Ion-selective Properties Research Articles

Antibiotic translocation through porins studied in planar lipid bilayers using parallel platforms.

In general, the method of choice to characterize the conductance properties of channel-forming bacterial porins is electrophysiology. Here, the classical method is to reconstitute single porins into planar lipid bilayers to derive functional information from the observed channel conductance. In addition to an estimated pore size, ion selectivity or transport properties in general are of importance. For the latter, measuring the ion current fluctuation can provide some information about the mode of transport of charged molecules penetrating the proteins. For instance, increasing the external voltage modifies the residence time in the channel: charged molecules with the ability to permeate through channels will travel faster whereas non-permeating molecules get pushed to the constriction zone with enhanced residence time. Here, we are interested in the ability of antibiotics to permeate channels and compare different techniques to reveal fast events.

Subdued, a TMEM16 family Ca2+-activated Cl− channel in Drosophila melanogaster with an unexpected role in host defense

TMEM16A and TMEM16B are calcium-activated chloride channels (CaCCs) with important functions in mammalian physiology. Whether distant relatives of the vertebrate TMEM16 families also form CaCCs is an intriguing open question. Here we report that a TMEM16 family member from Drosophila melanogaster, Subdued (CG16718), is a CaCC. Amino acid substitutions of Subdued alter the ion selectivity and kinetic properties of the CaCC channels heterologously expressed in HEK 293T cells. This Drosophila channel displays characteristics of classic CaCCs, thereby providing evidence for evolutionarily conserved biophysical properties in the TMEM16 family. Additionally, we show that knockout flies lacking subdued gene activity more readily succumb to death caused by ingesting the pathogenic bacteria Serratia marcescens, suggesting that subdued has novel functions in Drosophila host defense. DOI: http://dx.doi.org/10.7554/eLife.00862.001.

Ion-selective and complexation properties, vibrational spectra, and crystal and molecular structure of 1,5-bis[2-(hydroxyethoxyphosphoryl)phenoxy]-3-oxapentane

The ion-selective properties of 1,5-bis[2-(hydroxyethoxyphosphoryl)phenoxy]-3-oxapentane H2R2 have been described. The molecular and crystal structure of H2R2 has been determined by powder X-ray crystallography and IR spectroscopy. The molecule has the twofold symmetry axis, orthorhombic crystal system, space group Pbcn, a = 16.830(2) A, b = 8.866(15) A, c = 15.7190(2) A, V = 2346.1(6) A3, Z = 4. A new CuR2 · 2H2O complex has been synthesized and characterized.

Effect of nickel on ion-selective property of rust formed on low-alloying weathering steel

Effect of nickel on ion-selective property of rust formed on low-alloying weathering steel

Contribution of Ion Binding Affinity to Ion Selectivity and Permeation in KcsA, a Model Potassium Channel

Ion permeation and selectivity, key features in ion channel function, are believed to arise from a complex ensemble of energetic and kinetic variables. Here we evaluate the contribution of pore cation binding to ion permeation and selectivity features of KcsA, a model potassium channel. For this, we used E71A and M96V KcsA mutants in which the equilibrium between conductive and nonconductive conformations of the channel is differently shifted. E71A KcsA is a noninactivating channel mutant. Binding of K(+) to this mutant reveals a single set of low-affinity K(+) binding sites, similar to that seen in the binding of K(+) to wild-type KcsA that produces a conductive, low-affinity complex. This seems consistent with the observed K(+) permeation in E71A. Nonetheless, the E71A mutant retains K(+) selectivity, which cannot be explained on the basis of just its low affinity for this ion. At variance, M96V KcsA is a rapidly inactivating mutant that has lost selectivity for K(+) and also conducts Na(+). Here, low-affinity binding and high-affinity binding of both cations are detected, seemingly in agreement with both being permeating species in this mutant channel. In conclusion, binding of the ion to the channel protein seemingly explains certain gating, ion selectivity, and permeation properties. Ion binding stabilizes greatly the channel and, depending upon ion type and concentration, leads to different conformations and ion binding affinities. High-affinity states guarantee binding of specific ions and mediate ion selectivity but are nonconductive. Conversely, low-affinity states would not discriminate well among different ions but allow permeation to occur.

