Selectivity For Calcium Ions Research Articles

Na ion-exchanged zirconium phosphate crystal with high calcium ion selectivity.

We demonstrate hydrothermally grown sodium hydrogen zirconium phosphate ((Na,H)-ZrP) crystals exhibiting high calcium ion selectivity. The standard Gibbs free energies for Ca2+ exchange on (Na,H)-ZrP and γ-type ZrP were estimated to be -10.1 and -4.69 kJ mol-1, respectively. The high Ca2+ selectivity of (Na,H)-ZrP could be attributed to the size matching between the ion exchange site of (Na,H)-ZrP and Ca2+.

The insights into calcium ion selectivity provided by ancestral prokaryotic ion channels.

Prokaryotic channels play an important role in the structural biology of ion channels. At the end of the 20th century, the first structure of a prokaryotic ion channel was revealed. Subsequently, the reporting of structures of various prokaryotic ion channels have provided fundamental insights into the structure of ion channels of higher organisms. Voltage-dependent Ca2+ channels (Cavs) are indispensable for coupling action potentials with Ca2+ signaling. Similar to other proteins, Cavs were predicted to have a prokaryotic counterpart; however, it has taken more than 20 years for one to be identified. The homotetrameric channel obtained from Meiothermus ruber generates the calcium ion specific current, so it is named as CavMr. Its selectivity filter contains a smaller number of negatively charged residues than mutant Cavs generated from other prokaryotic channels. CavMr belonged to a different cluster of phylogenetic trees than canonical prokaryotic cation channels. The glycine residue of the CavMr selectivity filter is a determinant for calcium selectivity. This glycine residue is conserved among eukaryotic Cavs, suggesting that there is a universal mechanism for calcium selectivity. A family of homotetrameric channels has also been identified from eukaryotic unicellular algae, and the investigation of these channels can help to understand the mechanism for ion selection that is conserved from prokaryotes to eukaryotes.

Characteristics of Solid-State Calcium Ion Sensors Based on Photocurable and Selfplasticising Polyacrylate Matrices

New membrane materials based on cross-linked poly(n-butyl acrylate) (nBA), have been used successfully as calcium ion-selective membranes. These membrane materials possess selfplasticising property and hence do not require plasticisers. The photocurability and good adhesion characteristics of these polymer matrices enable workable solid-state calcium ion sensors to be fabricated by simple photocure procedures employing the calcium ionophore ETH5234 and a lipophilic additive as ion sensing components. The calcium ion-selectivity of the sensors can be controlled by varying the chemical composition of the photocured membrane. An optimum amount of the cross-linker 2,2-hexanedioldiacrylate (HDDA) and the incorporation of n-heptyl acrylate (nHA) led to improvement in the calcium ion-selectivity. The best calcium ion-selectivity was obtained from a copolymer membrane with composition: nBA = 74 wt-%, nHA = 20 wt-% and HDDA = 0.1 wt-%. The selectivity coefficients of calcium over major cations were: LogKCaPot,Na= -4.4, LogKCaPot,K = -3.6, LogKCa,PotLi = -5.9, LogKCaPot,Mg= -4.4 with a Nernstian slope (29.1 ± 0.8 mV/decade) under buffered conditions. This potentiometric performance is comparable to other solid-state calcium ion sensors with various plasticised polymer membranes.

An Optical Sensor Based on a Photonic Polymer Film to Detect Calcium in Serum

An optical calcium sensor is fabricated based on a cholesteric liquid crystalline (CLC) polymer containing benzoic acid metal binding sites. A chiral imprinted CLC polymer is made which is subsequently treated with KOH to yield a responsive green reflecting film. On investigation of various metal ions, the polymer film shows a high optical response, and selectivity for calcium ions, which is related to the preorganized binding sites in the ordered liquid crystalline phase, leading to a blue reflecting film. The photonic polymer film is sensitive to Ca2+ within the physiologically relevant concentration range of 10−4 to 10−2 m. Measurement of total calcium concentration in serum is also investigated using the film. The optical responses of normal serum and samples mimicking hypocalcemia and hypercalcemia can be clearly distinguished, providing a cheap, battery‐free, and easy‐to‐use alternative for calcium determination in clinical diagnostics.

