Selective Cation Binding Research Articles

Crystallographic Deciphering of Spontaneous Self-Assembly of Achiral Calciphores to Chiral Complexes.

Thiapyricins (TPC-A/B, 1 and 2), which are new metallophore scaffolds exhibiting selective divalent cation binding property, were produced in response to metal-deprived conditions by Saccharothrix sp. TRM_47004 isolated from the Lop Nor Salt Lake. TPCs represent a thiazolyl-pyridine skeleton of a calcium-binding natural product, calciphore, owing to the selectivity to calcium ions among diverse metal ions. The thiapyricins exhibited notable co-crystalline characteristics of the apo- and holo-forms with racemic enantiomers comprising a pair of space isomers in a Δ/Λ-form. Therefore, we postulated a mechanism for the four-hierarchical self-assembly of achiral natural products into chiral complexes. Furthermore, their metal-chelating trait aided the adaptation of the host during metal starvation by increasing the production of TPCs. This study presents a structural paradigm of a new calciphore, provides insight into the mechanism of natural product assembly, and highlights the causality between the production of the metallophore and metallic habitats.

Selective Phase Transfer Reagents (OxP-crowns) for Chromogenic Detection of Nitrates Especially Ammonium Nitrate.

Nitrogen and phosphorus-containing ions such as ammonium, nitrates and phosphates are anthropogenic pollutants while ammonium nitrate may be diverted for nefarious purposes in improvised explosive devices. Crown ether-oxoporphyrinogen conjugates (OxP-crowns) are used to selectively detect nitrates, especially their ion pairs with K+ and NH4 + , based on ion pair complexation of OxP-crowns under phase transfer conditions. The presence of phosphate and carbonate lead to deprotonation of OxP-crowns. OxP-1N18C6 is capable of extracting ion pairs with nitrate from aqueous phase leading to a selective chromogenic response. Deprotonation of the OxP moiety leads to [OxP- ]-1N18C6[K+ ] and is promoted by crown ether selective cation binding coupled with hydration of basic oxoanions, which are constrained to remain in the aqueous phase. This work illustrates the utility of molecular design to exploit partitioning and ion hydration effects establishing the selectivity of the chromogenic response.

Free-energy landscapes of membrane co-translocational protein unfolding

Protein post-translational translocation is found at the plasma membrane of prokaryotes and protein import into organellae. Translocon structures are becoming available, however the dynamics of proteins during membrane translocation remain largely obscure. Here we study, at the single-molecule level, the folding landscape of a model protein while forced to translocate a transmembrane pore. We use a DNA tag to drive the protein into the α-hemolysin pore under a quantifiable force produced by an applied electric potential. Using a voltage-quench approach we find that the protein fluctuates between the native state and an intermediate in the translocation process at estimated forces as low as 1.9 pN. The fluctuation kinetics provide the free energy landscape as a function of force. We show that our stable, ≈15 kBT, substrate can be unfolded and translocated with physiological membrane potentials and that selective divalent cation binding may have a profound effect on the translocation kinetics.

Selective cation and anion guest binding in host selenazamacrocycles.

We report selenazamacrocycle hosts that are the first system to change guest binding affinity from cation to anion depending upon macrocycle oxidation/reduction. Selective cation (Fe2+) or anion (BF4-) binding occurs with both ions present and under identical reaction conditions. We also report the first macrocyclic complex with a Fe-Se bond.

Structural change of nonionic surfactant self-assembling at electrochemically controlled HOPG/electrolyte interface

In this study, we have observed structural change of self-assembling nonionic pentaethylene glycol monododecyl ether (C12E5) surfactant formed on a potential-controlled highly oriented pyrolytic graphite (HOPG) surface by electrochemical frequency modulation atomic force microscopy (EC-FM-AFM). At a negative and a positive potential side with respect to the point of zero charge (pzc), a periodic hemicylindrical structure is formed on the whole surface, while a disordered single-layered thin film is formed at a potential around the pzc. This demonstrates that the electrochemical tuning of the electrode potential enables precise control of the nonionic self-assembly structure. We also found that the self-assembly structure is expanded by Na+ ion contained in C12E5 solution. This can be reasonably explained by selective cation binding of a C12E5 molecule like a crown ether. In addition, we estimated a response time of phase transition between the two phases by electrochemical impedance measurement.

