Enzyme-polyelectrolyte complex: Salt effects on the reaction of urease with polyallylamine

The effects of inorganic mono- and divalent salts of different types on how the cation polyelectrolyte polyallylamine hydrochloride (PAA) binds with the oligomer enzyme urease were studied. It was shown that in solutions of the monovalent salts NaCl, KCl, and NH₄Cl, polyelectrolyte-protein complexes formed by electrostatic interactions, which decreased monotonically as the salt concentrations increased according to the classic law of statistical physics, correlating the Debye radius with the ionic strength of the solution. In solutions of the divalent salts Na₂SO₄ and (NH₄)2_SO₄, the efficiency of the formation of the polyelectrolyte-protein complexes changed abruptly (the enzyme was drastically activated) at low salt concentrations (∼0.6–0.8 mM), which was not consistent with the classic theory of charge interactions in solutions with different ionic strengths. Turbidimetric titration at different salt concentrations in the given range revealed a high aggregative ability for sulfates and low ability for chlorides. It was concluded that the anomalies in the concentration dependence of the enzyme activity and aggregative ability were related to the formation of stable bonds PAA to the divalent SO₄²⁻ anion, which increased drastically when the ratio of anion concentration to the number of positively charged PAA monomers in solution reached 1: 2.     

Electrophoretic mobility of latex spheres in the presence of divalent ions: experiments and modeling

Electrophoretic mobilities (EPM) of negatively charged latex spheres were measured as a function of salt type and salt concentration. The measured values of EPM were analyzed using a standard electrokinetic model that includes double layer relaxation and the Poisson–Boltzmann model of diffuse double layer. Calculated values of EPM were in good agreement with experimental data taken in simple 1:1 (KCl) and 1:2 (Na₂SO₄) electrolyte solutions without using any fit parameters. For 2:1 electrolytes (CaCl₂ and MgCl₂), however, the magnitude of EPM calculated by the model was higher than the measured values of EPM at higher electrolyte concentrations. The difference between measured and calculated EPM was reduced by assuming the distance of slipping plane x _s?=?0.25 nm or by assuming the decrease of the magnitude of surface charge density from ?0.07 to ?0.025 C/m². These are probably due to the accumulation of divalent counterions in the vicinity of a particle’s surface.     

Comparative Study of Scattering and Osmotic Properties of Synthetic and Biopolymer Gels

Summary: The effect of monovalent/divalent cation exchange on the structure and osmotic properties of chemically cross-linked polyacrylate and DNA gels swollen in near physiological salt solutions has been investigated. Both systems exhibit a reversible volume phase transition in the presence of calcium ions. The small-angle neutron scattering spectra of these gels display qualitatively similar features. At low values of q surface scattering is observed, while in the intermediate q range the signal is characteristic of scattering from rod-like elements. At high values of q the scattering intensity is governed by the local (short-range) geometry of the polymer chains. The competition between monovalent and divalent cations has been studied by anomalous small-angle X-ray scattering (ASAXS). The ASAXS results reveal that the local concentration of the divalent counter-ions in the vicinity of the polymer chains significantly exceeds that of the monovalent counter-ions.     

Statistical-mechanical treatment of an adsorbed monolayer with local order: Part III. Its use in the interpretation of the adsorption behaviour of aliphatic compounds

The statistical-mechanical treatment of Part II of thi series is applied to a monolayer of water molecules and open-chain polymeric solute molecules consisting of structurally indistinguishable segments. Water molecules are assumed to interact between themselves both via dipole-dipole forces and via H-bonds, whereas water-solute and solute-solute interactions are considered to be of the dipole-dipole type. The resulting adsorption isotherm explains several features of the adsorption behaviour of aliphatic compounds, such as the Frumkin isotherm behaviour with an attractive interaction factor, the parabolic dependence of the standard Gibbs energy of adsorption ΔG°_ads on charge σ_m, the correct order of magnitude for b  ∂ ΔG°_ads/∂(σ_m  σ_max)², and the experimental dependence of b on the charge of maximum adsorption σ_max. The model also accounts satisfactorily for the shape of the experimental curves of the potential drop Δφ across the inner layer against surface coverage at constant charge. An explanation of two-dimensional condensation based on inhibition of H-bond formation between adsorbed and non-adsorbed water molecules is proposed.     

