Preference For Cations Research Articles

Structure of AgxNa1−xPO3 glasses by neutron diffraction and reverse Monte Carlo modelling

We have performed structural studies of mixed mobile ion phosphate glassesAgxNa1−xPO3 using diffraction experiments and reverse Monte Carlo simulations. This glass system isparticularly interesting as a model system for investigations of the mixed mobile ion effect,due to its anomalously low magnitude in the system. As for previously studiedmixed alkali phosphate glasses, with a much more pronounced mixed mobile ioneffect, we find no substantial structural alterations of the phosphorous–oxygennetwork and the local coordination of the mobile cations. Furthermore, the mobileAg+ and Na+ ions are randomly mixed with no detectable preference for either similar or dissimilar pairsof cations. However, in contrast to mixed mobile ion systems with a very pronouncedmixed mobile ion effect, the two types of mobile ions have, in this case, verysimilar local environments. For all the studied glass compositions the averageAg–O and Na–O distances in the first coordination shell are determined to be2.5 ± 0.1 and 2.5 ± 0.1 Å, and the corresponding average coordination numbers are approximately 3.2 and 3.7, respectively.The similar local coordinations of the two types of mobile ions suggests that the energy mismatch fora Na+ ion to occupy a site that previously has been occupied by aAg+ ion (and vice versa) is low, and that this low energy mismatch is responsible for theanomalously weak mixed mobile ion effect.

Insight into the selectivity and gating functions of Streptomyces lividans KcsA

Streptomyces lividans KcsA is a 160-aa polypeptide that oligomerizes to form a tetrameric potassium channel. The three-dimensional structure of the polypeptides has been established, but the selectivity and gating functions of the channel remain unclear. It has been shown that the polypeptides copurify with two homopolymers, poly[(R)-3-hydroxybutyrate] (PHB) and inorganic polyphosphate (polyP), which have intrinsic capacities for cation selection and transport. PHB/polyP complexes are highly selective for divalent cations when pH is greater than the pK(2) of polyP ( approximately 6.8), but this preference is lost when pH is < or =pK(2). It is postulated that KcsA polypeptides attenuate the divalent negative charge of the polyP end unit at physiological pH by strategic positioning of two C-terminal arginines. Here we mutate one or both of the C-terminal arginines and observe the effects on channel selectivity in planar lipid bilayers. We find that channels formed by KcsA polypeptides that retain a single C-terminal arginine remain highly selective for K(+) over Mg(2+), independent of medium pH; however, channels formed by KcsA polypeptides in which both C-terminal arginines have been replaced with neutral residues are selective for Mg(2+) when pH is >7 and for K(+) when pH is <7. Channel gating may be triggered by changes in the balance between the K(+) polyP(-) binding energy, the membrane potential, and the gradient force. The results reveal the importance of the C-terminal arginines to K(+) selectivity and argue for a supramolecular structure for KcsA in which the host polypeptides modify the cation preference of a guest PHB/polyP complex.

Oral secretions from herbivorous lepidopteran larvae exhibit ion channel-forming activities

Feeding insects introduce oral secretions (OS) into the wounded tissue of the attacked plant. Various OS-derived molecules must be involved in subsequent processes including the induction of plant defence reactions. Using the planar lipid bilayer membrane technique, isolated OS were analyzed with respect to their membrane activities. Transmembrane ion fluxes were generated by OS of eight different lepidopteran larvae, all of which form comparable ion channels in artificial membranes. Currents were characterized by long lasting open times and conductivities from 250 pS up to 1100 pS. Channels formed by Spodoptera exigua secretions showed a preference for cations over anions. OS also induced a transient increase of the cytosolic calcium concentration in soybean cells, determined by the aequorin technique. Known compounds of the OS, fatty-acid–glutamine conjugates, also interfered with the membrane but were unable to form stable channels. Since ion fluxes and depolarization are early responses upon insect feeding, OS-derived components may be involved in the elicitation process by direct interaction with the plant membranes.

