Group IIA Cations Research Articles

Host-guest interactions between p-sulfonatocalix[4]arene and p-sulfonatothiacalix[4]arene and group IA, IIA and f-block metal cations: a DFT/SMD study.

The molecular recognition in aqueous solution is extremely important because most biological processes occur in aqueous solution. Water-soluble members of the calix[n]arene family (e.g., p-sulfonato substituted) can serve as model systems for studying the nature and manner of interactions between biological receptors and small ions. The complex formation behavior of water-soluble p-sulfonatocalix[4]arene and thiacalix[4]arene and group IA, IIA and f-block metal cations has been investigated computationally by means of density functional theory computations in the gas phase and in aqueous environment. The calculated Gibbs free energy values of the complex formation reaction of these ligands with the bare metal cations suggest a spontaneous and energy-favorable process for all metal cations in the gas phase and only for Na+, Mg2+, Lu3+ cations in water environment. For one of the studied cations (La3+) a supramolecular approach with explicit solvent treatment has been applied in the study of the effect of metal hydration on the complexation process. The La3+ binding to the p-sulfonatocalix[4]arene host molecule (now in the metal’s second coordination shell) is still exergonic as evidenced by the negative Gibbs free energy values (ΔG1 and ΔG78). The combination of implicit/explicit solvent treatment seems useful in the modeling of the p-sulfonatocalix[4]arene (and thiacalix[4]arene) complexes with metal cations and in the prediction of the thermodynamic parameters of the complex formation reactions.

Theoretical study on the interaction of glutathione with group IA (Li+, Na+, K+), IIA (Be2+, Mg2+, Ca2+), and IIIA (Al3+) metal cations

The structures and energies of complexes obtained upon interaction between glutathione (GSH) and alkali (Li+, Na+, K+), or alkaline earth metal (Be2+, Mg2+, Ca2+), or group IIIA (Al3+) cations were studied using quantum chemical density functional theory. The characteristics of the interactions between GSH and the metal cations at different nucleophilic sites of GSH were examined selecting systematically, both mono- and multi-coordinating were taken into account. The results indicated that the heteroatom of GSH, the radius and charge of metal ion, and the coordination number of the metal cation with the ligand played important roles in determining the stability of these complexes. Moreover, the intramolecular hydrogen migration in GSH could be promoted by the metal cations during coordination reaction. Furthermore, the Al3+ cation might catalyze the decarboxylation reaction and stimulate the formation of covalent bond between S atom and adjacent O atom of GSH.

Whey protein nanofibrils: Kinetic, rheological and morphological effects of group IA and IIA cations

Self-assembly of whey proteins into amyloid-like fibrils during heating at pH 2 and low ionic strength is sensitive to the presence of NaCl and CaCl2. Our earlier work established that at 10–120 mm of these salts speeds up self-assembly and favours short, flexible fibrils over long semiflexible fibrils in a way that depends on cation concentration and cation type. Here we explored how other monovalent and divalent salts affected fibril morphology and the rheology of fibril dispersions. Divalent salts (MgCl2, CaCl2, BaCl2) had much stronger effects than monovalent salts (LiCl, NaCl, KCl) on gelation kinetics, and differences between salts of the same type were not large. No marked effects of salt type on fibril morphology were evident, but there were subtle differences in the size and extent of fibril networks with monovalent versus divalent salts, which may explain differences in bulk rheology.