Selective Cation Binding to the Gating-Ring Triggers Independent RCK Motions in the BK Channel

Large conductance voltage and calcium dependent potassium channels (BK channels) are important controllers of cell excitability. Functional BK channels are formed as tetramers of alpha subunits. Their open probability is regulated synergistically by changes in transmembrane voltage and intracellular concentration of divalent cations including Ca2+, Mg2+, Cd2+ and Ba2+. The voltage sensor resides within the transmembrane region of the alpha subunit, whereas a large C-terminal intracellular region called ‘gating ring’ senses divalent cation binding. The gating ring assembles as a large tetrameric structure that contains a total of eight Regulator of Conductance of K (RCK) domains, two from each subunit. In each RCK domain resides one high-affinity Ca2+ binding site. Even though the two RCK domains of each subunit (RCK1 and RCK2) are similar, their Ca2+ binding sites are not, particularly in their selectivity to divalent ions other than Ca2+. For example, Cd2+ effect is mediated exclusively through the RCK1 domains, while Ba2+ and Mg2+ (at low concentrations) prefer solely the site from the RCK2 domains. Using patch clamp fluorometry, we have shown previously that large rearrangements of the gating-ring occur in response to Ca2+ and voltage. By themselves, Cd2+, Mg2+ and Ba2+ induce movements of the gating-ring that are substantially smaller than those triggered by Ca2+. Full extent motions are achieved once binding sites from RCK1 and RCK2 domains are occupied by Cd2+ and Ba2+, or Cd2+ and Mg2+. These results suggest that the gating ring structure is flexible, potentially allowing independent movements of the RCK1 and RCK2 domains.

A Novel Ferrocenyl Naphthoquinone Fused Crown Ether as a Multisensor for Water Determination in Acetonitrile and Selective Cation Binding.

A multisensor which is based on a novel multifunctional triad molecule, ferrocenyl naphthoquinone fused crown ether (Fc-cnq) bearing ferrocene, quinone, and crown ether functional groups together, was synthesized and characterized in this study. Sensing performance of a trace amount of water and the selective cation binding capabilities of this multisensor were carried out by the electrochemical, spectroelectrochemical, and spectrophotometric titration techniques in acetonitrile (CH3CN). It was shown that the potential separation (E((Fc))1/2 - E((2))1/2) between the second reduction of naphthoquinone and the oxidation processes of ferrocene in the triad molecule Fc-cnq was proportional to the amount of water due to the hydrogen-bonding interactions between water and the doubly reduced species (Fc-cnq(2-)). This property enabled Fc-cnq to detect the trace amount of water in CH3CN. The half-wave potential (E((Fc))1/2) of the ferrocene in Fc-cnq was used as an internal reference potential, and it defined the accuracy of the detection. In addition, by using the UV-vis spectrophotometric titration technique in CH3CN, it was also shown that the Fc-cnq multisensor could bind Ba(2+) and Ca(2+) cations selectively. We proposed that the intramolecular charge-transfer (CT) transition which occurred between the donor ferrocene and the acceptor naphthoquinone was the principle mechanism for the selective binding property of this multisensor. Quantum chemical calculations were also performed to investigate optical and electronic properties of the Fc-cnq molecule.

Tuning Microbial Surfaces to Control Carbonate Mineralization

Carbon dioxide sequestration is an essential component of climate change mitigation, including geologic sequestration the underground storage CO2, and surface sequestration. Carbonate minerals are stable forms of CO2 storage, but their geologic formation is slow. Many microbes alter carbonate mineral morphology; however mechanisms of such mineralization are largely unknown. Hypothesized mechanisms include metabolic processes that alter pH and supersaturation, as well as cell surface properties that induce mineral nucleation. This work systematically investigates these mechanisms by allowing calcium carbonate (CaCO3) formation in the presence or absence of microbes with various native surfaces features including lipopolysaccharides, surface layer proteins (S-layers) and engineered surface features.Surprisingly, formation of stable crystalline CaCO3 was accelerated by the presence of all microbes relative to abiotic solutions. This rate acceleration also occurred for metabolically inactive bacteria, indicating that metabolic activity was not necessary. Rather, since CaCO3 crystals increased in number as the cell density increased, results indicate that many bacterial species accelerate the nucleation of CaCO3 crystals. To understand the role of specific biomolecules on nucleation, we engineered variants of surface layer proteins (S-layers), which are tractable models to evaluate surface interfaces through peptide display. Bacterial surface charge and cation binding was assessed and correlated to bacterial surface chemistry and biomineralization experiments. From these results, we propose that the S-layer surfaces that can selectively attract Ca2+ ions, serve as nucleation sites for CaCO3, thereby accelerating crystal formation, and that engineered variants with selective cation binding enhance this effect. These observations provide substantive evidence for a non-specific nucleation mechanism, and stress the importance of microbes, on the rate of formation of carbonate minerals. This work also indicates that microbes with tuned S-layer surfaces could be used to enhance the sequestration of CO2 as stable mineral carbonates. http://foundry.lbl.gov/afgroup/index.html

