Relevant Ionic Strength Research Articles

Removal of Copper (II) Ions from Aqueous Solutions using Na‐mordenite

The potential to remove copper (II) ions from aqueous solutions using Na‐mordenite, a common zeolite mineral, was thoroughly investigated. The effects of relevant parameters solution pH, adsorbent dose, ionic strength, and temperature on copper (II) adsorption capacity were examined. The sorption data followed the Langmuir, Freundlich, and Dubinin‐Radushkevich (D‐R) isotherms. The maximum sorption capacity was found to be 10.69 mg/g at pH 6, initial concentration of 40 mg/dm3, and temperature of 40°C. Different thermodynamic parameters viz., changes in standard free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) have also been evaluated and the results show that the sorption process was spontaneous and endothermic in nature. The dynamics of the sorption process were studied and the values of rate constant of adsorption, rate constant of intraparticle diffusion were calculated. The activation energy (Ea) was found to be 11.25 kJ/mol in the present study, indicating a chemical sorption process involving weak interactions between sorbent and sorbate. The interaction between copper (II) ions and Na‐mordenite is mainly attributable to ion exchange. The sorption capacity increased with the increase of solution pH and the decrease of ionic strength and adsorbent dose. The Na‐mordenite can be used to separate copper (II) ions from aqueous solutions.

Effect of ionic strength on the organization and dynamics of membrane-bound melittin

Melittin, a cationic hemolytic peptide, is intrinsically fluorescent due to the presence of a single functionally important tryptophan residue. We have previously shown that the sole tryptophan of melittin is localized in a motionally restricted environment in the membrane interface. We have monitored the effect of ionic strength on the organization and dynamics of membrane-bound melittin utilizing fluorescence and circular dichroism (CD) spectroscopic approaches. Our results show that red edge excitation shift (REES) of melittin bound to membranes is sensitive to the change in ionic strength of the medium. This could be attributed to a change in the immediate environment around melittin tryptophan with increasing ionic strength due to differential solvation of ions. Interestingly, the rotational mobility of melittin does not appear to be affected with change in ionic strength. In addition, fluorescence parameters such as lifetime and acrylamide quenching of melittin indicate an increase in water penetration in the membrane interface upon increasing ionic strength. Our results suggest that the solvent dynamics and water penetration in the interfacial region of the membranes are significantly affected at physiologically relevant ionic strength. These results assume significance in the overall context of the influence of ionic strength in the organization and dynamics of membrane proteins and membrane-active peptides.

Kinetics properties of Cu,Zn-superoxide dismutase as a function of metal content

The kinetics of bovine Cu,Zn superoxide dismutase were studied by pulse radiolysis. To ensure the absence of catalytically active free copper, commercially obtained holo-superoxide dismutase was demetallated, and the apo-superoxide dismutase concentrations were determined by isothermal titration calorimetry prior to reconstitution with defined amounts of copper and zinc. The catalytic rate constant was determined as a function of ionic strength over the range of 4–154 mM, and of the copper and zinc content. The catalytic rate constant increases with ionic strength up to (1.5 ± 0.2) × 10 9 M −1 s −1 at an ionic strength of 15 mM, and then decreases. At pH 7 and 50 mM ionic strength, k = (1.2 ± 0.2) × 10 9 M −1 s −1, and at a physiologically relevant ionic strength of 150 mM, it is (0.7 ± 0.1) × 10 9 M −1 s −1. The effect of ionic strength is ascribed to the inhomogeneous electric field generated by the surface charges of superoxide dismutase. The value of the catalytic rate constant at 50 mM is ca. 2-fold smaller than earlier values reported in the literature. The relationship between copper content and the catalytic rate constant shows that addition of more than a stoichiometric amount of copper cannot be masked efficiently by edta. The possibility exists that earlier reported values were based on experiments contaminated with trace amounts of copper.

Role of Hydrodynamic Drag on Microsphere Deposition and Re-entrainment in Porous Media under Unfavorable Conditions

Deposition and re-entrainment of 1.1 microm microspheres were examined in packed glass beads and quartz sand under both favorable and unfavorable conditions for deposition. Experiments were performed at environmentally relevant ionic strengths and flow rates in the absence of solution chemistry and flow perturbations. Numerical simulations of experimental data were performed using kinetic rate coefficients to represent deposition and re-entrainment dynamics. Deposition rate coefficients increased with increasing flow rate under favorable deposition conditions (in the absence of colloid-grain surface electrostatic repulsion), consistent with expected trends from filtration theory. In contrast, under unfavorable deposition conditions (where significant colloid-grain surface electrostatic repulsion exists), the deposition rate coefficients decreased with increasing flow rate, suggesting a mitigating effect of hydrodynamic drag on deposition. Furthermore, the re-entrainment rate was negligible under favorable conditions but was significant under unfavorable conditions and increased with increasing flow rate, demonstrating that hydrodynamic drag drove re-entrainment under unfavorable conditions. The drag torque resulting from hydrodynamic drag was found to be 1 order of magnitude or more lower than the adhesive torque based on pull-off forces from atomic force microscopy measurements. This result indicates that hydrodynamic drag was insufficient to drive re-entrainment of microspheres that were associated with the grain surface via the primary energy minimum and suggests that hydrodynamic drag drove re-entrainment of secondary-minimum-associated microspheres.