The effect of a low-molecular-mass salt on stoichiometric polyelectrolyte complexes composed of oppositely charged macromolecules with different solvent affinities

The effect of a low-molecular-mass salt on the thermodynamic stability of stoichiometric interpolymer complexes composed of oppositely charged macromolecules with different solvent affinities has been theoretically studied. It has been shown that the dissociation of such complexes with an increase in the concentration of the salt proceeds via several stages. At a low concentration of the salt, complexes retain their structure and dimensions. When a certain critical concentration of the salt n _s^cr is achieved, the dimensions of the complex increase abruptly. At this concentration, macromolecules involved in the complex begin to separate, and at concentration n _s^*, they fully move apart but remain soluble owing to the polyelectrolyte effect. Upon a further increase in the concentration of the salt, the polyelectrolyte effect is shielded and the dimensions of macromolecules decrease. The critical concentration of the low-molecular-mass salt, n _s^cr, increases with an increase in the degree of ionization of macromolecules and a decrease in the affinity of the hydrophilic component for water and diminishes with the degree of polymerization of macromolecules and the degree of hydrophobicity of a polycation. Because of the easy formation of soluble complexes from oppositely charged macromolecules differing in solvent affinities and their high stability in solutions of a low-molecularmass salt, such complexes are promising for wide use in medicine and pharmaceutical practice.     

The ionization constants of 2‐substituted 4,6‐diamino‐s‐triazines: The applicability to the hammett and taft equations

The ionization (or basicity) constants (pK_b) were determined for many 2‐substituted 4,6‐diamino‐s‐tri‐azines ( I ) by means of the electrometric titration. I includes 2‐alkoxy or aryloxy‐( Ia ), 2‐alkyl‐ or 2‐aryl‐( Ib ), and 2‐alkylamino‐ or 2‐arylamino‐4,6‐diamino‐s‐triazines ( Ic ). For the series with the same alkyl or aryl group, the order of the basicity was found to be Ic < Ib < Ia . A study was made of relationships between the pK_b, values of I , and the substituent constants, σ_p, σ_m, σ_p⁺, σ_m⁺, σ_p^O, σ_m^o, σ_I, σⁿ, and σ*. The Hammett relationships were observed between the pK_a values of I, and the substituent constants σ_m, (or the combination ones, [0.97σ_m + 0.03σ_p] as well as another [0.77σ_I + 0.23σ_R]). The Taft relationships were also found between the pK_a values of Ia , Ib , and Ic and the constants σ_*, respectively. Furthermore, in the case of Ic a linear relationship was observed between the pK_a values and Σσ⁸.     

Precipitation of oppositely charged polyelectrolytes in salt solutions

We study phase separation in symmetric solutions of weakly charged flexible chains of opposite sign. Precipitation is caused by effective attractions due to charge fluctuations and by short-range attractions between monomers. The contribution from charge fluctuations is computed within the random phase approximation (RPA), which takes into account the connectivity of charges in the polyions. The impenetrability of the ions is accounted for by using a modified Coulomb potential in the RPA. In good solvent conditions the precipitate monotonically swells and eventually dissolves upon addition of salt. However, near the theta-solvent condition, but still in the good solvent, the precipitate can be stable at any salt concentration. Moreover, the density of the precipitate after initial decrease can increase with addition of salt. This effect is a result of redistribution of salt between the precipitate and the supernatant, which is due to an interplay of electrostatic and hardcore interactions. For not too weakly charged polyions the precipitate properties become strongly dependent on temperature even in good solvent conditions.     

Modified model of electrical double layer for mixed solutions of constant ionic strength

A model is suggested, which generalized the Alekseev-Popov-Kolotyrkin model for the case of mixed solutions of constant ionic strength of the type mc KA_*⁻ + (1 − m)c KA, where A_*⁻ is the surface-active anion that lowers the electrical double layer capacitance as compared with that observed in the presence of surface-inactive anion A⁻. Calculations made for a real system mc NaNO₃ + (1 − m)c NaF showed that at 0.2 ≤ m ≤ 1 a transition occurred from quadratic to linear dependence of adsorption energy on the electrode potential (or charge). The conclusions are in agreement with experimental data obtained for the studied system.     