Emergence of Symmetry and Chirality in Crown Ether Complexes with Alkali Metal Cations

Crown ethers provide a valuable benchmark for the comprehension of molecular recognition mediated by inclusion complexes. One of the most relevant crown ethers, 18-crown-6 (18c6), features a flexible six-oxygen cyclic backbone that is well-known for its selective cation binding. This study employs infrared spectroscopy and quantum mechanical calculations to elucidate the structure of the gas-phase complexes formed by the 18c6 ether with the alkali metal cations. It is shown that symmetric and chiral arrangements play a dominant role in the conformational landscape of the 18c6-alkali system. Most stable 18c6-M(+) conformers are found to have symmetries C(3v) and C(2) for Cs(+), D(3d) for K(+), C(1) and D(3d) for Na(+), and D(2) for Li(+). Remarkably, whereas the bare 18c6 ether is achiral, chirality emerges in the C(2) and D(2) 18c6-M(+) conformations, both of which involve pairs of stable atropoisomers capable of acting as enantiomeric selective substrates.

Mechanism of Selective Cation Binding to Sodium-Coupled Transporters: Insights from Free Energy Simulations and QM/MM Simulations

Ion-coupled transport of neurotransmitter molecules by secondary amino-acids transporters plays pivotal role in the regulation of neuronal signaling. One of the major events in the transport cycle is ion-substrate coupling and formation of the high-affinity occluded state with bound ions and substrate. Molecular mechanisms of ion-substrate coupling, specificity for a particular cation and the corresponding ion-substrate stoichiometry in secondary transporters has yet to be understood. We have studied Li+/K+/Tl+/Na+ binding and/or selectivity to several transporters with available crystal structures such as the bacterial aspartate transporter GltPh, leucing transporter LeuT and maltose transporter vSGLT using free energy simulations and QM/MM minimization to evaluate the role of different factors in the observed selectivity and ion binding to the protein. Two different mechanisms were found to co-exist for crystallographically characterized binding sites Na1 and Na2 in LeuT and Glt. Furthermore, site Na1 appeared to be well conserved amongst members of different families. To evaluate the role of Na+ binding in the transporter function, we have performed free energy simulations to determine actual cation selectivity as well as binding affinity for sites Na1 and Na2 in the protein. QM/MM minimization was used to characterize the role of the electronic effects of the stabilization of non-native cations such as Li+ and Tl+ in the Na+-selective sites of different transporters. In the case of Tl+ binding to Glt transporter, neighboring residues from a second solvation shell provide a necessary stabilization to the larger cation due to polarization and charge transfer effects implying a rather large flexibility of the metal binding sites.

Magnetic exchange coupling tunable by means of selective cation binding into poly(radical-armed) azacrowns

Host-guest complexes of tris(NN-CH2)-substituted 1,4,7-triazacyclononane and tetrakis(NN-CH2)-substituted 1,4,7,10-tetraazacyclododecane afforded the folded structures of the hosts where the NN radical oxygen atoms were coordinated to the guest metal ion, leading to enhanced antiferromagnetic couplings in the solid state and an ESR line broadening in the solution phase (NN = 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide).

Selective Metal Cation Binding of Elastomeric Protein: Two Types of Binding Sites and Elastomeric Functions

Selective Metal Cation Binding of Elastomeric Protein: Two Types of Binding Sites and Elastomeric Functions

Modeling the Selectivity of Potassium Channels with Synthetic, Ligand-Assembled π Slides

A supramolecular ion channel model mediates transmembrane ion transport (shown schematically) with a selectivity topology similar to that of K+ channels. This supports the biological significance of flexible arene arrays as selective cation binding sites.