Designing biomimetic pores based on carbon nanotubes

Biomimetic nanopores based on membrane-spanning single-walled carbon nanotubes have been designed to include selectivity filters based on combinations of anionic and cationic groups mimicking those present in bacterial porins and in voltage-gated sodium and calcium channels. The ion permeation and selectivity properties of these nanopores when embedded in a phospholipid bilayer have been explored by molecular dynamics simulations and free energy profile calculations. The interactions of the nanopores with sodium, potassium, calcium, and chloride ions have been explored as a function of the number of anionic and cationic groups within the selectivity filter. Unbiased molecular dynamics simulations show that the overall selectivity is largely determined by the net charge of the filter. Analysis of distribution functions reveals considerable structuring of the distribution of ions and water within the nanopores. The distributions of ions along the pore axis reveal local selectivity for cations around filter, even in those nanopores (C0) where the net filter charge is zero. Single ion free energy profiles also reveal clear evidence for cation selectivity, even in the C0 nanopores. Detailed analysis of the interactions of the C0 nanopore with Ca(2+) ions reveals that local interactions with the anionic (carboxylate) groups of the selectivity filter lead to (partial) replacement of solvating water as the ion passes through the pore. These studies suggest that a computational biomimetic approach can be used to evaluate our understanding of the design principles of nanopores and channels.

Ion-selective properties of 1,8-bis [2-(dihydroxyphosphinyl)phenoxy]-3,6-dioxaoctane (H4L3), synthesis and vibrational spectra of copper and zinc complexes with H4L3, and crystal and molecular structure of [Cu(H4L3)(H2O)3][(H2L3)(H2O)

The ion-selective properties of 1,8-bis[2-(dihydroxyphosphinyl)phenoxy]-3,6-dioxaoctane (H4L3) have been studied and its potentiometric selectivity coefficients have been determined. New complexes [Cu(H4L3)(H2O)3][(H2L3)(H2O)] (I) and Zn(H4L3)(H2L3) · 3H2O, and Cu(H2L3) · 2(H2O) have been synthesized and characterized. The crystal and molecular structure of I has been determined by X-ray crystallography and vibrational spectroscopy. The crystals are monoclinic, a = 10.279(5) A, b = 26.532(13) A, c = 8.399(4) A, β = 99.270(8)°, V = 2260.8(7) A3, Z = 2, space group Cm, R = 0.0347 for 4325 reflections with I > 2σ(I). Ionic compound I is composed of the [Cu(H4L3)(H2O)3]2+ complex cations and [(H2L3)(H2O)]2− anions. In the cation, the Cu2+ cation located in the m plane is bound to a tetragonal pyramidal (TP) array. The equatorial plane of the TP is formed by two phosphoryl oxygen atoms of the podand (Cu(1)-O, 1.921(2) A) and two O atoms of two water molecules (av. Cu(1)-O, 1.981(3) A). The third water molecule is at the axial vertex of the TP at a considerably larger distance (Cu(1)-O, 2.139(3) A). The anion is of the host-guest type. The host is the deprotonated podand (H2L3)2−, and the guest is the water molecule. The latter is bound to the terminal hydroxyl groups of two phosphoryl groups of the podand by two acceptor hydrogen bonds and to two central ether oxygen atoms of the (H2L3)2− anion by one donor bifurcated hydrogen bond. The cations and anions in the structure are linked by hydrogen bonds to form chains parallel to the c axis.

Fabrication of a needle-type pH sensor by selective electrodeposition

This manuscript reports on the development of a needle-type sensor for in situ on-site measurement of pH. Conventional pH microelectrode sensors are fabricated by pulling glass pipettes and applying ion-selective films at the tip. However, these sensors suffer from low fabrication yields and short sensor lifetime due to disruption or loss of the ion-selective membrane after repeated use. The developed needle-type sensor is fabricated by meniscus etching and takes advantage of the MEMS batch fabrication techniques. The sensor is based on the ion-selective properties of iridium oxide film, which was deposited at the sensor tip by electrodeposition. The sensor exhibited a Nernstian response with sensitivity of ∼62 mV/pH in the pH 2 to 12 range. The sensor also exhibited a fast 5 s response and a lifetime of ∼2 months when stored in a pH7 buffer solution, which is substantially longer than that of the conventional pulled-pipette sensors.