Haemophilus influenzae Outer Membrane Protein P5 Is Associated with Inorganic Polyphosphate and Polyhydroxybutyrate

Outer membrane protein P5 of nontypeable (acapsulate) Haemophilus influenzae (NTHi P5) forms large pores in planar lipid bilayers between symmetric solutions that unpredictably display a nonzero reversal potential. Moreover, NTHi P5 has a high theoretical isoelectric point, calculated as 9.58, which is not in agreement with the experimental isoelectric point, determined as 6.3–6.8, or with its preference for cations, disproportionately strong at one side. These anomalous results intimate that NTHi P5 is associated with a polyanion. Chemical and immunological analyses revealed the presence of inorganic polyphosphate (polyP), and the amphiphilic, solvating polyester, poly-(R)-3-hydroxybutyrate, frequently associated with polyP. A sharp reduction in cation selectivity was observed after addition of Saccharomyces cerevisiae exopolyphosphatase X to the bilayer, providing functional evidence for the involvement of polyP in selectivity. The results suggest that NTHi P5 associates with polyP and poly-(R)-3-hydroxybutyrate to create large, cation-selective pores in the outer membrane of H. influenzae.

Mutational analysis of the GDD sequence motif of classical swine fever virus RNA-dependent RNA polymerases

To define the function of the GDD motif of the RNA-dependent RNA polymerase (RdRp) of classical swine fever virus (CSFV), single amino acid substitutions were introduced into the CSFV NS5B. All substitutions within the GDD motif were detrimental to the polymerase activity, the binding activity and the terminal nucleotidyl transferase activity of the NS5B protein. It was also found that the wild-type NS5B had higher RdRp activity with Mg(+2) than with Mn(+2) whereas some mutants worked better with Mn(+2) than with Mg(+2), suggesting that substitutions within the GDD motif modified the enzyme cation preferences and the GDD sequence of CSFV NS5B might be involved in polymerase-metal interaction. Therefore, the GDD amino acid sequence is important for the function of CSFV RdRp.

La–Zn Substituted Hexaferrites Prepared by Chemical Method

La-Zn substituted M-type Ba hexaferrite powders were prepared by sol-gel (Mx) and organometallic precursor (Sk) methods with Fe/Ba ratio of 11.6 and 10.8, respectively. The compositions (LaZn)xBa1−x Fe12−x O19 with 0.0 ≤ x ≤ 0.6 were annealed at 975°C/2 h. The cationic site preferences of nonmagnetic La3+ instead of Ba2+ ions and Zn2+ instead of Fe3+ ions were determined by Mossbauer spectroscopy. The La3+ ions substitute the large Ba2+ ions at 2a site and for x ≥ 0.4 also at 4f2 site. The nearly all Zn2+ ions are placed at the 4f1 sites. The thermomagnetic analysis of χ(ϑ) confirms that only the small substitutions for x ≤ 0.4 can be taken as a single-phase hexaferrites. The coercivity H c almost does not change at x = 0.2 for (Mx) samples and further decrease up to x = 0.6. For (Sk) samples at substitution x = 0.2 the values of H c are decreasing and at higher x the values nearly do not change. The Curie points, T c, slowly decrease with x for both (Mx) and (Sk) samples.

Influence of Co–Zr substitution on coercivity in Ba ferrites

Polycrystalline BaCo x Zr x Fe 11.5−2 x O 18.25 samples with 0 ⩽ x ⩽ 0.6 ions per formula units were prepared by modified citrate precursor method with the initial ratio of Ba:Fe equal to 1:11.5. The cationic site preferences of Co 2+ and Zr 4+ in Co–Zr substituted Ba ferrite were investigated by magnetic measurements and X-ray diffractometer (XRD) analysis. The coercivity H c was decreasing with increasing Co–Zr substitution. The datum showed that the max coercive force ( H c) was obtained when substitution of 0.2, while the best saturation magnetizations ( M s) was obtained when substitution of 0.4.

Flow‐Through and Batch Methods for Determining Calcium‐Magnesium and Magnesium‐Calcium Selectivity

Conducting binary‐exchange experiments is a common way to identify cationic preferences of exchanger phases and the two most common approaches are the flow‐through and batch methods. The objectives of this research were to provide the materials and methods for both flow‐through and batch equilibration techniques and to compare Ca‐Mg and Mg‐Ca selectivity when using these two methods on a montmorillonitic soil. The methods were evaluated by comparing both the Gibbs free energy values (ΔGex) and selectivity diagrams derived from the flow‐through and batch exchange reactions. The ΔGex values for the Ca‐Mg reaction were 634 and 444 J mol−1 as determined by the flow‐through and batch methods, respectively, indicating an exchanger preference for Ca. Exchanger preference for Ca was also evident in the Mg‐Ca reaction with ΔGex values of −882 and −784 J mol−1 for the flow‐through and batch methods, respectively. The flow‐through and batch methods worked very well for determining cation selectivity and results indicate no significant differences existed between the two methods.