Nanoporous Carbohydrate Metal–Organic Frameworks

The binding of alkali and alkaline earth metal cations by macrocyclic and diazamacrobicyclic polyethers, composed of ordered arrays of hard oxygen (and nitrogen) donor atoms, underpinned the development of host-guest supramolecular chemistry in the 1970s and 1980s. The arrangement of -OCCO- and -OCCN- chelating units in these preorganized receptors, including, but not limited to, crown ethers and cryptands, is responsible for the very high binding constants observed for their complexes with Group IA and IIA cations. The cyclodextrins (CDs), cyclic oligosaccharides derived microbiologically from starch, also display this -OCCO- bidentate motif on both their primary and secondary faces. The self-assembly, in aqueous alcohol, of infinite networks of extended structures, which have been termed CD-MOFs, wherein γ-cyclodextrin (γ-CD) is linked by coordination to Group IA and IIA metal cations to form metal-organic frameworks (MOFs), is reported. CD-MOF-1 and CD-MOF-2, prepared on the gram-scale from KOH and RbOH, respectively, form body-centered cubic arrangements of (γ-CD)(6) cubes linked by eight-coordinate alkali metal cations. These cubic CD-MOFs are (i) stable to the removal of solvents, (ii) permanently porous, with surface areas of ~1200 m(2) g(-1), and (iii) capable of storing gases and small molecules within their pores. The fact that the -OCCO- moieties of γ-CD are not prearranged in a manner conducive to encapsulating single metal cations has led to our isolating other infinite frameworks, with different topologies, from salts of Na(+), Cs(+), and Sr(2+). This lack of preorganization is expressed emphatically in the case of Cs(+), where two polymorphs assemble under identical conditions. CD-MOF-3 has the cubic topology observed for CD-MOFs 1 and 2, while CD-MOF-4 displays a channel structure wherein γ-CD tori are perfectly stacked in one dimension in a manner reminiscent of the structures of some γ-CD solvates, but with added crystal stability imparted by metal-ion coordination. These new MOFs demonstrate that the CDs can indeed function as ligands for alkali and alkaline earth metal cations in a manner similar to that found with crown ethers. These inexpensive, green, nanoporous materials exhibit absorption properties which make them realistic candidates for commercial development, not least of all because edible derivatives, fit for human consumption, can be prepared entirely from food-grade ingredients.

Differential Roles of Blocking Ions in KirBac1.1 Tetramer Stability

Potassium channels are tetrameric proteins that mediate K(+)-selective transmembrane diffusion. For KcsA, tetramer stability depends on interactions between permeant ions and the channel pore. We have examined the role of pore blockers on the tetramer stability of KirBac1.1. In 150 mm KCl, purified KirBac1.1 protein migrates as a monomer (approximately 40 kDa) on SDS-PAGE. Addition of Ba(2+) (K(1/2) approximately 50 microm) prior to loading results in an additional tetramer band (approximately 160 kDa). Mutation A109C, at a residue located near the expected Ba(2+)-binding site, decreased tetramer stabilization by Ba(2+) (K(1/2) approximately 300 microm), whereas I131C, located nearby, stabilized tetramers in the absence of Ba(2+). Neither mutation affected Ba(2+) block of channel activity (using (86)Rb(+) flux assay). In contrast to Ba(2+), Mg(2+) had no effect on tetramer stability (even though Mg(2+) was a potent blocker). Many studies have shown Cd(2+) block of K(+) channels as a result of cysteine substitution of cavity-lining M2 (S6) residues, with the implicit interpretation that coordination of a single ion by cysteine side chains along the central axis effectively blocks the pore. We examined blocking and tetramer-stabilizing effects of Cd(2+) on KirBac1.1 with cysteine substitutions in M2. Cd(2+) block potency followed an alpha-helical pattern consistent with the crystal structure. Significantly, Cd(2+) strongly stabilized tetramers of I138C, located in the center of the inner cavity. This stabilization was additive with the effect of Ba(2+), consistent with both ions simultaneously occupying the channel: Ba(2+) at the selectivity filter entrance and Cd(2+) coordinated by I138C side chains in the inner cavity.

FTIR Study on Molecular Motions of Benzene Adsorbed in ZSM-5 Zeolite: Role of Charge-Balancing Cations and Pore Size

Occlusion of benzene in NaZSM-5 zeolite is investigated using in situ FTIR spectroscopy as a function of substitution of Na+ with group IIA cations. At least four pairs of overlapping vibrational bands were observed in the region of out-of-plane C−H bending vibrations (2000−1800 cm-1) on adsorption of benzene in NaZSM-5 at room temperature. Whereas two pairs of these bands, e.g., one pair at around 2007 and 1986 cm-1 and the other at 1969 and 1956 cm-1, correspond to the 1960 cm-1 band of liquid benzene, the other two pairs, e.g., at 1874 and 1852 cm-1 and 1831 and 1810 cm-1, appear in place of the 1815 cm-1 band of liquid benzene in this region. No measurable difference was observed in the frequencies of these bands for adsorption in cation-exchanged samples, suggesting that any specific interaction between cations and benzene molecules is small compared to the effect of benzene−benzene interaction. These multiple bands are therefore attributed to the existence of at least two distinct clustered states o...

