Experimental data are in good agreement with theory. Selective interaction of anions with the ammonium copolymers has also been evidenced by viscometry. The selectivity follows the order For organic anions, in addition to electrostatic interactions, hydrophobic effects, which depend on the structure of the copolymer, play a significant role.     

Tuning substrate selectivity of a cationic enzyme using cationic polymers

Noncovalent interactions between an artificial molecular scaffold and a protein are interesting due to the possibility of reversible modulation of the activity of the protein. alpha-Chymotrypsin is a positively charged protein that has been shown to interact with negatively charged polymers. Here we show that positively charged polymers are also capable of electrostatically binding to this protein. The resulting experiments show that the ability of a polymer to bind a protein does not depend only on the pI of the protein. We also realized that the variations in charge density in the polymer backbone afford different selectivities of the enzyme toward charged substrates.     

Origins of selectivity in a colorimetric charge-transfer sensor for diols

Bispyridyl hydrogen bonding receptor 1 forms colored charge transfer (CT) complexes with complementary phenols and naphthols. Despite its low association constants of approximately 10(1) M(-1), receptor 1 was highly selective forming CT complexes of varying color and intensity with different diol guests. The selectivity of 1 was correlated with the ability of its CT band to simultaneously yield information about the association constant and the electronic structure of the phenols and naphthols.     

Influence of cationic antibiotics on phase behavior of rough-form lipopolysaccharide

The rough-form lipopolysaccharide (LPS) interacted with cationic antibiotic polymyxin B and gramicidin S in solution, and showed altered thermotropic phase behavior and viscoelasticity. The phase behavior was measured by differential scanning calorimetry and quartz crystal microbalance (QCM). Addition of polymyxin B of up to 0.5 mg/mL to the 5.0 mg/mL LPS solution increased gel-to-liquid crystalline phase transition enthalpy (ΔH) and raised the transition temperature (t_max). The further addition of polymyxin B reduced the ΔH value. Gramicidin S produced a different effect, whereby a minor addition reduced t_max and ΔH value of the LPS. The LPS film on the platinum electrode of the QCM indicated a downward shift of resonant frequency and an upward shift of resonant resistance when in contact with the antibiotic solution. An interpretation of these variations is that the LPS on the QCM electrode changed not only film weight, but also viscoelasticity owing to contact with the antibiotic solution. The different effects between the antibiotics between polymyxin B and gramicidin S on the LPS are induced by the difference of the governing effect. Polymyxin B interacts with the LPS electrostatically, whereas gramicidin S interacts by hydrophobic moieties.     

Origins of selectivity in pericyclic reaction cascades for the synthesis of gambogin and lateriflorone

[reaction: see text] Quantum mechanical calculations demonstrate that the second step of a Claisen-Diels-Alder reaction cascade controls regioselectivity that gives advanced intermediates for the synthesis of gambogin and 1-O-methyllateriflorone.     

Solvent enhancement of reaction selectivity: a unique property of cationic chiral dirhodium carboxamidates

1,3-Dipolar cycloaddition reactions of nitrones with α,β-unsaturated aldehydes catalyzed by a cationic chiral dirhodium(II,III) carboxamidate with (R)-menthyl (S)-2-oxopyrrolidine-5-carboxylate ligands in toluene increase reaction rates, give optimum regioselectivities, and enhance stereoselectivities compared to the same reactions performed in traditionally used halocarbon solvents. Rate and enantioselectivity enhancements were also obtained in hetero-Diels-Alder and carbonyl-ene reactions performed in toluene over those obtained in dichloromethane using the diastereomeric chiral cationic dirhodium(II,III) carboxamidate with (S)-menthyl (S)-2-oxopyrrolidine-5-carboxylate ligands. These enhancements are attributed to diminished or absent association of toluene with the catalyst which lessens the relative importance of the uncatalyzed background reaction, and they may also be a consequence of different coordination angles for aldehyde association with rhodium in the different solvent environments. Overall, the enhancement of reaction rates and selectivities with cationic chiral dirhodium(II,III) carboxamidates in toluene suggests broad applications for them in Lewis acid catalyzed reactions.     