Chelating resin containing hybrid calixpyrroles: New sorbent for noble metal cations

Novel chelating resins containing macrocyclic ligand calix[4]pyrrole[2]thiophene 6 immobilized on a cross-linked vinylbenzyl chloride/divinylbenzene copolymer are reported. The modification of the polymeric support by the macrocycle was followed by elemental and IR spectral analysis, indicating a significant degree of modification (up to 0.29 mmol of the macrocyclic ligand per gram resin after 7 days of modification – Resin 4). This material has shown a marked preference for cations of noble metals (Au(III), Ag(I), Pt(IV), Pd(II)) in static sorption studies over other metal cations, including Cu(II), Pb(II) or Cd(II). The affinity of the resin for noble metal cations is a result of the presence of “soft” electron pair donor atoms (sulfur) in the macrocycle, as well as the fact that the size of the macrocyclic receptor and metal cations are comparable. Langmuir-type isotherms were observed for platinum, palladium and silver on Resin 4, but the uptake of gold is probably driven by the reduction of Au(III) to metallic gold on the surface of the resin, therefore considerably high values of uptake were observed in this case (up to 335 mg/g resin). Gold was also found to be preferably sorbed and reduced in the presence of other noble metals cations; the selectivity coefficient in gold/silver binary mixture was 130.

Synthesis, NMR characterization and ion binding properties of 1,3-bridged p- tert-butyldihomooxacalix[4]crown-6 bearing pyridyl pendant groups

1,3-Di(2-pyridylmethoxy)- p- tert-butyldihomooxacalix[4]arene-crown-6 ( 2 ) was synthesized for the first time. 2 was isolated in a cone conformation in solution at room temperature, as established by NMR spectroscopy ( 1H, 13C and NOESY). Complete assignment of both proton and carbon NMR spectra was achieved by a combination of COSY, HSQC and HMBC experiments. The binding properties of ligand 2 towards alkali, alkaline earth, transition and heavy metal cations have been assessed by phase transfer and proton NMR titration experiments. The results are compared to those obtained with other dihomooxacalix[4]arene-crowns-6 and closely-related calix[4]arene-crown derivatives. 2 shows a preference for the soft heavy metal cations (except for Cd 2+), with a very strong affinity for Ag +. Some transition metal cations are also well extracted. 2 forms 1:1 complexes with K +, Ca 2+ and Ag +, and 1H NMR titrations indicate that they should be encapsulated into the cavity defined by the crown ether unit and by the two pyridyl pendant arms. A 1:2 (ML 2) complex is formed with Zn 2+ and two species, probably 1:1 and 1:2 complexes, are obtained with Pb 2+.

New Insights into the Mechanism of Permeation through Large Channels

The mitochondrial channel, VDAC, regulates metabolite flux across the outer membrane. The open conformation has a higher conductance and anionic selectivity, whereas closed states prefer cations and exclude metabolites. In this study five mutations were introduced into mouse VDAC2 to neutralize the voltage sensor. Inserted into planar membranes, mutant channels lack voltage gating, have a lower conductance, demonstrate cationic selectivity, and, surprisingly, are still permeable to ATP. The estimated ATP flux through the mutant is comparable to that for wild-type VDAC2. The outer membranes of mitochondria containing the mutant are permeable to NADH and ADP/ATP. Both experiments support the counterintuitive conclusion that converting a channel from an anionic to a cationic preference does not substantially influence the flux of negatively charged metabolites. This finding supports our previous proposal that ATP translocation through VDAC is facilitated by a set of specific interactions between ATP and the channel wall.

Biochemical and genetic characterization of Hmi1p, a yeast DNA helicase involved in the maintenance of mitochondrial DNA

The HMI1 gene encodes a DNA helicase that localizes to the mitochondria and is required for maintenance of the mitochondrial DNA (mtDNA) genome of Saccharomyces cerevisiae. Identified based on its homology with E. coli uvrD, the HMI1 gene product, Hmi1p, has been presumed to be involved in the replication of the 80 kb linear S. cerevisiae mtDNA genome. Here we report the purification of Hmi1p to apparent homogeneity and provide a characterization of the helicase reaction and the ATPase reaction with regard to NTP preference, divalent cation preference and the stimulatory effects of different nucleic acids on Hmi1p-catalysed ATPase activity. Genetic complementation assays indicate that mitochondrial localization of Hmi1p is essential for its role in mtDNA metabolism. The helicase activity, however, is not essential. Point mutants that lack ATPase/helicase activity partially complement a strain lacking Hmi1p. We suggest several possible roles for Hmi1p in mtDNA metabolism.