Effect of Charge Balancing Cations on the Entrapment of CO in Y-Zeolite: FTIR Spectroscopy Study

The effect of charge balancing cations on encapsulation of CO in the pores of Y-zeolite has been investigated using FTIR spectroscopy. The multiple vibrational bands in CO and CO2 stretching regions, originating from the clusters of these molecules and developed on exposure of CO on Na−Y, undergo an identical shift in frequency and relative intensity when Na+ are exchanged with group IIA cations. Thus, a constant blue shift of ∼18, 12, and 6 cm-1 was observed in all the ν(CO) bands for the samples exchanged with Ca2+, Sr2+, and Ba2+, respectively. The respective Δν(CO2) on Ca2+, Sr2+, and Ba2+ exchanged zeolite-Y were found to be around +6, +2, and −2 cm-1. The observed frequencies of all the CO and CO2 vibrational bands and their respective Δν values with respect to gas-phase frequency are found to show a close relationship with the ionic radius, pore volume, electrostatic potential (e/r), or the field strength (V/A) associated with the exchanging cations. The data reveal that the characteristics of zeol...

Structure and Nonlinear Optical Properties of Cationic Defects in Amorphous Silicon Dioxide. 1. Cluster Studies

We present the first systematic ab initio study of cationic defects in a-SiO2. Cluster models are employed to show that the presence of cationic defects (H+, Li+, Be2+, Na+, and Mg2+) can have large effects on both the structure and (hyper)polarizabilities of the material. The interaction of the proton with the cluster model demonstrates that it is unique compared to the other group IA and IIA cations in the study.

Subunit distribution of calcium-binding sites in Lumbricus terrestris hemoglobin.

The giant, approximately 3.6-MDa hexagonal bilayer hemoglobin (Hb) of Lumbricus terrestris consist of twelve 213-kDa globin subassemblies, each comprised of three disulfide-bonded trimers and three monomer globin chains, tethered to a central scaffolding of 36-42 linkers L1-L4 (24-32 kDa). It is known to contain 50-80 Ca and 2-4 Cu and Zn; the latter are thought to be responsible for the superoxide dismutase activity of the Hb. Total reflection X-ray fluorescence spectrometry was used to determine the Ca, Cu, and Zn contents of the Hb dissociated at pH approximately 2.2, the globin dodecamer subassembly, and linker subunits L2 and L4. Although the dissociated Hb retained 20 Ca2+ and all the Cu and Zn, the globin subassembly had 0.4 to approximately 3 Ca2+, depending on the method of isolation, and only traces of Cu and Zn. The linkers L2 and L4, isolated by reversed-phase high-pressure liquid chromatography at pH approximately 2.2, had 1 Ca per mole and very little Cu and Zn. Electrospray ionization mass spectrometry of linker L3 at pH approximately 2.2 and at neutral pH demonstrated avid binding of 1 Ca2+ and additional weaker binding of 7 Ca2+ in the presence of added Ca2+. Based on these and previous results which document the heterogeneous nature of the Ca2+-binding sites in Lumbricus Hb, we propose three classes of Ca2+-binding sites with affinities increasing in the following order: (i) a large number of sites (>100) with affinities lower than EDTA associated with linker L3 and dodecamer subassembly, (ii) approximately 30 sites with affinities higher than EDTA occurring within the cysteine-rich domains of linker L3 and dodecamer subassembly, and (iii) approximately 25 very high affinity sites associated with the linker subunits L1, L2, and L4. It is likely that the low-affinity type (i) sites are the ones involved in the effects of 1-100 mM Group IIA cations on Lumbricus Hb structure and function, namely increased stability of its quaternary structure and increased affinity and cooperativity of its oxygen binding.