Brownian Dynamics Simulation of Calcium Channels

Calcium-selective ion channels play a crucial role in many biological functions allowing the selective flux of calcium ions into the cytoplasm from storage sites inside the cell or from the extracellular environment. They feature high selectivity for calcium ions even if the calcium concentration is hundreds of times smaller than other monovalent cations (sodium, potassium).We studied ion permeation properties through calcium channels, using a simplified channel model and Brownian Dynamics simulations with Grand Canonical-Monte Carlo control regions in the baths.The physical mechanisms related to ion permeation were investigated under physiological conditions, with calcium ions only on one side of the membrane.The Brownian Dynamics approach was used to describe ion motion in the simulation domain. The electrostatic forces impinging on the ions were evaluated at run-time, solving Poisson's equation with the Induced Charge Computation method (ICC) that provided better accuracy in our hands than look-up tables. Different dielectric constants were assigned to the protein and the permeation pathway. Polarization effects were computed by the ICC method, which leads to an accurate description of the electrostatics. The transmembrane potential and the ion concentrations were imposed by a Grand Canonical Monte-Carlo algorithm.The carboxylate-rich selectivity filter of calcium channels was modeled with 8 independent half-charged oxygens. Oxygens were treated as ions, but they were confined in the central region of the pore. Transport properties were determined by the competition between electrostatic forces and steric repulsion due to charge crowding. This simple channel model allowed us to investigate the physical mechanisms responsible for the selective conduction of divalent cations over monovalent cations.

Characteristics of Solid-State Calcium Ion Sensors Based on Photocurableand Selfplasticising Polyacrylate Matrices

New membrane materials based on cross-linked poly(n-butyl acrylate) (nBA), have been used successfully as calcium ion-selective membranes. These membrane materials possess selfplasticising property and hence do not require plasticisers. The photocurability and good adhesion characteristics of these polymer matrices enable workable solid-state calcium ion sensors to be fabricated by simple photocure procedures employing the calcium ionophore ETH5234 and a lipophilic additive as ion sensing components. The calcium ion-selectivity of the sensors can be controlled by varying the chemical composition of the photocured membrane. An optimum amount of the cross-linker 2,2-hexanedioldiacrylate (HDDA) and the incorporation of n-heptyl acrylate (nHA) led to improvement in the calcium ion-selectivity. The best calcium ion-selectivity was obtained from a copolymer membrane with composition: nBA = 74 wt-%, nHA = 20 wt-% and HDDA = 0.1 wt-%. The selectivity coefficients of calcium over

Calcium ion selectivity and isotope effects studied by using benzo-18-crown-6 resins

The chromatography experiments using benzo-18-crown-6 resin were carried out to study isotope effects in calcium adsorption. By the elution technique with hydrochloric acid solution, it was found that the adsorption of calcium depends on the concentration of hydrochloric acid in solution and that 9 M HCl concentration is the appropriate condition for the adsorption of calcium to the cavity of benzo-18-crown-6 of which cavity size is a little larger than the diameter of calcium ion. Thereafter, the isotope fractionation of calcium in the complex formation reaction with 18-crown-6 ether was investigated by using the breakthrough technique of chromatography with above-mentioned synthesized resin and 9 M hydrochloric acid solution. The observed isotope separation coefficient ε = α − 1 was 1.9 × 10 − 3 for the isotopic pair 40Ca/ 48Ca.

An ion-selective electrode with a polymeric membrane containing an active chelating substance

An optimal new PVC membrane sensor for Cd2+ ions based on TRIAR3-(2′,4′ dihydroxyphenylazo-1 ')-1,2,4 triazol as the active substance was prepared. Its basic analytical parameters such as slope characteristics; detection limit; lifetime; response time; selectivity for zinc, nickel, cobalt, calcium, copper, barium, magnesium, manganese, potassium and sodium ions, as well as the dependence of the electrode potential on pH were established. The proposed membrane sensor can be used in flow injection potentiometric analysis of cadmium ions.