Chaotropic Monovalent Anion‐Induced Rectification Inversion at Nanopipettes Modified by Polyimidazolium Brushes

A nonintuitive observation of monovalent anion‐induced ion current rectification inversion at polyimidazolium brush (PimB)‐modified nanopipettes is presented. The rectification inversion degree is strongly dependent on the concentration and species of monovalent anions. For chaotropic anions (for example, ClO₄^?), the rectification inversion is easily observed at a low concentration (5 mm ), while there is no rectification inversion observed for kosmotropic anions (Cl^?) even at a high concentration (1 m ). Moreover, at the specific concentration (for example, 10 mm ), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl^?≥NO₃^?>BF₄^?>ClO₄^?>PF₆^?>Tf₂N^?). Estimation of the electrokinetic charge density (σ^ek) demonstrates that rectification inversion originates from the charge inversion owing to the over‐adsorption of chaotropic monovalent anion. To qualitatively understand this phenomenon, a concentration‐dependent adsorption mechanism is proposed.     

Adsorption of oppositely charged polyelectrolyte/gemini surfactant mixtures at the air/water interface and the effects of NaBr: a surface tension study

The adsorption of mixed solutions containing an anionic polyelectrolyte, carboxymethylchitosan (CMCH), and cationic gemini surfactants, alkanediyl-bis-(dimethyldodecyl-ammonium bromide) (C₁₂-s-C₁₂, s?=?2, 6, 12), has been investigated by surface tension method. The oppositely charged polyelectrolyte and the surfactants co-adsorb at the surface to form highly surface-active complexes. Combining the surface tension data with the Gibbs equation, it is referred that the surface layers of the mixed solutions have the multi-level structure, which includes the sublayers beneath an outermost layer. The gemini surfactant spacer with different length takes different conformations in the surface layers. The salt (NaBr) effects on the adsorption of the mixtures have also been studied. The spacer length of C₁₂-s-C₁₂ influences the responses of CMCH/C₁₂-s-C₁₂ mixtures to the salt effects. The comprehensive salt effects depend on the competition between the salt-enhancing effect and the salt-weakening effect.     

Mixed brushes consisting of oppositely charged Y-shaped polymers in salt free,monovalent, and divalent salt solutions

The conformation and the internal stratification of mixed brushes formed from oppositely charged Y(−) and Y(+)-shaped chains in salt free, monovalent, and divalent salt solutions were studied by means of molecular dynamics simulations using the primitive model. Scaling relations of mixed brush height with respect to the grafting surface per chain, the ratio of the total positive to the total negative charge of polyelectrolyte chains, and salt concentrations were obtained. The simulations predicted that mixed brushes show a unique response to divalent salt (1:2) solutions. For symmetric brushes having the same spacer lengths, number of chains and charged units fractions the increase of the salt concentration leads to the enrichment of the outer brush surface with Y(+) units and the lamella microphase separation. For asymmetric brushes in high salt concentration cylindrical domain microphases are formed.     

Reactivities of N-alkylitaconimides in radical copolymerizations with styrene or methyl methacrylate

The radical copolymerizations of N-alkylitaconimide (RII, R = CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, n-C₄H₉, i-C₄H₉, CH₂CH₂Cl, CH₂C₆H₅) (M₁) with styrene (ST) (M₂) or methyl methacrylate (MMA)-(M₂) were carried out at 60°C, using azobisisobutyronitrile as an initiator in tetrahydrofuran in order to clarify the substituent effect on the copolymerizations. The monomer reactivity ratios r₁, r₂ and the Q₁ and e₁ values were determined from the results obtained. It was found that the relative reactivities 1/r₂ of RII toward an attack by a poloystyryl radical could be correlated not by the steric-substituent constant E_s of the alkyl group in RII but by the polar-substituent constant σ^* in Taft's equation: log(1/r₂) = ρ^*σ^* + δ E_s. According to the above equation, the ρ^* and δ values were obtained as 0.55 and 0, respectively, in the RII-ST system, while in the RII-MMA system, the ρ^* and δ values were obtained as 0.49 and 0, respectively. It was also observed that the Q₁values for RII were proportional to σ^* constants and that the e₁ values for RII were independent of σ^* substituent constant. It was also found that the weight-average molecular weights of the copolymers are between 8.5 × 10⁴ and 32.5 × 10⁴.     