Independent Folding and Ligand Specificity of the C2 Calciumdependent Lipid Binding Domain of Cytosolic Phospholipase A2

The Ca(2+)-dependent lipid binding domain of the 85-kDa cytosolic phospholipase A2 (cPLA2) is a homolog of C2 domains present in protein kinase C, synaptotagmin, and numerous other proteins involved in signal transduction. NH2-terminal fragments of cPLA2 spanning the C2 domain were expressed as inclusion bodies in Escherichia coli, extracted with solvent to remove phospholipids, and refolded to yield a domain capable of binding phospholipid vesicles in a Ca(2+)-dependent manner. Unlike other C2 domains characterized to date, the cPLA2 C2 domain bound preferentially to vesicles comprised of phosphatidylcholine in response to physiological concentrations of Ca2+. Binding of the cPLA2 C2 domain to vesicles in the presence of excess Ca2+ chelator was induced by high concentrations of salts that promote hydrophobic interactions. Despite the selective hydrolysis of arachidonyl-containing phospholipid vesicles by cPLA2, the cPLA2 C2 domain did not discriminate among phospholipid vesicles containing saturated or unsaturated sn-2 fatty acyl chains. Moreover, the cPLA2 C2 domain bound to phospholipid vesicles containing sn-1 and -2 ether linkages and sphingomyelin at Ca2+ concentrations that caused binding to vesicles containing ester linkages, demonstrating that the carbonyl oxygens of the sn-1 and -2 ester linkage are not critical for binding. These results suggest that the cPLA2 C2 domain interacts primarily with the headgroup of the phospholipid. The cPLA2 C2 domain displayed selectivity among group IIA cations, preferring Ca2+ approximately 50-fold over Sr2+ and nearly 10,000-fold over Ba2+ for vesicle binding. No binding to vesicles was observed in the presence of greater than 10 mM Mg2+. Such strong selectivity for Ca2+ over Mg2+ reinforces the view that C2 domains link second messenger Ca2+ to signal transduction events at the membrane.

Unusual behaviour of 7-diethylamino-3-(3,4-ethylen-dioxybenzoyl)coumarin towards Group IIA cations: A potential photoactive probe for magnesium

The titled compound showed, upon spectrophotometric titrations with Group IIA cations, a spectacular bathochromic shift ( c.a. 50 nm) with Mg 2+ but not with the other cations. This result is understood in terms of internal charge-transfer species expressly favored by Et 2N and ArCO substituents and best matching of ionic diameter and charge density of Mg 2+. This finding may lead to new simple photoactive probes.

Foundations of chemical microscopy. 1. Solid-state characterization of 5-nitrobarbituric acid (dilituric acid) and its complexes with Group IA and Group IIA cations

5-Nitrobarbituric acid (dilituric acid) has been used as a chemical microscopic reagent for the qualitative identification of alkali metal (Group IA) and alkaline earth (Group IIA) cations. This methodology was based on the characterization of observed crystal morphologies, since a unique crystal habit could be associated with each adduct product. To understand the scientific foundations which permitted chemical microscopy to function as a useful analytical technique, the products formed between dilituric acid and the Group IA and IIA cations were characterized using polarizing optical microscopy, powder X-ray diffraction, thermal analysis and solid-state nuclear magnetic resonance. It was found that the origins of the different crystal morphologies associated with each of the adduct arose from the ability of the systems to form various hydrate species, which could also contain structural variations due to cation/diliturate packing patterns.

The Role of the Dodecamer Subunit in the Dissociation and Reassembly of the Hexagonal Bilayer Structure of Lumbricus terrestris Hemoglobin

The dissociation of the approximately 3500-kDa hexagonal bilayer (HBL) hemoglobin (Hb) of Lumbricus terrestris upon exposure to Gdm salts, urea and the heteropolytungstates [SiW11O39]8- (SiW), [NaSb9W21O86]18- (SbW) and [BaAs4W40O140]27- (AsW) at neutral pH was followed by gel filtration, SDS-polyacrylamide gel electrophoresis, and scanning transmission electron microscopy. Elution curves were fitted to sums of exponentially modified gaussians to represent the peaks due to undissociated oxyHb, D (approximately 200 kDa), T+L (approximately 50 kDa), and M (approximately 25 kDa) (T = disulfide-bonded trimer of chains a c, M = chain d, and L = linker chains). OxyHb dissociation decreased in the order Gdm*SCN > Gdm.Cl > urea > Gdm.OAc and AsW > SbW > SiW. Scanning transmission electron microscopy mass mapping of D showed approximately 10-nm particles with masses of approximately 200 kDa, suggesting them to be dodecamers (a+b+c)3d3. OxyHb dissociations in urea and Gdm.Cl and at alkaline pH could be fitted only as sums of 3 exponentials. The time course of D was bell-shaped, indicating it was an intermediate. Dissociations in SiW and upon conversion to metHb showed only two phases. The kinetic heterogeneity may be due to oxyHb structural heterogeneity. Formation of D was spontaneous during HBL reassembly, which was minimal (</= 10%) without Group IIA cations. During reassembly, maximal (approximately 60%) at 10 mM cation, D occurs at constant levels (approximately 15%), implying the dodecamer to be an intermediate.