Structural studies of ion permeation and Ca 2+ blockage of a bacterial channel mimicking the cyclic nucleotide-gated channel pore

Cyclic nucleotide-gated (CNG) channels play an essential role in the visual and olfactory sensory systems and are ubiquitous in eukaryotes. Details of their underlying ion selectivity properties are still not fully understood and are a matter of debate in the absence of high-resolution structures. To reveal the structural mechanism of ion selectivity in CNG channels, particularly their Ca(2+) blockage property, we engineered a set of mimics of CNG channel pores for both structural and functional analysis. The mimics faithfully represent the CNG channels they are modeled after, permeate Na(+) and K(+) equally well, and exhibit the same Ca(2+) blockage and permeation properties. Their high-resolution structures reveal a hitherto unseen selectivity filter architecture comprising three contiguous ion binding sites in which Na(+) and K(+) bind with different ion-ligand geometries. Our structural analysis reveals that the conserved acidic residue in the filter is essential for Ca(2+) binding but not through direct ion chelation as in the currently accepted view. Furthermore, structural insight from our CNG mimics allows us to pinpoint equivalent interactions in CNG channels through structure-based mutagenesis that have previously not been predicted using NaK or K(+) channel models.

Biomimetic Membranes with Aqueous Nano Chanels but without Proteins: Impedance of Impregnated Cellulose Ester Filters

Earlier we have shown that many important properties of ionic aqueous channels in biological membranes can be imitated using simple biomimetic membranes. These membranes are composed of mixed cellulose ester-based filters, impregnated with isopropyl myristate or other esters of fatty acids, and can be used for high-throughput drug screening. If the membrane separates two aqueous solutions, combination of relatively hydrophilic polymer support with immobilized carboxylic groups results in the formation of thin aqueous layers covering inner surface of the pores, while the pore volume is filled by lipid-like substances. Because of these aqueous layers biomimetic membranes even without proteins have a cation/anion ion selectivity and specific (per unit of thickness) electrical properties, which are similar to typical properties of biological membranes. Here we describe frequency-dependent impedance of the isopropyl myristate-impregnated biomimetic membranes in the 4-electrode arrangement and present the results as Bode and Nyquist diagrams. When the membranes are placed in deionized water, it is possible to observe three different dispersion processes in the frequency range 0.1 Hz to 30 kHz. Only one dispersion is observed in 5 mM KH(2)PO(4) solution. It is suggested that these three dispersion features are determined by (a) conductivity in aqueous structures/channels, formed near the internal walls of the filter pores at high frequencies, (b) dielectric properties of the whole membrane at medium frequencies, determined by polymer support, aqueous layers and impregnating oil, and, finally, (c) by the processes in hydrated liquid crystal structures formed in pores by impregnating oil in contact with water at low frequencies.

Ferrocene Bound Poly(vinyl chloride) as Ion to Electron Transducer in Electrochemical Ion Sensors

We report here on the synthesis of poly(vinyl chloride) (PVC) covalently modified with ferrocene groups (FcPVC) and the electrochemical behavior of the resulting polymeric membranes in view of designing all solid state voltammetric ion sensors. The Huisgen cycloaddition ("click chemistry") was found to be a simple and efficient method for ferrocene attachment. A degree of PVC modification with ferrocene groups between 1.9 and 6.1 mol % was achieved. The chemical modification of the PVC backbone does not significantly affect the ion-selective properties (selectivity, mobility, and solvent casting ability) of potentiometric sensing membranes applying this polymer. Importantly, the presence of such ferrocene groups may eliminate the need for an additional redox-active layer between the membrane and the inner electric contact in all solid state sensor designs. Electrochemical doping of this system was studied in a symmetrical sandwich configuration: glassy carbon electrode |FcPVC| glassy carbon electrode. Prior electrochemical doping from aqueous solution, resulting in a partial oxidation of the ferrocene groups, was confirmed to be necessary for the sandwich configuration to pass current effectively. The results suggest that only approximately 2.3 mol % of the ferrocene groups are electrochemically accessible, likely due to surface confined electrochemical behavior in the polymer. Indeed, cyclic voltammetry of aqueous hexacyanoferrate (III) remains featureless at cathodic potentials (down to -0.5 V). This indicates that the modified membrane is not responsive to redox-active species in the sample solution, making it possible to apply this polymer as a traditional, single membrane. Yet, the redox capacity of the electrode modified with this type of membrane was more than 520 microC considering a 20 mm(2) active electrode area, which appears to be sufficient for numerous practical ion voltammetric applications. The electrode was observed to operate reproducibly, with 1% standard deviation, when applying pulsed amperometric techniques.

