Influence of divalent ions on composition and viscoelasticity of polyelectrolyte complexes

The addition of monovalent salts to polyelectrolyte complexes (PECs) comprising oppositely charged polyelectrolytes results in diminishing propensity for complexation, leading to complexes with higher water contents and lower moduli. However, the corresponding influence of multivalent ions on polyelectrolyte complexation has not yet been explored beyond enhanced screening effects. Here, we elucidate the significant impact of the valency of the salt cation on the composition, ion partitioning, and viscoelasticity of charge-matched PECs comprising sodium salt of poly(acrylic acid) and poly(allylamine hydrochloride). Notably, preferential partitioning of divalent cations (Ca²⁺ and Sr²⁺) into the complexes is observed, in stark contrast to the depletion of monovalent ions (Na⁺) from the complexes. Concomitantly, electrostatic bridging of polyanion chains by divalent ions is found to hinder their relaxation, manifesting as a non-monotonic evolution of the shear moduli of the complexes with increasing divalent salt concentrations. Relatedly, a failure of time-salt and time-ionic strength superposition approaches in presence of divalent ions is demonstrated, highlighting the nontrivial influence of these ions on chain relaxation behavior.     

Structures of ionic species and electronic interactions in radical ions; ketyls with divalent cations

EPR and optical spectra of magnesium and calcium ketyls were investigated in MTHF and mixtures of DMF and MTHF at various concentrations. It was shown that these ketyls form aggregates in ethereal solvents and dissociate into biradicals in more polar solutions. Models of ion aggregate to explain the observed EPR and optical spectra are suggested.     

Complexation between oxonic acid and divalent metal cations. Influence of metal salt addition on decarboxylation kinetics under acidic conditions

The dissociation constants of oxonic acid are determined by UV spectrometry (pKa₁ = 0.8 ± 0.1) and potentiometry (pKa₂ = 6.85 ± 0.06). The N(3)H tautomer seems to be the predominant form of HL²⁻, the oxonic acid dianion. This latter species binds with divalent metal ions to form complexes. Their formation constants, K_M^HL, are determined by UV spectrometry and/or potentiometry for Ca²⁺, Mg²⁺, Co²⁺, Ni²⁺ and Cu²⁺. The rate of decarboxylation decreases when metal salt is added to acidic solutions; this is explained by the formation of a complex, H₂LM⁺, between a metal ion and the monoanion of oxonic acid. The stability constants are determined for complexes formed with Ca²⁺, Mg²⁺, Co²⁺ and Ni²⁺. These experimental results serve as a basis for assumptions concerning enzymatic phosphoribosylation and decarboxylation.     

Influence of different divalent metal ions on the properties of alginate microcapsules and microencapsulated cells

Different divalent metal ions (Ba²⁺, Sr²⁺, Ca²⁺, Zn²⁺) were selected as crosslinkers. The mechanical properties, cytocompatibility, histocompatibility, cell proliferation and long-term cultivation were investigated. The resulting microcapsules had good sphericity, smooth surface and particle size distribution of 300–400 μm. Sr²⁺ microcapsules exhibited a better mechanical strength. The molecular weights cut-off of all membranes were between 24 and 67 kDa. All microcapsules had no cytotoxicity. After intraperitoneal transplantation, the recovered microcapsules still maintained good mechanical strength and morphology with no fibrosis or macrophage aggregation phenomena. The microencapsulated PC12 cells showed no significant variation after recultivation following microcapsule breaking. The cell activity sequence of different microcapsules after 72 h was as follows: bare control cell >Sr²⁺ > Ca²⁺ > Ba²⁺ > Zn²⁺. After 9 weeks’ in vitro culture, the cell survival rate was higher than 80 %. This paper will be of scientific interests for the basic research and clinical application of alginate/chitosan microcapsules embedded with drugs or cells.     

Tuning supramolecular G-quadruplexes with mono- and divalent cations

Supramolecular G-quadruplexes (SGQs) are formed via the cation promoted self-assembly of guanine derivatives into stacks of planar hydrogen-bonded tetramers. Here, we present results on the formation of SGQs made from the 8-(m-acetylphenyl)-2′-deoxyguanosine (mAGi) derivative in the presence of various mono- and divalent cations. NMR and HR ESI-MS data indicate that varying the cation can efficiently tune the molecularity, the fidelity and stability (thermal and kinetic) of the resulting SGQs. The results show that, parallel to the previously reported potassium-templated hexadecamer (mAGi₁₆·3K⁺), Na⁺, Rb⁺ and NH₄⁺ also promote the formation of similar supramolecules with high fidelity and molecularity. In contrast, the divalent cations Pb²⁺, Sr²⁺ and Ba²⁺ template the formation of octamers (mAGi₈), with the latter two inducing higher thermal stabilities. Molecular dynamics simulations for the hexadecamers containing monovalent cations enabled critical insights that help explain the experimental observations.     