Intrinsic Surface Reaction Constant in 1-pK Model of Mg-Fe Hydrotalcite-like Compounds

The relationship among intrinsic surface reaction constant (K) in 1-pK model, point of zero net charge (PZNC) and structural charge density (σst) for amphoteric solid with structural charges was established in order to investigate the effect of σst on pK. The theoretical analysis based on 1-pK model indicates that the independent PZNC of electrolyte concentration (c) exists for amphoteric solid with structural charges. A common intersection point (CIP) should appear on the acid-base titration curves at different c, and the pH at the CIP is pHPZNC. The pK can be expressed as pK=-pHPZNC log[(1 2αPZNC)/(1-2αPZNC)], where αPZNC≡σst/eNANs, in which e is the elementary charge, NA the Avogadro‘s constant and Ns the total density of surface sites. For solids without structural charges, pK=-pHPZNC. The pK values of hydrotalcite-like compounds (HTlc) with general formula of [Mg1-xFex(OH)2](Cl,OH)x were evaluated. With increasing x, the pK increases, which can be explained based on the affinity of metal cations for H^- or OH^- and the electrostatic interaction between charging surface and H^- or OH^-.     

Effect of counterion condensation on the acid-base equilibrium of a pH-sensitive merocyanine dye covalently attached to polyelectrolytes

Effects of monovalent and divalent counterions on the acid-base equilibrium of a pH-sensitive merocyanine dye covalently attached to copolymers of acrylic acid and acrylamide with varying charge densities (0.28 < ξ < 2.8) were investigated. Added chloride salts of Li⁺, Na⁺, K⁺, and NH⁺₄ (< 0.2 mM) had essentially no effect on pK observed (pK^obs) for the equilibrium. By contrast, the salts of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ caused a significant decrease in pK^obs for the copolymers with larger ξ. With smaller ξ, most likely when ξ < 0.5, no decrease in pK^obs was observed upon addition of the salts of divalent cations. A competitive effect of Ca²⁺ and Na²⁺ ions on pK^obs in the presence of an excess of Na⁺ ions implied that Ca²⁺ ions at very low concentrations were preferentially, and therefore exhaustively, condensed on the polyanions with sufficiently large ξ probably until effective charge density was lowered to 0.5. The observed difference in the influence of the monovalent and divalent cations on pK^obs was discussed in terms of the difference in the microscopic behavior of the condensed monovalent and divalent cations. © 1993 John Wiley & Sons, Inc.     

The surface adsorption and micelle formation of the mixed aqueous solutions of fluorocarbon and hydrocarbon surfactants: II. Sodium perfluorooctanoate-sodium decylsulfate system

The adsorption and micellar behavior of the mixed solutions of sodium perfluorooctanoate (7CFNa) and sodium decylsulfate (C₁₀SNa) have been studied at constant ion strength of 0.1m. The adsorption was calculated from the surface (and interfacial) tension-concentration curves by applying Gibbs equation. It was found that the cmc's of 7CFNa and C₁₀SNa are of nearly the same value (1.66 × 10⁻²m and 1.45 × 10⁻²m, respectively), but γ_cmc of 7CFNa solution is ∼23 mNm⁻¹, which is much lower than that of C₁₀SNa solution. This implies that 7CFNa would have a much higher surface activity than C₁₀SNa and be adsorbed preferentially; for instance, 7CFNa has a surface mole fraction of about 0.8 in the saturated adsorption layer of the 1:1 mixed solution. The mole fractions of 7CFNa at the surface are always greater than those in the bulk solutions. The adsorption at the n- heptane-aqueous solution interface is quite different from that at the air-solution surface. Here C₁₀SNa is preferentially adsorbed owing to “Mutual phobicity” between the HC-chain of n-heptane and the FC-chain of 7CFNa at the interface. All the cmc's obtained from the γ-log m relations of 7CFNa (or C₁₀SNa) in the mixed solutions have nearly the same value and the cmc-x curves show a positive deviation from the ideal case. This further indicates that in the mixed solutions of fluorocarbon and hydrocarbon surfactants no completely miscible micelle but essentially the individual micelle of each surfactant exist due to the “Mutual phobicity” between FC- and HC-chain in the micellization process.     

Charged polypeptide diffusion at a very high ionic strength

Two charged polypeptides of opposite charge, poly(glutamic acid) (negative charge) and polylysine (positive charge), were end-labeled with Alexa fluorescent dyes, and their translational diffusion coefficient (D) values in dilute solutions (∼10⁻⁴ mg mL⁻¹) were studied at the biological pH with fluorescence correlation spectroscopy as a function of the ionic strength (C_s) mediated by the addition of NaCl. At a moderate ionic strength, D increased consistently with expected chain contraction because of electrostatic screening. At a very high ionic strength, D of poly(glutamic acid) increased more rapidly, following the empirical power law R_H ∼ C_s^−1/2 over a limited range of C_s, where the changes in D were interpreted as changes in the hydrodynamic radius, R_H. However, D of polylysine at first decreased but eventually passed through a maximum followed by a decrease. These large increases implied that R_H decreased considerably, in turn implying a strong contraction of the chain conformations even though the polymer remained soluble and showed no evidence of aggregation. For polylysine, the unexpected minimum R_H value may be related to the salting-in phenomenon. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3497–3502, 2005     