Exploring the Ion Selectivity Properties of a Large Number of Simplified Binding Site Models

The ability to discriminate between different cations efficiently is essential for the proper physiological functioning of many membrane transport proteins. One obvious mechanism of ion selectivity is when a binding site is structurally constrained by the protein architecture and its geometry is precisely adapted to fit an ion of a given size. This mechanism is not effective in the case of flexible protein binding sites that are able to deform structurally or to adapt to a bound ion. In this study, the concept of nontrivial ion selectivity arising in a highly flexible protein binding site conceptually represented as a microdroplet of ligands confined to a small volume is explored. The environment imposed by the spatial confinement is a critical feature of the reduced models. A large number of reduced binding site models (1077) comprising typical ion-coordinating ligands (carbonyl, hydroxyl, carboxylate, water) are constructed and characterized for Na+/K+ and Ca2+/Ba2+ size selectivity using free energy perturbation molecular dynamics simulations. Free energies are highly correlated with the sum of ion-ligand and ligand-ligand mean interactions, but the relative balance of those two contributions is different for K+-selective and Na+-selective binding sites. The analysis indicates that both the number and the type of ligands are important factors in ion selectivity.

Ion-selective properties of nanostructured alkali-metal-doped vanadium oxide

We have studied thin-film electrodes based on nanoparticulate vanadium oxide doped with alkalimetal cations. The hydrogen function of the electrodes manifests itself in the pH range 2–5, with a slope of 55, 54, and 54 ± 2 mV/pH for MxV2O5·nH2O with M = Li, Na, and K, respectively. Their sensitivity range is 10−4 to 10−1 mol/l. The electrode potential as a function of concentration follows the Nernst equation with a slope of 55, 55, and 57 ± 2 mV/pC for M = Li, Na, and K, respectively. The Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ selectivity coefficients of the electrodes are determined.

Synthesis, vibrational spectra, and structure of 4′-(4″-benzo-15-crown-5)oxy-2,2′:6′,2″-terpyridine (L) and its transition metal complexes. Extraction and ion-selective properties of L

Complexes of Co(II), Ni(II), Zn(II), and Cu(II) perchlorates and hexafluorophosphates with 4′-(4″-benzo-15-crown-5)oxy-2,2′:6′,2″-terpyridine (L) [M(L)2](ClO4)2 · 3H2O and [M(L)2](PF6)2 · 2H2O were synthesized. The spectral criteria of ligand coordination through the terpyridine nitrogen atoms were established. An assumption concerning the benzo-15-crown-5 conformation in the ligand molecule in the synthesized complexes was made. The extraction and ion-selective properties of L were studied.

LiMn 2O 4 spinel direct synthesis and lithium ion selective adsorption

The cubic phase LiMn 2O 4 spinel is synthesized via a directly soft chemistry method via hydrothermal reaction of Mn(NO 3) 2, LiOH and H 2O 2 at 383 K for 5–10 h, more favorable to control the nanocrystalline structure with well-defined pore-size distribution and high surface area than traditional solid-phase reaction at high temperature. Further, the 1D MnO 2 nanorod ion-sieves with lithium ion selective adsorption property is prepared by the acid treatment process to completely extract lithium ions from the LiMn 2O 4 lattice. The effects of hydrothermal conditions on the nanostructure, chemical stability and ion-exchange property of the LiMn 2O 4 spinel and MnO 2 ion-sieve are examined via powder X-ray diffraction (XRD), N 2 adsorption–desorption at 77 K, high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and lithium ion selective adsorption measurements. The results show that the 1D MnO 2 nanorods might be utilized in lithium extraction from aqueous environment including brine, seawater and waste water.