Conversion of methanol on a high-silica zeolite with divalent cations

The influence of divalent cations introduced into a high-silica zeolite by an ion exchange on its acidity and catalytic properties has been studied. The nature of the divalent cations has an influence on the distribution of the acid sites according to their strength, as well as on the activity and selectivity of action of the zeolite in the conversion of methanol. The primary conversion products of methanol, viz., dimethyl ether and ethylene, form on the acid sites of moderate strength. The hydrogen-redistribution reactions take place predominantly on the strong acid sites.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 114–118, January–February, 1986.     

Removal and regeneration of aqueous divalent cations by boehmite

Boehmite (Al(OH)O) was employed for the removal of aqueous Mg(2+), Cu(2+), Cd(2+), Pb(2+), and Co(2+) at 298 K. Although boehmite was able to remove these divalent cations, the greater removal rate with boehmite of Pb(2+) (28.7%) than with Mg(2+), Cu(2+), Cd(2+), and Co(2+) (5.6, 25.3, 10.9, and 13.3%, respectively) was observed under acidic conditions. Under stronger alkaline conditions, in which the lead species was completely dissolved, a greater removal rate of Pb(2+) (more than 80%) was observed under the corresponding conditions employed for the acidic conditions. The removed lead species could not be dissolved from boehmite in an acidic solution while an evident dissolution of lead species was detected using an aqueous NaOH solution. The results shown in the present study reveal that boehmite can be employed as a reagent for the removal and regeneration of aqueous metal cations.     

Influence of methanol as a buffer additive on the mobilities of organic cations in capillary electrophoresis

The mobilities of a series of aromatic ammonium ions, ranging in charge from +1 to + 3, were investigated by capillary electrophoresis using buffers consisting of 0-75% v/v methanol. This is an extension of our previous studies involving anion mobility in methanol-water media [1]. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. For all of the buffer compositions studied, the ionic strength effect increased with increasing cation charge, and varied as a function of solvent 1/eta epsilon (1/2) as predicted by the electrophoretic term within the Pitts' equation. In the presence of methanol, the ionic strength effects became more dramatic. The absolute mobilities of the cations were altered by the addition of methanol to the electrophoretic media. For example, at 75% MeOH, a migration order reversal was observed between the + 2 and + 3 ammonium ions. These solvent-induced selectivity changes are attributed to dielectric friction. As predicted by the Hubbard-Onsager dielectric friction model, dielectric friction increased with increasing methanol content and with increasing analyte charge. Further, the changes in cation mobility correlated to the changes in solvent relaxation time (tau), epsilon and eta. Although not predicted by the Hubbard-Onsager theory, the + 3 ammonium ion experienced more dielectric friction than the - 3 sulfonate and - 3 carboxylate investigated previously [1]. This apparent failure of the Hubbard-Onsager model results from its continuum nature, whereby ion-solvent interactions are not taken into account.     

Sorption of divalent metal ions on CrPO4

The divalent metal ion sorption (Cu(2+), Cd(2+), Ni(2+), and Pb(2+)) on chromium phosphate (CrPO(4)) was studied as a function of pH, temperature, and concentration of metal ions. The sorption of metal ions is observed to increase with the increase in pH, temperature, and concentration of metal ions in solution. The mechanism of sorption is found to be the exchange of the hydrolyzed metal cations with the protons from solid at high temperature. The sorption at low temperature is found to be accompanied by the precipitation of the corresponding metal phosphates such as Pb(3)(PO(4))(2).     

Electroconduction of rubidium pyrophosphate modified by divalent cations

Solid solutions based on rubidium pyrophosphate are synthesized in the Rb_{4 -2x}M_xP₂O₇ systems (M = Ca, Sr, Ba, Pb). The temperature and concentration dependences of their rubidium-cation conductivity are investigated. The X-ray data for the low and high-temperature forms of Rb₄P₂O₇ and also for solid solutions based on it are reported. The effect exerted by modifying cation M²⁺ on electrical properties of synthesized solid solutions is considered     

Effect of divalent cations on DMPC/DHPC bicelle formation and alignment

Many important classes of biomolecules require divalent cations for optimal activity, making these ions essential for biologically relevant structural studies. Bicelle mixtures composed of short-chain and long-chain lipids are often used in solution- and solid-state NMR structure determination; however, the phase diagrams of these useful orienting media and membrane mimetics are sensitive to other solution components. Therefore, we have investigated the effect of varying concentrations of four divalent cations, Ca(2+), Mg(2+), Zn(2+), and Cd(2+), on cholesterol sulfate-stabilized DMPC/DHPC bicelles. We found that low concentrations of all the divalent ions are tolerated with minimal perturbation. At higher concentrations Zn(2+) and Cd(2+) disrupt the magnetically aligned phase while Ca(2+) and Mg(2+) produce more strongly oriented phases. This result indicates that divalent cations are not only required to maintain the biological activity of proteins and nucleic acids; they may also be used to manipulate the behavior of the magnetically aligned phase.     

