Low-salt Regime Research Articles

Particle/wall electroviscous effects at the micron scale: comparison between experiments, analytical and numerical models

We report a experimental study of the motion of 1 μm single particles interacting with functionalized walls at low and moderate ionic strengths conditions. The 3D particle’s trajectories were obtained by analyzing the diffracted particle images (point spread function). The studied particle/wall systems include negatively charged particles interacting with bare glass, glass covered with polyelectrolytes and glass covered with a lipid monolayer. In the low salt regime (pure water) we observed a retardation effect of the short-time diffusion coefficients when the particle interacts with a negatively charged wall; this effect is more severe in the perpendicular than in the lateral component. The decrease of the diffusion as a function of the particle–wall distance h was similar regardless the origin of the negative charge at the wall. When surface charge was screened or salt was added to the medium (10 mM), the diffusivity curves recover the classical hydrodynamic behavior. Electroviscous theory based on the thin electrical double layer (EDL) approximation reproduces the experimental data except for small h. On the other hand, 2D numerical solutions of the electrokinetic equations showed good qualitative agreement with experiments. The numerical model also showed that the hydrodynamic and Maxwellian part of the electroviscous total drag tend to zero as h → 0 and how this is linked with the merging of both EDL’s at close proximity.

Looping-in complexation and ion partitioning in nonstoichiometric polyelectrolyte mixtures.

A wide variety of intracellular membraneless compartments are formed via liquid-liquid phase separation of charged proteins and nucleic acids. Understanding the stability of these compartments, while accounting for the compositional heterogeneity intrinsic to cellular environments, poses a daunting challenge. We combined experimental and theoretical efforts to study the effects of nonstoichiometric mixing on coacervation behavior and accurately measured the concentrations of polyelectrolytes and small ions in the coacervate and supernatant phases. For synthetic polyacrylamides and polypeptides/DNA, with unequal mixing stoichiometry, we report a general "looping-in" phenomenon found around physiological salt concentrations, where the polymer concentrations in the coacervate initially increase with salt addition before subsequently decreasing. This looping-in behavior is captured by a molecular model that considers reversible ion binding and electrostatic interactions. Further analysis in the low-salt regime shows that the looping-in phenomenon originates from the translational entropy of counterions that are needed to neutralize nonstoichiometric coacervates.

Reentrant liquid condensate phase of proteins is stabilized by hydrophobic and non-ionic interactions

Liquid–liquid phase separation of proteins underpins the formation of membraneless compartments in living cells. Elucidating the molecular driving forces underlying protein phase transitions is therefore a key objective for understanding biological function and malfunction. Here we show that cellular proteins, which form condensates at low salt concentrations, including FUS, TDP-43, Brd4, Sox2, and Annexin A11, can reenter a phase-separated regime at high salt concentrations. By bringing together experiments and simulations, we demonstrate that this reentrant phase transition in the high-salt regime is driven by hydrophobic and non-ionic interactions, and is mechanistically distinct from the low-salt regime, where condensates are additionally stabilized by electrostatic forces. Our work thus sheds light on the cooperation of hydrophobic and non-ionic interactions as general driving forces in the condensation process, with important implications for aberrant function, druggability, and material properties of biomolecular condensates.

Reentrant Liquid Condensate Phase of Proteins is Stabilized by Hydrophobic and Non-Ionic interactions

Many cellular proteins demix spontaneously from solution to form liquid condensates. These phase-separated structures have wide-ranging roles in health and disease. Elucidating the molecular driving forces underlying liquid-liquid phase separation (LLPS) is therefore a key objective for understanding biological function and malfunction. Here we show that proteins implicated in cellular LLPS, such as FUS, TDP-43, Brd4, Sox2, and Annexin A11, which form condensates at low salt concentrations, can reenter a phase-separated regime at high salt concentrations. Through experiments and simulations, we demonstrate that phase separation in the high-salt regime is mainly driven by hydrophobic and non-ionic interactions, and is mechanistically distinct from the low-salt regime, where condensates are additionally stabilized by electrostatic forces. Our work thus provides a new view on the cooperation of hydrophobicity and non-ionic interactions as non-specific driving forces for the condensation process, with important implications for aberrant function, druggability, and material properties of biomolecular condensates.