Site location of Co in β-FeSi2

In order to reveal cationic site preference in β-FeSi2, Co-substituted samples synthesized by various techniques such as molecular beam epitaxy, ion implantation, and chemical vapor transport were investigated by Fe57 conversion electron Mössbauer (CEM) as well as Co57 Mössbauer emission (ME) spectroscopy. Literature on the structure of β-FeSi2 is somewhat contradictory, especially on the point of the population of the two iron sites in the orthorhombic structure. Co57 ME and Fe57 CEM spectra both showed two quadrupole split spectral components in the crystalline phase. Hyperfine parameters indicate that Co atoms substitute Fe in both Fe positions in the orthorhombic lattice. The aim of the present study was to get reliable results on the relative population of the two iron sites and determine the substitution of the iron sites by Co atoms in the β-FeSi2 lattice. The relative intensities of the two components in the absorption and emission Mössbauer spectra were found to be very similar for the samples prepared by different techniques, indicating a homogeneous distribution of Co on both Fe sites.

DFT Study of Structure and Binding Energies of Fe−Corannulene Complex

Density functional theory (DFT) based theoretical calculations are performed to identify the ground-state geometries, the spin multiplicities, and the relevant energetics of neutral and positively charged Fe-corannulene complexes. Our calculations show that the on-top site of the six-membered ring (eta(6)) of corannulene molecule is the most preferred binding site for both Fe atom and Fe(+) ion. The electrostatic potential (ESP) surface picture is employed to explain the preference of the eta(6)- over the eta(5)-binding site (on-top site of central pentagon) of corannulene. Though in both neutral and cationic species the eta(6)-site is the most preferred binding site, the ground-state geometries of these complexes are different. The Fe(+) cation prefers to bind to the convex face of the corannulene, whereas the neutral Fe atom prefers slightly the concave to the convex face. The ionization-induced structural changes are reflected in the large energy difference between the vertical and adiabatic ionization potential values. We also show that the dissociation of Fe(+)-corannulene complex to corannulene + Fe(+) is just as likely as that to Fe + (corannulene)(+).

Gene expression and characterization of isoprene synthase from Populus alba

Isoprene synthase cDNA from Populus alba (PaIspS) was isolated by RT-PCR. This PaIspS mRNA, which was predominantly observed in the leaves, was strongly induced by heat stress and continuous light irradiation, and was substantially decreased in the dark, suggesting that isoprene emission was regulated at the transcriptional level. The subcellular localization of PaIspS protein with green fluorescent protein fusion was shown to be in plastids. PaIspS expressed in Escherichia coli was characterized enzymatically: it had an optimum pH of approximately 8.0, and an optimum temperature 40°C. Its preference for divalent cations for its activity was also studied.

Computational investigation of Al/Si and Al/Mg ordering in aluminous tremolite amphiboles

AbstractThe [4]Al/Si and [6]Al/Mg order-disorder behaviour of minerals in the tremolite-tschermakite solid solution (namely, end-member tschermakite and the 50:50 composition, magnesiohornblende) has been investigated by Monte Carlo simulation, using a model Hamiltonian in which atomic interaction parameters Ji were derived from empirical lattice energy calculations, and chemical potential terms μj (to express the preferences of cations for particular crystallographic sites) were derived from ab initio methods. The simulations performed were increasingly complex. Firstly, ordering in one tetrahedral double chain with Al:Si = 1:3 (tschermakite) was simulated. Although the low-temperature cation distribution in this system was ordered, there was no phase transition (due to the quasi-one-dimensional nature of the system). Next, interactions between tetrahedral Al:Si = 1:3 double chains were included, and a phase transition was observed, with the cation distribution in one double chain lining up with respect to that in the next. Finally, interactions between tetrahedral and octahedral sites were incorporated, to model the whole unit cell, and compositions corresponding to tschermakite and magnesiohornblende were investigated. The whole-cell simulation results compare favourably with experimental conclusions for magnesiohornblende, in that Al at T1 is preferred over Al at T2, and Al at M2 is favoured over that at M1 and M3, but the significant amount of Al at M1 is at odds with experimental observation.