Selective sensing of guanidinium and tetraalkylammonium ions using lipophilic cyclodextrins

A range of lipophic cyclodextrin derivatives, including 2,6-didodecyl-α-and-β-cyclodextrins, and 2,3,6-tri-O-ethyl-β-cyclodextrin, has been exmined as ionophores in electrode response studies of the detection of guanidine, alkylated guanidines, creatinine and several tetraalkylammonium ions including choline, acetylcholine and the long-chain cationic surfactants CnH2n+1NMe3+(n= 8 → 16). Interference from Group Ia and IIa cations is minimal for alkylammonium ions (–log kpot > 4 typically) and in certain cases binding of the ion has been further defined by electrospray mass sectrometric and NMR methods.

Parallel and antiparallel G-DNA structures from a complex telomeric sequence.

We investigated the formation in vitro of higher order structures by a DNA oligomer containing the terminal motif TGTG3TGTGTGTG3, derived from the Saccharomyces telomeric consensus, in order to (a) understand why certain cations favor the formation of parallel-stranded (G4 and G8) G-DNA structures, while others favor foldback, antiparallel structures (G'2) and (b) probe the structures of G-DNAs formed by this telomeric sequence, which is more complex than its well-studied counterparts from the protozoans oxytricha and tetrahymena. We find that dramatic switches in the formation of G4 versus G'2 structures occur in solutions of not only the group Ia cations, Li(+)-Cs+, but also in those of the group IIa cations, Mg(2+)-Ba2+. These data and the temperature-dependent formation and destruction of the different structures lend support to the kinetic scheme of Sen and Gilbert (1990), by which rapidly forming G'2 structures accumulate in highly stabilizing potassium (and strontium) solutions at the expense of the thermodynamically more stable G4 structures. Both the G4 and the G'2 complexes formed by the Saccharomyces sequence show novel structural features. Protection and interference experiments with dimethyl sulfate and potassium permanganate reveal that the core of alternating thymines and guanines within the telomeric motif plays a critical role in the stabilization of the parallel G4 structure, but not of the antiparallel G'2. Very likely, in the G4 complex, this GT core forms a novel higher order arrangement of alternating G and T quartets, the latter possibly comparable to the U quartets described by Cheong and Moore (1992) in their NMR study of the higher order structure formed by rUG4U.

The role of metal ions in the conformation of the four-way DNA junction.

Metal ions fold DNA junctions into a compact conformation that confers protection of all thymine bases to modification by osmium tetroxide. In the absence of the cation the arms of the junction are fully extended in an approximately square-planar configuration. Group IIa cations are effective in achieving a folded conformation of the junction at 80-100 microM, and there is an excellent agreement between the ionic concentrations that fold the junctions as deduced from gel electrophoretic experiments, and those that prevent osmium tetroxide reaction at the junction. Hexamminecobalt(III) achieves full folding at 2 microM, while spermine and spermidine are effective at 25 microM. Some transition metal ions such as Ni(II) may replace the group IIA cations. Monovalent ions of group IA are only partially effective in folding the junctions. Very much higher concentrations are necessary, gel electrophoretic mobilities suggest that a less symmetrical conformation is adopted and thymine bases at the junction remain reactive to osmium tetroxide. Charge-charge interactions at the centre of the junction are structurally extremely important. Substitution of junction phosphate groups by uncharged methyl phosphonates severely perturbs the structure of the junction. If just two phosphates are substituted, diametrically facing across the junction, the structure always folds in order to place the electrically neutral phosphate on the exchanging strands. We suggest that folding of the junction into the stacked X-structure generates electronegative clefts that can selectively bind metal ions, depending on the chemistry, size and charge of the ion. Moreover, occupation of these cavities is essential for junction folding, in order to reduce electrostatic repulsion.(ABSTRACT TRUNCATED AT 250 WORDS)

The voltage sensor of excitation-contraction coupling in skeletal muscle. Ion dependence and selectivity.