A thermodynamic evaluation of antibody-surface interactions in multimodal cation exchange chromatography.

In this study, the thermodynamics of binding of two industrial mAbs to multimodal cation exchange systems was investigated over a range of buffer and salt conditions via a van't Hoff analysis of retention data. Isocratic chromatography was first employed over a range of temperature and salt conditions on three multimodal resins and the retention data were analyzed in both the low and high salt regimes. While mAb retention decreased with salt for all resins at low salts, retention increased at high salts for two of the resins, suggesting a shift from electrostatic to more hydrophobic driven interactions. The retention data at various temperatures were then employed to generate non-linear van't Hoff plots which were fit to the quadratic form of the van't Hoff equation. At low salts, retention of both mAbs decreased with increasing temperature and the van't Hoff plots were concave downward on Capto MMC and Nuvia cPrime, while being concave upward on Capto MMC ImpRes. Different trends were observed on some of the resins with respect to both the concavity of the van't Hoff plots as well as the impact of temperature on the favorable enthalpies in the low salt regime. Interestingly, while increasingly favorable enthalpy with temperature was observed with Capto MMC and Nuvia cPrime at low salt, favorable enthalpy decreased with temperature for Capto MMC ImpRes. At high salts, binding of both mAbs on the two Capto resins were consistently entropically driven, consistent with desolvation. While the negative heat capacity data at low salts indicated that desolvation of polar/charged groups were important in Capto MMC and Nuvia cPrime, the positive data suggested that desolvation of non-polar groups were more important with Capto MMC ImpRes. Finally, the data at high salts indicated that desolvation of non-polar groups was the major driver for binding of both mAbs to the Capto resins under these conditions.

Computationally Mapping pKa Shifts Due to the Presence of a Polyelectrolyte Chain around Whey Proteins.

Experimental studies have shown the formation of soluble complexes in the pure repulsive Coulombic regime even when the net charges of the protein and the polyelectrolyte have the same sign ( De Kruif et al. Curr. Opin. Colloid Interface Sci. 2004 , 9 , 340 ; De Vries et al. J. Chem. Phys. 2003 , 118 , 4649 ; Grymonpre et al. Biomacromolecules 2001 , 2 , 422 ; Hattori et al. Langmuir 2000 , 16 , 9738 ). This attractive phenomenon has often been described as "complexation on the wrong side of pI". While one theory assumes the existence of "charged patches" on the protein surface from ion-dipole interactions, thus allowing a polyelectrolyte to bind to an oppositely heterogeneous charged protein region, another theoretical view considers the induced-charge interactions to be the dominant factor in these complexations. This charge regulation mechanism can be described by proton fluctuations resulting from mutual rearrangements of the distributions of the charged groups, due to perturbations of the acid-base equilibrium. Using constant-pH Monte Carlo simulations and several quantitative and visual analysis tools, we investigate the significance of each of these interactions for two whey proteins, α-lactalbumin (α-LA) and lysozyme (LYZ). Through physical chemistry parameters, free energies of interactions, and the mapping of amino acid pKa shifts and polyelectrolyte trajectories, we show the charge regulation mechanism to be the most important contributor in protein-polyelectrolyte complexation regardless of pH, dipole moment, and protein capacitance in a low salt regime.