Thermodynamics of ion exchange of H+ by Na+ or NH4 + on ion-exchange resins based on C-tetramethylcalix[4]resorcinarene

The corrected selectivity coefficients of the ion exchange H+-Na+ and H+-NH4+ on ion-exchange resins based on C-tetramethylcalix[4]resorcinarene were calculated from the experimental data obtained from studying ion-exchange equilibria. The preference of the ion-exchange resins for cations increases in the sequence: Na+ < NH4+ < < H+, and the ion-exchange resin based on (2-furyl)hydroxymethyltetramethylcalix[4]resorcinarene has a higher preference for ammonium cations. According to the results of microcalorimetric measurements, the exchange H+-Na+ on this ion-exchange resin is accompanied by the highest change in the differential enthalpy. It follows from the quantum-chemical calculations that the introduction of a (2-furyl)hydroxymethyl group into the structure of the polymer induces additional electrostatic interactions between an ammonium cation and an elementary unit of the ion-exchange resin.

Theoretical model of copper Cu(I)/Cu(II) hydration. DFT and ab initio quantum chemical study

Hydration study of both Cu + and Cu 2+ cations in variable water environment was performed using the DFT method. After optimization using B3PW91 functional, stabilization energies with and without ligand repulsion were calculated using B3LYP functional. It was found that optimal Cu + coordination involves two directly bonded solvent molecules while Cu 2+ cation prefers 4 (or 5) coordinated waters in the first solvation shell. Higher coordination corresponds to lower stabilization energies. Morokuma's energy decomposition (for Cu + complexes only) was used to elucidate bonding characteristics in detail. NBO partial charges and MO's analyses support the explanation for these energy results.

Peptidyl linkers for protein heterodimerization catalyzed by microbial transglutaminase.

Specific peptidyl linkers that result in the heterodimerization of functional proteins, which is catalyzed by microbial transglutaminase from Streptomyces mobaraensis (MTG), were generated based on a ribonuclease S-peptide using site-directed mutagenesis. The peptidyl linkers designated as Lys-tag and Gln-tag were designed to possess sole reactive Lys or Gln residue that was amenable for selective Lys-Gln cross-linkage of different proteins. Green fluorescent protein variants, ECFP and EYFP, were employed as model proteins, and those Lys- and Gln-tags were fused to the N-termini of ECFP and EYFP, respectively. As a result, we succeeded in solely obtaining the ECFP-EYFP heterodimer without forming multiply cross-linked byproducts. It was found that the reactivity of peptidyl linkers varied according to the type of amino acid to be replaced. Peptidyl linkers with a basic amino acid (Arg) exhibited the highest reactivity in the cross-linking reaction, suggesting the cationic residue substrate preference of MTG. Kinetic analysis utilizing fluorescent resonance energy transfer (FRET), that is only observed upon the heterodimeric ECFP-EYFP conjugation, revealed that the amino acid replacement contributed to the acceleration of cross-linking reactions by increasing catalytic turnover (k(cat)), rather than substrate binding affinity (K(m)). Finally, using a ribonuclease S-protein, the manipulation of enzymatic protein cross-linking based on specific S-peptide:S-protein interactions was explored. Since newly designed Lys- and Gln-tags retained binding affinities to the S-protein, the heterodimerization was perfectly restrained by wrapping them with the S-protein. The results suggest the possibility of limited protein conjugation by tuning steric hindrance against the MTG. Tailoring enzymatic posttranslational modifications with either engineering peptidyl substrates or by taking specific peptide-protein interactions into consideration may facilitate the development of a new sequential protein conjugation method for the preparation of multifunctional protein.

Residue Asp-189 Controls both Substrate Binding and the Monovalent Cation Specificity of Thrombin

Residue Asp-189 plays an important dual role in thrombin: it defines the primary specificity for Arg side chains and participates indirectly in the coordination of Na(+). The former role is shared by other proteases with trypsin-like specificity, whereas the latter is unique to Na(+)-activated proteases in blood coagulation and the complement system. Replacement of Asp-189 with Ala, Asn, Glu, and Ser drastically reduces the specificity toward substrates carrying Arg or Lys at P1, whereas it has little or no effect toward the hydrolysis of substrates carrying Phe at P1. These findings confirm the important role of Asp-189 in substrate recognition by trypsin-like proteases. The substitutions also affect significantly and unexpectedly the monovalent cation specificity of the enzyme. The Ala and Asn mutations abrogate monovalent cation binding, whereas the Ser and Glu mutations change the monovalent cation preference from Na(+) to the smaller cation Li(+) or to the larger cation Rb(+), respectively. The observation that a single amino acid substitution can alter the monovalent cation specificity of thrombin from Na(+) (Asp-189) to Li(+) (Ser-189) or Rb(+) (Glu-189) is unprecedented in the realm of monovalent cation-activated enzymes.