Synergic solvent extraction of divalent cations with decafluoroheptanedione and di-n-butylsulfoxide

The synergic solvent extraction of iron(II), cobalt(II), nickel(II) and lead(II) with l,l,l,2,2,6,6,7,7,7-decafluoro-3,5-heptanedione(H(FHD)) and di-n-butylsulfoxide (DBSO), is described. The divalent cations are extracted with 99.9% efficiency at pH 5.5 in extraction times of less than 15 min. The extracted species was shown to be M(FHD)₂·2 DBSO by mass action studies and elemental analyses. The extraction of copper(II) was also studied. No DBSO adduct was found in the copper extraction.     

The sorption of divalent metal ions on AlPO4

The sorption of Cu(2+), Pb(2+), Ni(2+), and Cd(2+) ions on the aluminum(III) phosphate was observed to increase with increases in the concentration, temperature, and pH of the system. The apparent dissociation (pK(a)), binding (pK(b)) and exchange (pK(ex)) constants of aluminum(III) phosphate were evaluated and found to be dependent upon the temperature and nature of the metal cations. The values of the dissociation constants (pK(a)) followed the order Pb(2+)相似文献   

Removal of divalent cations reduces fouling of ultrafiltration membranes

The effect of divalent ions on hydraulic irreversible fouling of ultrafiltration membranes was studied. Not only the effect of removing divalent ions by pretreatment of raw water with ion exchange is quantitatively studied, but also the effects of different types of backwash water are considered. By replacing divalent ions with sodium in cation exchange, the amount of hydraulic irreversible fouling (remaining fouling after backwashing) is reduced by at least 60%. When adding either calcium or magnesium to water treated with cation exchange, a linear relation is found between the ion concentration and the irreversible fouling rate. The effects of calcium and magnesium are identical when the concentrations are expressed in mol/L. Removing divalent ions from the backwash water does not affect irreversible fouling, but when using MilliQ water as backwash water, irreversible fouling can (almost) completely be prevented.     

Charge density of divalent metal cations determines RNA stability

RNA molecules are exquisitely sensitive to the properties of counterions. The folding equilibrium of the Tetrahymena ribozyme is measured by nondenaturing gel electrophoresis in the presence of divalent group IIA metal cations. The stability of the folded ribozyme increases with the charge density (zeta) of the cation. Similar scaling is found when the free energy of the RNA folded in small and large metal cations is measured by urea denaturation. Brownian dynamics simulations of a polyelectrolyte show that the experimental observations can be explained by nonspecific ion-RNA interactions in the absence of site-specific metal chelation. The experimental and simulation results establish that RNA stability is largely determined by a combination of counterion charge and the packing efficiency of condensed cations that depends on the excluded volume of the cations.     

Complex hydrogen bonded cations: III. Asymmetric ions

Spectroscopic evidence is presented for the occurrence of a number of asymmetric complex cations, of formula (B¹B²)⁺ where B¹ and B² differ, and may be pyridine, 4-methylpyridine, 2,6- and 3,5-dimethylpyridine, 2,4,6-trimethyl-pyridine, 2-fluoropyridine, 3-chloropyridine or quinoline. In the H-bridged cations, the NH stretching band v_s is doubled, the ratio of the intensities of the components varying regularly with AΔpK_a of the component bases. In the D-bridged cations, only a single v_s band is observed. All features in the spectra evolve continuously from those of the symmetric cations (B¹ = B²).     

Interaction of divalent metal cations and nucleotides: A computational study

In this study, the anionic phosphate group of nucleotides was found to be the best site to bind the divalent metal cations Be²⁺, Mg²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ to form the most stable complexes. Molecular orbital calculations at the semiempirical level were performed on nucleotidemetal cation complexes to report energies of complexation reactions, geometrical parameters of complexes and charge distributions on the complexes. In the discussion, complexational preferences of divalent metal cations, the charge transfer involved in the binding of the metal cations to the phosphate anion of the nucleotides and their conformational effects are included.     

Metallocenyl cations : I. Stable manganese-stabilised carbonium ions

Stable α-cymantrenylcarbonium ions have been identified by solution IR spectroscopy. The substitution of tert-phosphine for CO markedly stabilises the carbonium ions.