Salt-induced reentrant stability of polyion-decorated particles with tunable surface charge density

The electrostatic complexation between DOTAP/DOPC unilamellar liposomes and an oppositely charged polyelectrolyte (NaPA) has been investigated in a wide range of the liposome surface charge density. We systematically characterized the reentrant condensation and the charge inversion of polyelectrolyte-decorated liposomes by means of dynamic light scattering and electrophoresis. We explored the stability of this model polyelectrolyte/colloid system at different values of the surface charge of the bare liposomes and by changing two independent control parameters of the suspensions: the polyelectrolyte/colloid charge ratio and the ionic strength of the aqueous suspending medium. The progressive addition of neutral DOPC lipid within the liposome membrane gave rise to an interesting phenomenon which has not been observed previously: the stability diagram of the suspensions showed a novel reentrance due to the crossing of the desorption threshold of the polyelectrolyte. Indeed, at fixed charge density of the bare DOTAP/DOPC liposomes and for a wide range of polyion concentrations, we showed that the simple electrolyte addition first (low salt regime) destabilizes the suspensions because of the enhanced screening of the residual repulsion between the complexes, and then (high salt regime) determines the onset of a new stable phase, originated by the absence of polyelectrolyte adsorption on the particle surfaces. We show that the observed phenomenology can be rationalized within the modified Velegol–Thwar model for heterogeneously charged particles and that the polyelectrolyte desorption fits well the predictions of the adsorption theory of Winkler and Cherstvy [1]. Our findings unambiguously support the picture of the reentrant condensation as driven by the correlated adsorption of the polyelectrolyte chains on the particle surface, providing interesting insights into possible mechanisms for tailoring complex colloids via salt-induced effects.

Thermal unbinding and ordering of amphiphile bilayers in the presence of salt

We have studied the effect of KBr on the electrostatically stabilized fluid lamellar phase of the ionic surfactant, C12-alkenylsuccinic acid (ASA). Three distinct regimes are found in the temperature - salt phase diagram of this system at a fixed ASA concentration of 20wt.%. A collapsed lamellar phase is formed in the low-salt regime, which exhibits an unbinding transition into uncorrelated bilayers on heating. In the intermediate salt regime the opposite trend is observed, with the uncorrelated bilayers present at low temperatures ordering into a lamellar phase at higher temperatures. As far as we are aware, this is the first report of such an ordering transition of uncorrelated bilayers. In the high salt regime, the topology of the bilayer changes, resulting in a lamellar–sponge transition. All the three transitions are reversible and the corresponding transition temperatures decrease with increasing salt concentration. The occurrence of these transitions in a single amphiphile system demonstrates the strong influence of salt on the bilayer elastic moduli as well as on the inter-bilayer interactions in the present system.

Polyelectrolyte Adsorption onto Oppositely Charged Interfaces: Image-Charge Repulsion and Surface Curvature

We analyze theoretically the influence of low-dielectric boundaries on the adsorption of flexible polyelectrolytes onto planar and spherical oppositely charged surfaces in electrolyte solutions. We rationalize to what extent polymer chains are depleted from adsorbing interfaces by repulsive image forces. We employ the WKB (Wentzel-Kramers-Brillouin) quantum mechanical method for the Green function of the Edwards equation to determine the adsorption equilibrium. Scaling relations are determined for the critical adsorption strength required to initiate polymer adsorption onto these low-dielectric supports. Image-force repulsion shifts the equilibrium toward the desorbed state, demanding larger surface charge densities and polyelectrolyte linear charge densities for the adsorption to take place. The effect is particularly pronounced for a planar interface in a low-salt regime, where a dramatic change in the scaling behavior for the adsorption-desorption transition is predicted. For the adsorbed state, polymers with higher charge densities are displaced further from the interface by image-charge repulsions. We discuss relevant experimental evidence and argue about possible biological applications of the results.