Spatial Variation in Deposition Rate Coefficients of an Adhesion-Deficient Bacterial Strain in Quartz Sand

The transport of bacterial strain DA001 was examined in packed quartz sand under a variety of environmentally relevant ionic strength and flow conditions. Under all conditions, the retained bacterial concentrations decreased with distance from the column inlet at a rate that was faster than loglinear, indicating that the deposition rate coefficient decreased with increasing transport distance. The hyperexponential retained profile contrasted againstthe nonmonotonic retained profiles that had been previously observed for this same bacterial strain in glass bead porous media, demonstrating that the form of deviation from log-linear behavior is highly sensitive to system conditions. The deposition rate constants in quartz sand were orders of magnitude below those expected from filtration theory, even in the absence of electrostatic energy barriers. The degree of hyperexponential deviation of the retained profiles from loglinear behavior did not decrease with increasing ionic strength in quartz sand. These observations demonstrate thatthe observed low adhesion and deviation from log-linear behavior was not driven by electrostatic repulsion. Measurements of the interaction forces between DA001 cells and the silicon nitride tip of an atomic force microscope (AFM) showed that the bacterium possesses surface polymers with an average equilibrium length of 59.8 nm. AFM adhesion force measurements revealed low adhesion affinities between silicon nitride and DA001 polymers with approximately 95% of adhesion forces having magnitudes < 0.8 nN. Steric repulsion due to surface polymers was apparently responsible for the low adhesion to silicon nitride, indicating that steric interactions from extracellular polymers controlled DA001 adhesion deficiency and deviation from log-linear behavior on quartz sand.

Nonmonotonic Variations in Deposition Rate Coefficients of Microspheres in Porous Media under Unfavorable Deposition Conditions

The transport of carboxylate-modified polystyrene latex microspheres was examined in packed quartz sand under a variety of environmentally relevant ionic strength and flow conditions. The retained concentrations of microspheres in the sediment increased first, and then decreased with transport distance, indicating that the deposition rate coefficient changed nonmonotonically over the transport distance. This finding demonstrates the ubiquity of spatial variation in deposition rate coefficients under unfavorable deposition conditions, and in addition indicates that the previously recognized monotonic decrease with transport distance is not the sole form of spatial variations in deposition rate coefficients. In contrast, the deposition rate coefficients of similarly sized microspheres with different surface group densities were shown to decrease monotonically with transport distance in the same porous media, indicating that the form of spatial variation in deposition rate coefficient is highly sensitive to system conditions. The ubiquity and sensitivity of the spatial variation of deposition rate coefficients indicate that current practices that utilize log-linear extrapolation of discreet measurements of colloid attenuation to determine colloid removal with distance from source are not valid (for both biological and nonbiological colloids). The retained colloid profiles hold the promise to reveal processes governing colloid deposition under unfavorable conditions that are yet to be identified.

Binding of Oxidized and Reduced Cytochrome c2 to Photosynthetic Reaction Centers: Plasmon-Waveguide Resonance Spectroscopy

The dissociation constants for the binding of oxidized and reduced wild-type cytochrome c(2) from Rhodobacter capsulatus and the lysine 93 to proline mutant of cytochrome c(2) to photosynthetic reaction centers (Rhodobacter sphaeroides) has been measured to high precision using plasmon-waveguide resonance spectroscopy. For the studies reported, detergent-solubilized photosynthetic reaction center was exchanged into a phosphatidylcholine lipid bilayer to approximate the physiological environment. At physiologically relevant ionic strengths ( approximately 100 mM), we found two binding sites for the reduced wild-type cytochrome (K(D) = 10 and 150 nM), with affinities that decrease with decreasing ionic strength (2-5-fold). These results implicate nonpolar interactions as an important factor in determining the dissociation constants. Taking advantage of the ability of plasmon-waveguide resonance spectroscopy to reslove the contribution of changes in mass and of structural anisotropy to cytochrome binding, we can demonstrate very different properties for the two binding sites. In contrast, the oxidized wild-type cytochrome only binds to a single site with a K(D) of 10 nM at high ionic strength, and this site has properties similar to the low-affinity site for binding the reduced cytochrome. The binding of oxidized cytochrome c(2) has a strong ionic strength response, with the affinity decreasing approximately 30-fold in going from high to low ionic strength. The K93P mutant binds to a single site in both redox states, which is similar, in terms of mass and structural anisotropy, to the oxidized wild-type site, with the affinity of the mutant oxidized state being approximately 30-fold weaker than that of the oxidized wild-type cytochrome at high ionic strength. Thus, reduced wild-type cytochrome can bind to both the high- and low-affinity sites, while the oxidized wild-type cytochrome and both redox states of the mutant cytochrome can only bind to the low-affinity site, possibly the consequence of the more stable structure of reduced wild-type cytochrome. In aggregate, the results are consistent with a model in which a transient conformational change in the region 88-102 in the cytochrome three-dimensional structure, the so-called hinge region, drives the dissociation of the oxidized cytochrome from the reaction center-cytochrome complex, facilitating turnover.