Manifestations of excitation-contraction (EC) coupling of skeletal muscle were studied in the presence of metal ions of the alkaline and alkaline-earth groups in the extracellular medium. Single cut fibers of frog skeletal muscle were voltage clamped in a double Vaseline gap apparatus, and intramembrane charge movement and myoplasmic Ca2+ transients were simultaneously measured. In metal-free extracellular media both charge movement of the charge 1 type and Ca transients were suppressed. Under metal-free conditions the nonlinear charge distribution was the same in depolarized (holding potential of 0 mV) and normally polarized fibers (holding potentials between -80 and -90 mV). The manifestations of EC coupling recovered when ions of groups Ia and IIa of the periodic table were included in the extracellular solution; the extent of recovery depended on the ion species. These results are consistent with the idea that the voltage sensor of EC coupling has a binding site for metal cations--the "priming" site--that is essential for function. A state model of the voltage sensor in which metal ligands bind preferentially to the priming site when the sensor is in noninactivated states accounts for the results. This theory was used to derive the relative affinities of the various ions for the priming site from the magnitude of the EC coupling response. The selectivity sequence thus constructed is: Ca greater than Sr greater than Mg greater than Ba for group IIa cations and Li greater than Na greater than K greater than Rb greater than Cs for group Ia. Ca2+, the most effective of all ions tested, was 1,500-fold more effective than Na+. This selectivity sequence is qualitatively and quantitatively similar to that of the intrapore binding sites of the L-type cardiac Ca channel. This provides further evidence of molecular similarity between the voltage sensor and Ca channels.

Synthesis and binding studies of neutral dioxydiamide ionophores -III

The syntheses of several bis-(1,2-phenylenedioxydiscetamides) with eight binding sites (four ethers, four amides) are described. It had been anticipated that these bis-compounds would be much stronger binders for Group IIA cations than are members of our previously described 1,2-phenylenedioxydiacetamide system, e.g. 1-3, which give isolable complexes usually of 2:1 ligand/metal cation stoichiometry. Binding constants for the new diesters 4 and 5 were determined in methanol using UV absorption changes and the Scatchard method. The binding strength of 4 was concentration dependent and only moderately greater than that for 1 or for the more closely related 4-hydroxymethyl compound 3. Diester 5 was a weaker binder for Group IIA cations than was either 1 or 3. Cooperativity of the two sets of binding sites with either Sr 2+ or Ba 2+ was demonstrated for 4 but not for 5. Electrochemical selectivity values (K pot ij) as determined by Simon et al for 4, 5, and 6, in liquid membrane electrodes are reported for various cations. High ion selectivity for Na + vs either Ca 2+ or K + were found, especially for 6.

Calixarene-Based Amide Ionophores for Group IIA Metal Cations

Abstract Amide function-containing oligopodands synthesized from calixarenes exhibited characteristic selectivities toward Group IIA cations over Group IA cations in solvent extraction.

The preparation and properties of neutral diamide ionophores for group IIA metal cations-II

The elaboration of a series of neutral ligands featuring vicinal ether and N,N-dialkylacetamido groups is described. Several of these ligands were prev with K app=10 3-10 5 in methanol of the Group IIA cations for which they are sensitive. Binding constants by the Scatchard method are reported for a number of ligands mainly with Ca, Sr, Mn and Ba. The ligands in methylene chloride solution extract these cations via their picrate salts from water. The electrochemical K ij pot (selectivity) values for some of the ligands when they are incorporated into liquid membrane electrodes and tested with various cations as determined by Simon . (ETH Zurich) are reported. Selectivity ratios of over 100:1 for Na vs Ca were found for several piperidenyl amides of 1,2-phenylenedioxydiacetic acid. Incorporation of N-methylamino instead of ether oxygen groups into the basic structure gives a stronger cation binder which is still selective for Group IIA vs Group IA cations but with binding capacity for transition metal cations also. Limitations of the Scatchard plots for these ligands and the non-correspondance of electrochemical selectivities with the ordering of binding of cations in single liquid phase are discussed.