331 One year impact of a specific low salt educational program in heart failure patients

Heart failure is a serious disease despite recent therapeutic advances. A third of hospital admissions are therefore preventable. Dietetic education about low salt cooking is crucial for these patients. Changes in lifestyle are difficult to implement and ill reflexes can take over. We wanted to determine whether reflexes towards salt were persistent 12 months after an initial training session. 398 patients with systolic heart failure have been trained in heart failure in our center between 2003 and 2008. All have followed an education cursus focusing about low salt cooking. This education took place in the context of care, including integrated educational individual educational diagnosis, disease education, training in self rehabilitation and put into practical situations. Education and collective diet was designed to teach the patient's autonomy concerning the management of its little salty. Reflexes of the patients were analyzed by means of questionnaires based on clinical cases and a new questionnaire was sent to the patient an average of 11 months after initial training. The percentages were analyzed using the Chi 2 . A p <0.05 was considered significant. Mean age of patients was 65 (± 4), LVEF of 34 (± 4), the average BNP of 235 (± 346). Patient answers were higher than what was envisaged with a true response rate of 60%. (10-100%). In pre-training answers it is interesting to note that a high rate of patient considers potassium salt as a correct salt substitute and that the low salt scheme involves cooking everything with water (40% of responses). Within one year of training, the same clinical cases were asked and the answers were as good (81.5% of correct answers p<0.01 vs the initial questionnaire)) and comparable to the results after training. The implementation of an education in dietary salt-free diet allows sustainably improvement of heart failure patients’ reactions about the low salt regime.

Electrophoresis of Colloidal Dispersions in the Low-Salt Regime

We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a function of the colloidal concentration. Using an effective particle charge and a reduced screening parameter, we map the data for systems with different particle charges and sizes, including numerical simulation data with full electrostatics and hydrodynamics and experimental data for latex dispersions, on a single master curve. We observe two different volume fraction-dependent regimes for the electrophoretic mobility that can be explained in terms of the static properties of the ionic double layer.

Electrokinetic behaviour of overlapped electric double layers in nanofluidic channels

The ionic concentration variation arising in overlapped electric double layers innanochannels presents different electrokinetic behaviour from that in microchannels. Usinga modified concentration approach, the results reveal that the surface charge density isinsensitive to the concentration in the low-salt regime, and a nearly constant streamingconductance is predicted. A scaling analysis confirms that the streaming potential also hasa constant value because of length scale effects, but not due to the accumulated charge inthe double layer.

Macroion correlation effects in electrostatic screening and thermodynamics of highly charged colloids

We study macroion correlation effects on the thermodynamics of highly charged colloidal suspensions using a mean-field theory and primitive model computer simulations. We suggest a simple way to include the macroion correlations into the mean-field theory as an extension of the renormalized jellium model of Trizac and Levin [Phys. Rev. E 69, 031403 (2004)]. The effective screening parameters extracted from our mean-field approach are then used in a one-component model with macroions interacting via a Yukawa-like potential to predict macroion distributions. We find that inclusion of macroion correlations leads to a weaker screening and hence smaller effective macroion charge and lower osmotic pressure of the colloidal dispersion as compared to other mean-field models. This result is supported by comparison to primitive model simulations and experiments for charged macroions in the low-salt regime, where the macroion correlations are expected to be significant.

Ionizable polyelectrolyte brushes: brush height and electrosteric interaction

Semi-analytical scaling theory is used to describe quenched and annealed (weakly charged, ionizable, charge-regulating) polyelectrolyte brushes in electrolyte solutions of arbitrary salt concentration. An Alexander-De Gennes box model with homogeneous distribution of polymer segments and the free ends located at the edge of the brush is assumed, as is local electroneutrality in the brush. For annealed polyelectrolyte and in the low-salt regime, the theory predicts that for sufficiently dense brushes, the salt concentration has a small influence on brush height, while the brush expands with increasing grafting density, in agreement with experiment. Expressions are presented for the interaction free energy of compressed ionizable and quenched polyelectrolyte brushes (proportional to the force between particles or curved surfaces). In all cases, the required prefactors are explicitly stated. The theory is compared directly with published experiments on the influence of salt concentration, pH, and grafting density on the thickness and interaction force of polystyrene sulfonate (quenched) and poly(meth)acrylic acid (annealed) brushes. In general, trends are well reproduced but significant deviations remain.