Direct Hydrothermal Synthesis of Ternary Li‐Mn‐O Oxide Ion‐Sieves

Spinel-type ternary LiMn(2)O(4) oxide precursor was synthesized by direct hydrothermal synthesis of Mn(NO(3))(2), LiOH, and H(2)O(2) at 383 K for 8 h, a better technique for controlling the nanocrystalline structure with well-defined pore size distribution and high surface area than the traditional solid state reaction method. The final low-dimensional MnO(2) nanorod ion-sieve with a lithium ion selective adsorption property was further prepared by an acid treatment process to completely extract lithium ions from the Li-Mn-O lattice. The effects of hydrothermal reaction conditions on the nanostructure, chemical stability, and ion-exchange property of the LiMn(2)O(4) precursor and MnO(2) ion-sieve were systematically examined via powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and lithium ion selective adsorption measurements. The results show that this new kind of low-dimensional MnO(2) nanorod can be used for lithium extraction from aqueous environments, including brine, seawater, and waste water.

Building And Controlling Networks Of Droplet Interface Bilayers

One goal of synthetic biology is the manufacture of micromachines from simple parts. Such machines would be motile, able to generate, store and use energy, capable of sensing and carrying out computation, and able to take up substrates and convert them to products. We have found that aqueous droplets can be connected by lipid bilayers to form networks in a hydrocarbon environment [1]. We propose to use these networks for the construction of “soft” micromachines (or “prototissues”, by analogy with efforts to build protocells). Proteins can be incorporated in to the bilayers of the networks [2]. Therefore, we expect that membrane proteins will play a role in the functioning of droplet-based micromachines, notably by allowing the droplets to communicate and exhibit emergent properties. Towards this end, we have engineered the staphylococcal alpha-hemolysin pore to endow it with a variety of capabilities. We have been able to alter the pore size, and its ion selectivity and rectification properties. We have also altered the pore so that it is regulated by chemicals, light and temperature. With these components, we have shown that droplet networks can behave like simple electrical circuits [3], be used to form tiny batteries [1] and respond to light [1]. With these subsystems in place, the manufacture of the proposed micromachines may soon be in the offing.1. Holden MA, Needham D, Bayley H: Functional bionetworks from nanoliter water droplets. J. Am. Chem. Soc. 2007, 129:8650-8655.2. Bayley H, Cronin B, Heron A, Holden MA, Hwang W, Syeda R, Thompson J, Wallace M: Droplet interface bilayers. Mol. BioSystems 2008:published ASAP, DOI: 10.1039/b808893d.3. Hwang WL, Holden MA, White S, Bayley H: Electrical analysis of protein pore insertion and blockade in droplet interface bilayer networks. J. Am. Chem. Soc. 2007, 129:11854-11864.

Copper N-(4′-benzo-15-crown-5)-2-(amino-N-tosyl)phenylaldiminate: Complexing and ion-selective properties

The lithium, sodium, and potassium complexes with zinc N-(4′-benzo-15-crown-5)-2-(amino-N-tosyl)phenylaldiminate (ZnL2) have been synthesized. Their IR spectra have been studied, and selected vibrational frequencies of the ligand and complexes have been assigned. Based on spectroscopy and elemental analysis data, the structure of the complexes has been suggested. The ion-selective properties of ZnL2 have been studied by potentiometry.

Synthesis and study of ion-selective properties of 2-azo-[2′-(diphenylphosphinylmethyloxy)phenyl]-4-tert-butylphenol

Synthesis and study of ion-selective properties of 2-azo-[2′-(diphenylphosphinylmethyloxy)phenyl]-4-tert-butylphenol

Fluorescent Sensing Properties of Benzo-18-crown-6 Ethers Containing Thiazoles

The binding properties of dibenzo-18-crown-6 ether (BTB, 1) and napthobenzo-18-crown-6 ether (NTB, 2), containing two thiazole units toward mono, and divalent cations, are outlined. The ion-selective properties of 1 and 2 were studied by measuring their fluorescent emission responses to alkali, alkaline earth, and transition metal ions. BTB (1) showed the highest binding constant toward Ag+ over Pb2+, Hg2+, and Cu2+. NTB (2) revealed a high selectivity toward Ag+ over Pb2+, Hg2+, and Cu2+.