Reactions following electron transfer: Origins of selectivity in the reaction of radicals with carbanions

The regiochemistry observed upon arylation of ambident carbanions by phenyl radicals can be readily explained by consideration of the site of greatest basicity. Thus 1,3-diphenylindenyl anion is arylated to give predominantly 1,1,3-triphenylindene despite the presumed greater stability of the intermediate leading to 1,2,3-triphenyl indene, that is, the 1,2,3-triphenylisoindene radical anion. This explanation may also account for the preference of C-alkylation rather than O-alklation of nitrocarbanions and for the absence of O-arylation in the S_RN1 arylation of enolates. The lack of reactivity of other carbanions is rationalized on thermodynamic grounds.     

Studies on the cationic selectivity of the potentiometric membrane containing thiamine metatungstate as ionophore

An unconventional structure was obtained by extraction of K⁺ ions with nitrobenzene solution of thiamine metatungstate. This shows that K⁺ penetration into the organic phase is not an exchange process. The solution of thiamine metatungstate in nitrobenzene acts as a spacial structure that entraps K⁺ ions in a molar ratio of 1 : 2. The absence of a chemical reaction between K⁺ and the metatungstic anion (proved experimentally) and the specificity of the interaction with K⁺, suggests that the formation of the structure potassium – thiamine metatungstate (KTM) could be controlled by dimensional criteria. The nitrobenzene solution of KTM shows a remarkable potentiometric selectivity for K⁺ compared with any other alkaline and alkaline earth cations. The potentiometric K⁺-selective electrode, based on KTM as ionophore, responds linearly in the range 3.8 × 10^–5–1.0 × 10^–1 mol/L, with a slope of 56 mV/ decade and a detection limit of 1.2 × 10^–5 mol/L. The electrode maintained these response characteristics over a period of more than two months. Received: 12 May 1998 / Revised: 28 July 1998 / Accepted: 30 July 1998     

Studies on the cationic selectivity of the potentiometric membrane containing thiamine metatungstate as ionophore

An unconventional structure was obtained by extraction of K⁺ ions with nitrobenzene solution of thiamine metatungstate. This shows that K⁺ penetration into the organic phase is not an exchange process. The solution of thiamine metatungstate in nitrobenzene acts as a spacial structure that entraps K⁺ ions in a molar ratio of 1 : 2. The absence of a chemical reaction between K⁺ and the metatungstic anion (proved experimentally) and the specificity of the interaction with K⁺, suggests that the formation of the structure potassium – thiamine metatungstate (KTM) could be controlled by dimensional criteria. The nitrobenzene solution of KTM shows a remarkable potentiometric selectivity for K⁺ compared with any other alkaline and alkaline earth cations. The potentiometric K⁺-selective electrode, based on KTM as ionophore, responds linearly in the range 3.8 × 10^–5–1.0 × 10^–1 mol/L, with a slope of 56 mV/ decade and a detection limit of 1.2 × 10^–5 mol/L. The electrode maintained these response characteristics over a period of more than two months. Received: 12 May 1998 / Revised: 28 July 1998 / Accepted: 30 July 1998     

Block ionomer complexes containing cationic antibiotics to kill intracellular Brucella melitensis in vitro

Brucellosis, caused by Brucella spp., is the most common zoonotic disease worldwide. It is difficult to eradicate because the pathogen resides partially within phagocytic host cells and the polar antibiotics that are recommended for treatment do not enter cells efficiently. Core‐shell block ionomer complexes (BICs) carrying antibiotics in their cores were designed to transport these drugs into cells. Polyether–polyacrylate copolymers were condensed with cationic aminoglycoside antibiotics to form BICs with diameters of 170–340 nm in water. An anionic poly(acrylate‐b‐ethylene oxide‐b‐propylene oxide‐b‐ethylene oxide‐b‐acrylate) copolymer blended with a poly(ethylene oxide‐b‐acrylate) diblock was condensed with gentamicin to afford complexes containing up to 42 wt% of the antibiotic. The poly(propylene oxide) contributed hydrophobic interactions that enhanced stability of the complexes in aqueous media, whereas the hydrophilic blocks provided a steric brush to keep the structures dispersed. In vitro efficacy of the BICs to reduce intracellular Brucella was studied in murine macrophage‐like cells. Significant bacterial reductions of 2.78 and 2.85 logs were obtained relative to only 0.75 logs for the free drug. This suggests that the BICs are efficient transporters of polar antibiotics into phagocytic cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Mechanism of selectivity in ion-pair high-performance liquid chromatography of aminoglycoside antibiotics using perfluorinated pairing ions