Hydrophilic Silica Nanoparticles at the PDMS Droplet−Water Interface

The adsorption behavior of hydrophilic silica nanoparticles (fumed Aerosil 380) at the polydimethylsiloxane (PDMS) droplet−water interface has been investigated through particle adsorption isotherms, with complementary studies of the adsorbed layer structure by freeze−fracture SEM. The influence of solution conditions (pH and electrolyte concentration) has indicated the magnitude of particle−droplet and particle−particle interactions, and the influence of droplet cross-linking (deformability) indicated the role of particle penetration at the droplet−water interface. The silica particles adsorb onto PDMS droplets with plateau surface coverages that correspond to their effective particle size (hard sphere radius + double-layer thickness), i.e., lateral silica−silica interaction controls particle packing. Free energies of adsorption (ΔGads) are in the range −15 to −23 kJ mol-1 and concordant with a physical adsorption mechanism. The plateau surface coverages ΔGads and particle packing at the interface are only weakly influenced by pH, but significantly influenced by salt addition. Droplet cross-linking results in a reduction in particle adsorption, but only at higher salt concentrations: this was attributed to the increased likelihood of silica particles wetting PDMS and interfacial penetration. Freeze−fracture SEM revealed that in the low-salt regime individual silica particles are adsorbed at the droplet interface with negligible interfacial aggregation. Densely packed adsorbed particle layers are only observed when the double-layer thickness is reduced to a few nanometers, and even in the presence of a closely packed particle layer, droplets are not resistant to coalescence by excess salt. These findings lead to an improved understanding of particle adsorption to stabilize emulsion droplets.

Complexation between a Semiflexible Polyelectrolyte and an Oppositely Charged Sphere

We study theoretically the interaction of a charged, semiflexible polymer with an oppositely charged sphere. Both the effects of added salt (leading to a finite screening length) and of a bare stiffness of the polymer are taken into account. For intermediate salt concentration and high enough sphere charge we obtain a strongly bound complex where the polymer completely wraps around the sphere. The complex may or may not exhibit charge reversal, depending on the sphere charge and salt concentration. The low-salt regime is dominated by the polymer−polymer repulsion and leads to a characteristic hump shape: the polymer partially wraps around the sphere, and the two polymer arms extend parallel and in opposite directions from the sphere. In the high-salt regime we find bent solutions, where the polymer partially wraps the sphere and the polymer ends extend in arbitrary directions from the sphere; in this regime, the wrapping transition is strongly discontinuous. This wrapping behavior agrees qualitatively we...

Effect of Polyelectrolyte Adsorption on Intercolloidal Forces

The behavior of polyelectrolytes between charged surfaces immersed in semi-dilute solutions is investigated theoretically. A continuum mean field approach is used for calculating numerically concentration profiles between two electrodes held at a constant potential. A generalized contact theorem relates the inter-surface forces to the concentration profiles. The numerical results show that over-compensation of the surface charges by adsorbing polyelectrolytes can lead to effective attraction between equally charged surfaces. Simple scaling arguments enable us to characterize qualitatively the inter-surface interactions as function of the fraction of charged monomers p and the salt concentration c_b. In the low salt regime we find strong repulsion at short distances, where the polymers are depleted from the inter-surface gap, followed by strong attraction when the two adsorbed layers overlap. The magnitude of this attraction scales as p^{1/2} and its dominant length scale is proportional to a/p^{1/2}, where a is the monomer size. At larger distances the two adsorbing surfaces interact via a weak electrostatic repulsion. For strong polyelectrolytes at high salt concentration the polymer contribution to attraction at short distances scales as p/c_b^{1/2} and the length scale is proportional to kappa*a^2/p, where kappa^{-1} is the Debye-Huckel screening length. For weak polyelectrolytes at high salt concentration the interaction is repulsive for all surface separations and decays exponentially with a decay length equal to kappa^{-1}. The effect of irreversible adsorption is discussed as well and it is shown that inter-surface attraction can be obtained in this case as well.