In situ detection of calcium ions with chemically modified microcantilevers

We report a novel technique of micromechanical detection of trace amounts of calcium ions by using microcantilevers modified with ion-selective self-assembled monolayers (SAMs). The SAM-modified microcantilevers undergo bending due to selective adsorption of calcium ions. Experiments conducted under flow conditions show that the modified cantilevers respond sensitively to calcium ions (Ca 2+); a Ca 2+ concentration of 10 −9 M can be detected with this technique. Other cations, such as Na + and K +, do not have any effect on the deflection of these cantilevers. We demonstrate two different kinds of SAMs having selectivity for calcium ions.

The brain α7 nicotinic receptor may be an important therapeutic target for the treatment of Alzheimer's disease: studies with DMXBA (GTS-21)

A large decrease in brain nicotinic receptor levels occurs in Alzheimer's disease, relative to muscarinic and other receptors. Neurons possessing high affinity nicotinic receptors seem particularly vulnerable. The low affinity nicotinic receptors which selectively bind α-bungarotoxin are not significantly affected. The major nicotinic receptor subtype which binds this toxin is a homo-oligomer composed of α7 subunits. Due to its exceptionally high calcium ion selectivity, this particular receptor can be considered as a ligand-gated calcium channel. α7 receptors are found in regions of the brain which are important for cognition, including cerebral cortex and hippocampus. Hippocampal receptors are largely confined to GABAergic interneurons. α7 receptors seem less likely than α4–β2 receptors to be up-regulated in number and down-regulated in function as a result of chronic agonist exposure. A family of nicotinic agonists based upon the marine animal toxin anabaseine have been synthesized and investigated. One of these compounds, DMXBA [3-(2,4-dimethoxybenzylidene)-anabaseine; code name GTS-21] has displayed promising characteristics during phase I clinical tests. In the rat DMXBA is selectively agonistic upon α7 nicotinic receptors. In addition it is a moderately potent antagonist at α4–β2 receptors. DMXBA enhances a variety of cognitive behaviors in mice, monkeys, rats and rabbits. It also displays neuroprotective activity upon cultured neuronal cells exposed to β-amyloid or deprived of NGF. The compound is much less toxic than nicotine and does not affect autonomic and skeletal muscle systems at doses which enhance cognitive behavior. Phase I clinical tests indicate that large doses can be safely administered orally without adverse effects. Psychological tests on healthy young male subjects indicate a positive effect of DMXBA on some measures of cognition. While DMXBA is a much weaker partial agonist on human α7 receptors than upon rat α7 receptors, its 4-hydroxy metabolite has been shown to have excellent efficacy on both receptors. Thus, some of the physiological and behavioral effects of GTS-21 may be due to the actions of this primary metabolite.

Pronounced calcium binding selectivity with diamide derivatives of 12-N2O2and the importance of desolvation

Abstract The complexation of alkali and alkaline-earth cations with two N-substituted amide derivatives of 1,7-dioxa-4,10-diazacyclododecane has been studied in methanol and water by calorimetric and potentiometric methods. High stability (e.g. log KCaL = 11.2: 298 K, MeOH) and selectivity (107.1 over Na+) for calcium ions in methanol is shown by the secondary amide derivative, and although significantly higher enthalpies of complexation are found for the tertiary amide analog (resulting from lesser differential ligand solvation), accompanying lower entropies of complexation lead to somewhat lower binding free energies for IIa cations. In water, both the complex stability and the selectivity for the more charge dense ions is reduced.

Analytical performances of lipophilic diamides based alkaline earth ion‐selective electrodes

AbstractBased on complex chemical considerations, lipophilic diamide derivatives have been designed and synthesized as ionophores for liquid membrane electrodes. The structure‐selectivity studies revealed that the majority of the ligands exhibit calcium ion selectivity, except the compound bis‐N,N‐dicyclohexyl‐malonamide. The latter has an almost equal selectivity for Ca2+ and Mg2+ and sufficiently rejects sodium ions, thus it lends itself to water hardness measurements.

Cation-exchange selectivity for potassium(1+), magnesium(2+) and calcium(2+) ions on sodium-substituted taeniolite

Cation-exchange selectivity for potassium(1+), magnesium(2+) and calcium(2+) ions on sodium-substituted taeniolite

Lipophilic Cyclic Hexapeptide for Calcium Ion-Selective-Electrode

Abstract The cyclic hexapeptides containing artificial and lipophilic amino acid, α-aminomyristic acid were designed. The poly(vinyl chloride) (PVC) membrane electrode based on a cyclic hexapeptide exhibited high calcium ion selectivity over sodium and potassium ions.