The behaviour of perfluorinated carboxylic acids as pairing ions for the chromatography of the aminoglycoside antibiotics was studied. As with perhydrogenated pairing ions, the capacity factor can be modified according to the percentage of organic modifier and the nature and concentration of perfluorinated pairing ion in the mobile phase. The special selectivity effect observed with trifluoroacetic acid was investigated and interpreted as a concerted mechanism involving ionic and hydrophobic interactions.     

Interaction of cationic liposomes with cell membrane models

The colloidal dispersion stability of nano-sized graphene sheets in supercritical fluid (SCF) media is very important for developing SCF-based exfoliation and dispersion technologies for stabilization and solubilization of graphenes. We carried out molecular dynamics simulations to elucidate the stability mechanism of graphene in supercritical CO(2) (scCO(2)). The potential of mean force (PMF) between two graphene nanosheets in scCO(2) was simulated, and the effect of scCO(2) density and temperature on the PMF behavior has been investigated. The simulation results demonstrate that there exists a free energy barrier between graphenes in the scCO(2) fluid, possibly obstructing the aggregation of graphenes. The single-layer confined CO(2) molecules between the graphene sheets can induce a dominating repulsion interaction between graphene sheets. At higher scCO(2) fluid density, there are more confined CO(2) molecules within the interplate regions, resulting in a stronger repulsive free energy barrier. The effect of temperature on the PMF is relatively minor. The scCO(2) solvent structure shows layered confined arrangement in the interfacial region near the graphene nanosheets, which is correlated well with the PMF profile curve.     

A new paradigm for anion trapping in high capacity and selectivity: crystal-to-crystal transformation of cationic materials

We describe a new methodology to the selective trapping of priority pollutants that occur inherently as oxo-anions (e.g., perchlorate, chromate, arsenate, pertechnetate, etc.) or organic anions (e.g., salicylate, pharmaceuticals, and their metabolites, which are often chlorinated into potentially more harmful compounds). The typical approach to trapping anions is exchange into cationic hosts such as resins or layered double hydroxides. Both capacity and selectivity are limited by the equilibrium of the process and moreover are often subject to interference, e.g. by carbonate that is always present in water from atmospheric CO(2). Our approach takes advantage of the metastability of our cationically charged materials to instead trap by recrystallization to a new structure. Exceptionally high adsorption capacities for permanganate and perrhenate--studied as models for pertechnetate--were found for a Ag(I)-based cationic extended framework. The exchange capacity reached 292 and 602 mg/g, respectively, over five times the exchange capacity compared to conventional layered double hydroxides. Our cationic material can also selectively trap these and other toxic oxo-anions when nontoxic anions (e.g., nitrate, carbonate) were present in an over 100-fold excess concentration.     

Synthesis and cell transfection properties of cationic uracil-morpholino tetramer

Synthesis and cell transfection properties of guanidinium-functionalized uracil morpholino tetramer have been reported for the first time. Due to the basic nature of guanidinium groups they remain protonated under physiological conditions. Such cationic tetramer exhibits efficient cellular uptake properties as visualized by microscopy imaging using fluorescent dye BODIPY. 7′-End of this morpholino tetramer was functionalized with an azide group for conjugation with various types of biomolecules or drugs for cellular delivery.     