Electroviscous Effects in Ceramic Nanofiltration Membranes

Membrane permeability and salt rejection of a γ‐alumina nanofiltration membrane were studied and modeled for different salt solutions. Salt rejection was predicted by using the Donnan‐steric pore model, in which the extended Nernst–Planck equation was applied to predict ion transport through the pores. The solvent flux was modeled by using the Hagen–Poiseuille equation by introducing electroviscosity instead of bulk viscosity. γ‐Alumina particles were used for ζ‐potential measurements. The ζ‐potential measurements show that monovalent ions did not adsorb on the γ‐alumina surface, whereas divalent ions were highly adsorbed. Thus, for divalent ions, the model was modified, owing to pore shrinkage caused by ion adsorption. The ζ‐potential lowered the membrane permeability, especially for membranes with a pore radius lower than 3 nm, a ζ‐potential higher than 20 mV, and an ionic strength lower than 0.01 m . The rejection model showed that, for a pore radius lower than 3 nm and for solutions with ionic strengths lower than 0.01 m , there is an optimum ζ‐potential for rejection, because of the concurrent effects of electromigration and convection. Hence, the model can be used as a prediction tool to optimize membrane perm‐selectivity by designing a specific pore size and surface charge for application at specific ionic strengths and pH levels.     

Consequences of acceptor effects on the polarization of oxo and imine bonds in acyl compounds and certain characteristic derivatives CH3-C(=N-NH-A)-X: I. Betainylhydrazones (A=CO-CH2N+Me3) and semicarbazones (A=CO-NH2)

Measurements at half-wave potentials of betainylhydrazones and semicarbazones of acyl compounds of the type CH₃-CO-X enabled determinations to be made of values for Taft polar substituent constants σ_x/^*. The acceptor effects of these substituents can be compared by the increasing value of the substituent constant σ_x/^*: COO^? (+0.78)<COOH (+1.10)<CO-CH₃ (+1.30)<CH=N-NH-CO-(+1.50)<COOR (+1.58)<C≡N and COSR (σ^* not measured).     

HYDROPHOBIZING EFFECT OF CATIONS ON ACIDIC PHOSPHOLIPID MEMBRANES

By means of contact angle measurements with water and aqueous salt solutions, it is shown that plurivalent cations increase the hydrophobicity of negatively charged phospholipid vesicle membranes (consisting of phosphatidic acid, PA, or of phosphatidylserine, PS), but does not influence the hydrophobicity of neutral phospholipid membranes, (e.g., phosphatidylcholine, PC, at up to 200 mM of CaCl₂). The hydrophobizing action of cations on PA and PS membranes is concomitant with the reduction in (negative) zeta potential with increasing cation concentrations. Trivalent cations, La³⁺, showed more effective in hydrophobizing negatively charged phospholipid membranes than divalent and monovalent cations. Except for hydrogen ions, monovalent cations do not show any appreciable hydrophobizing effect on lipid vesicle membranes at concentrations less than 1 M. The hydrophobizing effect on phospholipid membranes can also be used to explain the induction of lateral phase separation into patches of different phospholipids as well as cell fusion.     

Salt-catalyzed addition reaction of monoepoxides with philosamia cynthia ricini and bombyx mori silk fibroins

The heterogeneous addition reaction of various monoepoxides with silk fibroins of Philosamia cynthia ricini and Bombyx mori was investigated at 45–75°C by use of aqueous solutions of various salts as padding catalysts. The effects of salt on the epoxide–silk fibroin reactions were attributed mainly to the nucleophilicity of the anions and also to the acidity or the electronegativity of the cations. The effect of the substituent of the epoxide on the add-ons was elucidated by the modified Taft equation, (log W ? log W₀)/σ* = ρ_p + ρ_sE_s/σ*, where W₀ and W are the add-ons for the reaction of a given compound and of its substituted derivatives, σ* and E_s are the polar and the steric substituent constants, ρ_p and ρ_s are the polar and the steric reaction constants, respectively. Histidine, lysine, arginine, tyrosine, serine, and acidic amino acids were found to react. The reactivity difference between Philosamia cynthia ricini and Bombyx mori fibroins towards the epoxide was discussed in the light of the observed phenomena.