Modeling the Selectivity of Potassium Channels with Synthetic, Ligand-Assembled π Slides.

A supramolecular ion channel model mediates transmembrane ion transport (shown schematically) with a selectivity topology similar to that of K+ channels. This supports the biological significance of flexible arene arrays as selective cation binding sites.

Selective cation binding of crown ether acetals in electrospray ionization mass spectrometry

Download options Please wait... Article information DOI https://doi.org/10.1039/A703286B Article type Paper Download Citation J. Chem. Soc., Perkin Trans. 2, 1998, 145-148 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Selective cation binding of crown ether acetals in electrospray ionization mass spectrometry T. Oshima, F. Matsuda, K. Fukushima, H. Tamura, G. Matsubayashi and R. Arakawa, J. Chem. Soc., Perkin Trans. 2, 1998, 145 DOI: 10.1039/A703286B To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Takumi Oshima Fumikazu Matsuda Kazuaki Fukushima Hatsue Tamura Gen-etsu Matsubayashi Ryuichi Arakawa

A Crown Ether Bearing Fulgide: The Regulation of Photochromism by Supramolecular Effects

AbstractA novel benzo‐15‐crown‐5 modified fulgide (CF) has been prepared. The absorption spectra of CF and its colored form (CFDHN) are hypsochromically shifted and the decolouring rate of CFDHN is apparently decreased through selective cation binding.

Selective cation binding with cis,cis-1,3,5-trioxycyclohexyl based ligands: application to ion transport and electrochemical detection and assessment of complexation by electrospray mass spectrometry

The synthesis is reported of two sets of oxa-amide ionophores based on 2-phenylglycerol and cis,cis-cyclohexane-1,3,5-triol, with ligand coordination numbers of four, five and six. Ionophores based on the hexadentate cyclohexyl triamide 9 show excellent Na+/K+ selectivity (-log KpotNa,K = 3.1), and the pentadentate analogue 10 shows good Li+/Na+ selectivity (-log KpotLi,Na = 2.2). Solution NMR and liquid–liquid extraction studies confirmed the formation of 1∶1 complexes and IR studies pinpointed the presence of amide binding. Ligands based on 2-phenylglycerol exhibited good Ca2+ selectivity which was highest for the hexadentate triamide, 6. Detailed ESMS studies revealed similar trends in ion-binding, performed under controlled conditions.

A New Type of Photochromic Spirodihydroindolizines and Their Ability of Cation Binding

Abstract The photophysical properties of photochromic spirodihydro-indolizines (DHI) with podand structures can be tuned by selective cation binding. Complexation of alkaline earth metal ions by the DHI provokes changes in the electronic states of the host molecules, which is evident in changes in the absorption spectra. Thus binding constants could be determined by uv spectroscopy. Two methods have been used to evaluate the titration data:. The double-reciprocal Benesi- Hildebrand plot and the direct fit of the observation equation by nonlinear least-squares analysis.

Cation recognition properties of polypyrrole 3-substituted by azacrown ethers

The synthesis of new functionalized conducting polypyrroles bearing azacrown ethers at C-3 is described; these show selective cation binding on voltammetric cycling in organic media.

Ion behaviour in plant cell walls. IV. Selective cation binding by Sphagnum russowii cell walls

Selective cation binding by Sphagnum russowii cell walls was investigated using data from bicationic potentiometric titrations of isolated cell walls. Selectivity measurements were interpreted in the context of Manning condensation. In titrations with cations of different valency, selectivity favoured the higher valency cation, as expected in Manning condensation. This selectivity generally increased with bathing pH until the wall-bound polyuronic acids became fully ionized (pH > 5). The selectivity coefficient order at full ionization was Na+–Ca2+ > Na+–La3+ > Ca2+–La3+, as predicted by the weak acid Donnan–Manning (WADM) model. Other phenomena also appear to influence binding selectivity. A small population of isolated binding sites are more effectively neutralized by univalent (or divalent) than divalent (or trivalent) cations. High selectivity for cations of lower valency at low pH also suggests a site isolation effect. In bicationic titrations involving divalent cations only, Sr24+ and Ca2+ were bound by the cell wall with approximately equal effectiveness, as predicted by the WADM model. However, cation or binding site specificities probably account for the favoured binding of Ca2+ over Mg2+ by the cell wall. Key words: ion exchange, cell wall, selectivity.