Calcium complexation with a highly calcium selective chelator: crystal structure of ca(CaFBAPTA) ·5H 2O

The structure of the complex of calcium ions with 5,5'-difluoro-1,2- bis( o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (FBAPTA) has been determined. The calcium complex, with stoichiometry Ca(CaFBAPTA) · 5H 2O crystallizes in a monoclinic space group P2 1/ c with 4 formula units/unit cell with a(Å) = 9.767, b(Å) = 26.309, c(Å) = 11.769; β (deg) = 111.57. The structure may be considered as the calcium salt of a CaFBAPTA2- complex in which the octacoordinate calcium ion is complexed with the two ether oxygen atoms, the two nitrogens, and coordinated in a unidentate manner with the four acetate carboxyl groups. The geometry of the nitrogen atoms is nearly tetrahedral and provides a basis for the interpretation of the optical perturbations which result from calcium complexation with BAPTA type ligands. In addition, it is clear from the distance and relative orientation of the aromatic rings that ring current contributions to the fluorine chemical shifts are of negligible importance, so that such effects cannot be used in the design of NMR active indictators. Although BAPTA and analogs such as PBAPTA and quin2 have found extensive physiological applications as a consequence of the high degree of selectivity for calcium ions over magnesium ions which they exhibit, this is the first reported structural data for any of these complexes.

Enhanced calcium ion selectivity of a macrocyclic polyether-amide with two polyether rings

The macrocyclic polyether -amide (1) is shown to be mere selective than its corresponding monocyclic analogue (4) for the Ca2+ ion; the potentiometric selectivity for Ca2+ relative to Mg2+ is improved by more than one order of magnitude.

Calcium ion-selective electrode studies: covalent bonding of organic phosphates and phosphonates to polymer matrices

The covalent linking of organophosphates and phosphonates to the copolymer VAGH (a partially hydrolysed copolymer of vinyl chloride and vinyl acetate) is described along with attempts to phosphonate polystyrene by Friedel - Crafts and free-radical processes. The various products obtained have been evaluated for use in calcium ion-selectiv electrode membranes. The best membranes were all based on VAGH, a product from each type being selected for further study for their calcium ion-sensing qualities, namely, VAGH PI, VAGH PII and VAGH PIII.VAGH PI is based on the covalent bonding of monodecylphosphate to VAGH, while VAGH PII resulted from the binding of mono-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate. Viable electrode membranes required additional poly(vinyl chloride) support and dioctyl phenylphosphonate solvent mediator. VAGH PIII was obtained by a synthesis amounting to covalently binding monooctyl phenylphosphonate to VAGH and was used with some additional dioctyl phenylphosphonate solvent mediator and calcium bis{di-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} sensor in electrode membranes.The VAGH PI electrode matched those based on equivalent ungrafted didecylphosphate sensors, except that calcium ion selectivity was poor in a high sodium ion background. The VAGH PII electrode, on the other hand, did not fulfil the performance of electrodes based on free calcium bis{di-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} as ion sensor, and there was less selectivity for calcium over sodium and magnesium. Some loss of selectivity was also characteristic of the VAGH PIII electrode, but its slightly lengthened lifetime was insufficient to warrant synthesis of the phosphonated polymer matrix.

Studies of calcium ion-selective electrodes in the presence of anionic surfactants