Improvement of the separation selectivity in capillary zone electrophoresis of synthetic cationic dyes by polyvinylpyrrolidone as pseudo-stationary phase

Capillary zone electrophoresis is applied to separate under acidic conditions (pH 3, citric acid/TRIS) five synthetic organic dyes used as colourants for textiles. These dyes are migrating as cations under the given conditions, but are not fully resolved in this buffer system. 1% Polyvinylpyrrolidone added to the background electrolyte solution and acting as a pseudophase significantly improved the selectivity and enabled full separation of all analytes.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

Anion-selective membrane electrodes based on metalloporphyrins: The influence of lipophilic anionic and cationic sites on potentiometric selectivity

The role of lipophilic anionic and cationic additives on the potentiometric anion selectivities of polymer membrane electrodes prepared with various metalloporphyrins as anion selective ionophores is examined. The presence of lipophilic anionic sites (e.g. tetraphenylborate derivatives) is shown to enhance the non-Hofmeister anion selectivities of membranes doped with In(III) and Sn(IV) porphyrins. In contrast, membranes containing Co(III) porphyrins require the addition of lipophilic cationic sites (e.g. tridodecylmethylammonium ions) in order to achieve optimal anion selectivity (for nitrite and thiocyanate) as well as rapid and reversible Nernstian response toward these anionic species. These experimental results coupled with appropriate theoretical models that predict the effect of lipophilic anion and cation sites on the selectivities of membranes doped with either neutral or charged carrier type ionophores may be used to determine the operative ionophore mechanism of each metalloporphyrin complex within the organic membrane phase.     

Synthesis and cationic selectivity studies of novel calix[4]arene derivatives containing heteroatom at the lower rim

A series of calixarene derivatives 2―5 containing heteroatom at the lower rim have been synthesized. 1H NMR studies and crystallographic structures demonstrated that the calix[4]arene derivatives adopted cone conformations. Their cationic binding abilities and selectivities towards heavy and tran- sition metal ions have been evaluated by solvent extraction of aqueous metal picrates. The obtained results indicated that the introduction of nitrogen, sulfur, and/or phosphor atoms to the calix[4]arene framework could effectively enhance their binding ability and selectivity for heavy and transition metal ions, such as Pb2 or Ag .     

Covalent cell surface recruitment of chemotherapeutic polymers enhances selectivity and activity

Synthetic macromolecular chemotherapeutics inspired by host defence peptides can disrupt cell membranes and are emerging as agents for the treatment of cancer and infections. However, their off-target effects remain a major unmet challenge. Here we introduce a covalent recruitment strategy, whereby metabolic oligosaccharide engineering is used to label targeted cells with azido glycans, to subsequently capture chemotherapeutic polymers by a bio-orthogonal click reaction. This results in up to 10-fold reduction in EC₅₀ and widening of the therapeutic window. Cell death is induced by not only membrane leakage, but also by apoptosis due to the conjugated chemotherapeutic being internalised by glycan recycling. Covalent recruitment also lead to increased penetration and significant cell death in a 3-D tumour model in just 3 hours, whereas doxorubicin required 24 hours. This conceptual approach of ‘engineering cells to capture polymers’ rather than ‘engineering polymers to target cells’ will bring new opportunities in non-traditional macromolecular therapeutics.

Chemotherapeutic polymers are targeted to cells by introduction of unnatural glycans to their glycocalyx, enhancing their cytotoxic effect.     

Ferrocene-containing cationic lipids: influence of redox state on cell transfection

A ferrocene-containing, redox-active cationic lipid that can be transformed using electrochemical methods yields large differences in cell transfection depending on the oxidation state of the lipid. Expression of enhanced green fluorescent protein and firefly luciferase occurs at very high levels when DNA lipoplexes are formulated using the lipid in the reduced state. In contrast, transfection is negligible when oxidized lipid is used. These observations suggest the basis of a general method that could be used to transform inactive lipoplex formulations to an active form through the application of externally applied electrical potentials. The ability to activate lipoplexes toward transfection electrochemically and "on demand" could create new opportunities to deliver DNA in vitro and in vivo with both spatial and temporal control.