Apparent Decreases in Colloid Deposition Rate Coefficients with Distance of Transport under Unfavorable Deposition Conditions: A General Phenomenon

The transport of polystyrene microspheres was examined in packed glass beads under a variety of environmentally relevant ionic strength and flow conditions. The observed profiles of numbers of retained microspheres versus distance from the column entrance were much steeper than expected based on a constant rate coefficient of deposition acrossthe length of the column, indicating apparent decreases in deposition rate coefficients with transport distance. Deviation in the profile from log-linear decreases with distance was greatest under highly unfavorable conditions (low ionic strength), relatively reduced under mildly unfavorable conditions (high ionic strength), and was eliminated under favorable conditions. The generality of apparent decreases in deposition rate coefficients with distance of transport among microspheres, bacteria, and viruses leads to the conclusion that such effects reflect processes that are fundamental to filtration under unfavorable conditions. Numerical simulations of experiments that were performed under unfavorable conditions utilized a log-normal distribution of deposition rate coefficients among the colloid population in orderto simulate the effluent curves and retained profiles simultaneously. It is shown that while straining could be a significant contributor to the steep retained profiles at low ionic strength, where overall retention is low, distribution in interaction potentials among the population was a viable mechanism that can yield apparent decreases in deposition rate coefficients with distance of transport.

Superior duplex DNA strand invasion by acridine conjugated peptide nucleic acids.

DNA helix invasion by P-loop forming peptide nucleic acids (PNAs) is extremely sensitive to increased ionic strength as this stabilizes the DNA duplex. To address this, the DNA intercalator 9-aminoacridine was conjugated to helix invading PNAs, and the duplex DNA binding efficiency of such constructs was measured at different ionic strength conditions by electrophoretic mobility shift analysis. Remarkably, at physiogically relevant ionic strength (140 mM K+/10 mM Na+, 2 mM Mg2+), acridine conjugated PNAs showed 20-150-fold superior binding to a cognate sequence target as compared to the conventional PNAs. This enhancement occurred without compromising the sequence specificity of binding. Thus, simply conjugating the DNA intercalator 9-aminoacridine to PNA represents a major step toward the development of helix invading constructs for in vivo applications such as gene targeting.

Maurotoxin: a potent inhibitor of intermediate conductance Ca2+-activated potassium channels.

Maurotoxin, a 34-amino acid toxin from Scorpio maurus scorpion venom, was examined for its ability to inhibit cloned human SK (SK1, SK2, and SK3), IK1, and Slo1 calcium-activated potassium (K(Ca)) channels. Maurotoxin was found to produce a potent inhibition of Ca(2+)-activated (86)Rb efflux (IC(50), 1.4 nM) and inwardly rectifying potassium currents (IC(50), 1 nM) in CHO cells stably expressing IK1. In contrast, maurotoxin produced no inhibition of SK1, SK2, and SK3 small-conductance or Slo1 large-conductance K(Ca) channels at up to 1 microM in physiologically relevant ionic strength buffers. Maurotoxin did inhibit (86)Rb efflux (IC(50), 45 nM) through, and (125)I-apamin binding (K(i), 10 nM) to SK channels in low ionic strength buffers (i.e., 18 mM sodium, 250 mM sucrose), which is consistent with previous reports of inhibition of apamin binding to brain synaptosomes. Under similar low ionic strength conditions, the potency for maurotoxin inhibition of IK1 increased by approximately 100-fold (IC(50), 14 pM). In agreement with its ability to inhibit recombinant IK1 potassium channels, maurotoxin was found to potently inhibit the Gardos channel in human red blood cells and to inhibit the K(Ca) in activated human T lymphocytes without affecting the voltage-gated potassium current encoded by Kv1.3. Maurotoxin also did not inhibit Kv1.1 potassium channels but potently blocked Kv1.2 (IC(50), 0.1 nM). Mutation analysis indicates that similar amino acid residues contribute to the blocking activity of both IK1 and Kv1.2. The results from this study show that maurotoxin is a potent inhibitor of the IK1 subclass of K(Ca) potassium channels and may serve as a useful tool for further defining the physiological role of this channel subtype.