Eighteen membrane systems, based on a calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} ion sensor have been compared in potentiometric studies with a membrane based on the sensor with a dioctyl phenylphosphonate solvent mediator with respect to interferences of calcium ion-selective electrodes by anionic surfactants, especially by sodium dodecylsulphate (SDS) and sodium tetradecylbenzenesulphonates (ABS). Electrodes made from poly(vinyl chloride) matrix membranes of the sensor with trioctyl phosphate solvent mediator are far superior to the other membrane systems in resisting interference by anionic surfactants and are shown to yield calcium ion levels in the presence of a wash liquor to match the expected values.Some improvement over dioctyl phenylphosphonate is also offered by decan-1-ol, dodecan-1-ol and tetradecan-1-ol but the use of such solvent mediators impairs calcium ion selectivity and poly(vinyl chloride) plasticising qualities. The use of alternative polymer matrices based on poly(vinylidene chloride) and VAGH copolymer (hydrolysed vinyl chloride - vinyl acetate) offer no advantages.A nineteenth membrane, obtained from a commercial supplier, exhibited interference by SDS.An interesting effect is the increase (rather than the normal decrease) in e.m.f. observed for those electrodes from membranes in which the amount of free active sensor was reduced to low levels. By using optimum levels of sensor it was possible to fabricate electrodes exhibiting a zero e.m.f. change when changing the background 10–2M calcium chloride to one that is also 10–3M is SDS. Such membranes are based on decyl phosphate grafted to VAGH copolymer.X-ray fluorescence and chromatographic studies on membranes of the sensor with dioctyl phenylphosphonate in poly(vinyl chloride) show SDS to be a significant agent in leaching membrane components, especially of dioctyl phenylphosphonate. Such observations are indicative that the interference of calcium ion-selective electrodes by anionic surfactants may be the result of the different solubilities of calcium - surfactant complexes in the solvent mediator of the membrane.

Sodium docosyl sulfate monolayers at liquid air interfaces: Counterions effect

The adsorption of radioactive calcium ions ( 45Ca 2+) has been investigated in competition with cesium, sodium, or lithium cations. The model system consisted of the monolayers of sodium docosyl sulfate spread on buffered aqueous supports. The monolayers were of different surface concentrations of the surfactant; calcium concentration in the aqueous solution was variable and the concentration of a particular monovalent cation was maintained always constant. It was found that the calcium ion selectivity increases following the hydration sequence Li + > Na − > Cs +. For all studied monovalent cations and for a given calcium concentration, the ratio of the docosyl sulfate negative sites and the adsorbed calcium tends to the same value, 2. The exchange mechanism is discussed and a model, assuming constant local surface concentration, is proposed to calculate the corresponding exchange constants for the film concentrations lower than the collapse surface concentration. The measurements of the surface potential have been parallelly performed on the same systems and also on the sodium dodosyl sulfate monolayers spread on nonbuffered subsolutions. In the latter case the surface concentration of the surfactant was maintained equal to the collapse surface concentration. For these systems the dependence of the surface potential on the continuously changing concentration of the cation (sodium, lithium, or calcium) in the solution was measured. A study was also made of the surface potential variation of the solution containing two cations (sodium and calcium or lithium and calcium); the concentration of one of them (sodium or lithium) was maintained constant and of the other one (calcium) was variable. The surface potential measurements confirm the adsorption results. Taking into account different experimental conditions, a model of the counterion distribution is proposed when the surface concentration of the surfactant is equal to the collapse surface concentration. The counterions may form either a compact layer or ion-pairs with the negative charges of the spread monolayer. The model is discussed in terms of the stability of colloidal particles. Thus, further interpretation of the effects induced by a long chain alcohol on the properties of sodium docosyl sulfate monolayer was possible.

Calcium ion-selective electrodes based on calcium bis[di(p-1,1,3,3-tetramethylbutylphenyl)-phosphate] sensor and trialkyl phosphate mediators

Calcium ion-selective electrodes based on PVC matrices with membranes composed of calcium bis[di(p-1,1,3,3-tetramethylbutylphenyl)phosphate] sensor and trialkyl phosphate solvent mediator are described and compared with electrodes of the same sensor with di-n-octyl phenylphosphonate mediator. Trialkyl phosphates of sufficiently high viscosity, for example, tri-n-butyl phosphate, tri-n-amyl phosphate, tri(1,1,3,3-tetramethylbutyl) phosphate and tri-n-octyl phosphate make good solvent mediators and maintain the calcium ion selectivity of the sensor for the range of alkali metal and bivalent ions studied, namely, Na+, K+, Mg2+, Sr2+, Ba2+, Mn2+, Cu2+, Ni2+ and Zn2+. The trialkyl phosphate mediators provide commercially available alternatives to di-n-octyl phenylphosphonate, the best in the range examined being tri-n-amyl phosphate.