Acceleration of Amyloid Fibril Formation by Specific Binding of Aβ-(1–40) Peptide to Ganglioside-containing Membrane Vesicles

The interaction of Alzheimer's Abeta peptide and its fluorescent analogue with membrane vesicles was studied by spectrofluorometry, Congo Red binding, and electron microscopy. The peptide binds selectively to the membranes containing gangliosides with a binding affinity ranging from 10(-6) to 10(-7) M depending on the type of ganglioside sugar moiety. This interaction appears to be ganglioside-specific as under our experimental conditions (neutral pH, physiologically relevant ionic strength), no Abeta binding was observed to ganglioside-free membranes containing zwitterionic or acidic phospholipids. Importantly, the addition of ganglioside-containing vesicles to the peptide solution dramatically accelerates the rate of fibril formation as compared with that of the peptide alone. The present results strongly suggest that the membrane-bound form of the peptide may act as a specific "template" (seed) that catalyzes the fibrillogenesis process in vivo.

The interaction between Alzheimer amyloid β(1–40) peptide and ganglioside G M1-containing membranes

The interaction between Alzheimer amyloid peptide Aβ(1–40) and membrane lipids was studied by circular dichroism spectroscopy under the conditions of physiologically relevant ionic strength and neutral pH. The peptide binds to the membranes containing ganglioside G M1 and upon binding undergoes a conformational transition from random coil to an ordered structure rich in β-sheet. This interaction appears to be ganglioside-specific as no changes in Aβ(1–40) conformation were found in the presence of various phospholipids or sphingomyelin. The isolated oligosaccharide moiety of the ganglioside was ineffective in inducing alterations in the secondary structure of Aβ(1–40). No interaction was observed between ganglioside G M1 and the N-terminal peptide fragment Aβ(1–28). Binding to the ganglioside is likely to modulate the neurotoxic and/or amyloidogenic properties of Aβ(1–40).

Electrostatic as well as hydrophobic interactions are important for the association of Cpn60 (groEL) with peptides

The interactions of groEL with five N-dansyl peptides were investigated by means of a fluorescence binding assay. The peptides studied (Bamph, Bhphil, Aamph, Ahphil, Namph) were designed and synthesised as systematic variants of each other in terms of their patterns of charge and hydrophobicity. Fluorescence data were analysed using a fluorescence modified, y-reciprocal linearised form of the Benesi–Hildebrand equation which was derived from first principles and verified by theoretical simulations. Under optimal conditions, apparent dissociation constants, Kd, were obtained in the µM range. At physiologically relevant ionic strengths, only two peptides (basic amphiphilic Bamph and neutral amphiphilic Namph) interacted with groEL whilst a third peptide (acidic amphiphilic Aamph) was able to interact but only at very high ionic strength (> 1 mol kg-1). Thermodynamic (van’t Hoff) analysis of the tightest binder, basic amphiphilic Bamph peptide, revealed endothermic binding and a large positive entropy, ΔSobind, consistent with a mixed binding mode involving both hydrophobic and electrostatic interactions. At physiologically relevant ionic strengths, positively charged amino acid residues appear to augment hydrophobic binding interactions with groEL by electrostatic attraction whilst negatively charged amino acid residues oppose short-range hydrophobic interactions with electrostatic repulsion. In conclusion, whilst a principal means of interaction between groEL and a peptide or partially folded protein substrate is certainly hydrophobic, electrostatic effects can modulate or even overwhelm this interaction.

Stern-Volmer in Reverse: 2:1 Stoichiometry of the Cytochrome c-Cytochrome c Peroxidase Electron-Transfer Complex

A reverse protocol for measurements of molecular binding and reactivity by excited-state quenching has been developed in which the quencher, held at a fixed concentration, is titrated by a photoexcitable probe molecule whose decay is monitored. The binding stoichiometries, affinities, and reactivities of the electron-transfer complexes between cytochrome c (Cc) and cytochrome c peroxidase (CcP) were determined over a wide range of ionic strengths (4.5 to 118 millimolar) by the study of photoinduced electron-transfer quenching of the triplet excited state of zinc-substituted Cc (ZnCc) by Fe3+CcP. The 2:1 stoichiometry seen for the binding of Cc to CcP at low ionic strength persists at the physiologically relevant ionic strengths and likely has functional significance. Analysis of the stoichiometric binding and rate constants confirms that one surface domain of CcP binds Cc with a high affinity but with poor electron-transfer quenching of triplet-state ZnCc, whereas a second binds weakly but with a high rate of electron-transfer quenching.