Structural and electrical properties of poly(ethylene oxide)-cadmium sulphate complexes

Solid complexes of poly(ethylene oxide), PEO and 3CdSO 4.8H 2O varying from 0 to 25 wt% corresponding to solubility limit Cd 2+/EO=0 to 0.019 were prepared by the solution cast technique. X-ray diffraction (XRD) technique shows the presence of a new crystalline phase at Cd 2+/EO>0.009. Two new peaks are observed at 2 θ≈16° and 27° which is not attributable to PEO or 3CdSO 4.8H 2O. These peaks increase in intensity and are accompanied by other peaks as indicated in the diffractograms for samples with higher doping concentration. At low salt concentrations Cd 2+/EO<0.009, X-ray diffraction infers the disruption of the crystalline order of the PEO and their conversion to the amorphous state. The cations Cd 2+ and anions [SO 4] 2− are therefore dispersed in the amorphous phase of the PEO. The increase in salt concentration in the low salt regime (Cd 2+/EO<0.009) increases the amorphous nature of the PEO. This is verified by the variation of the coherent or Scherrer length with Cd 2+/EO ratio and implies that the highest conducting film is the least crystalline in nature. Absorption spectroscopy shows the occurrence of polymer–salt complexation which involves an interaction between the cation of the salt and the lone pair electrons of the ether oxygen atom. This can be inferred from the peaks present at 1395 and 1912 nm, respectively. Infrared spectra show some shifts in the spectral bands which could be attributed to complexation. Overlapping of some PEO and 3CdSO 4.8H 2O bands in the region 1000–1500 cm −1 makes such analysis difficult. Ionic conductivity in these electrolytes was studied by impedance measurement in the frequency range between 40 Hz and 100 kHz. The highest conductivity at room temperature was 5.5×10 −7 Scm −1 for the polymer film with solubility limit of Cd 2+/EO=0.009 at 5 MPa pressure. The transference number is ∼0.7. Some electrochemical cells were fabricated.

Investigations of ion absorption during NH4+ exposure I. Relationship between H+ efflux and NO3- absorption

Root NO3− absorption was examined under steady state conditions in the presence and absence of NH4+ using intact tomato plants (Lycopersicon esculentum cv. T-5). Plants grown under a low-salt regime showed much higher rates of NO3− absorption than plants grown under a high-salt regime, but the presence of NH4+ at concentrations less than 200 µM increased the capacity for net NO3− uptake for both the low- and high-salt conditions. Simultaneous changes in net NO3− , K+, and H+ exchanges were continuously monitored for 3 h prior to and up to 7 h following exposure to NH4+ . Upon first exposure to 50 or 100 µM NH4+, NO3− absorption remained constant; but during the subsequent 6 to 7 h, NO3− absorption continually increased. Net K+ absorption decreased iµMediately following its first exposure to NH4+ , but gradually recovered during the 7 h following first exposure. Changes in K+ absorption were not correlated with changes in NO3− absorption. Proton efflux gradually increased under NH4+ exposure and was significantly correlated with the observed increase in NO3− absorption. When roots absorbing NO3− were exposed to 5000 µM NH4+, NO3− absorption declined throughout the entire observation period.

Investigations of ion absorption during NH4+ exposure I. Relationship between H+ efflux and NO3- absorption

Root NO 3 - , absorption was examined under steady-state conditions in the presence and absence of NH 4 + using intact tomato plants (Lycopersicon esculentum cv. T-5). Plants grown under a low-salt regime showed much higher rates of NO 3 - , absorption than plants grown under a high-salt regime, but the presence of NH 4 + at concentrations less than 200 μM increased the capacity for net NO 3 - , uptake for both the low- and high-salt conditions. Simultaneous changes in net NO 3 - , K + , and H + exchanges were continuously monitored for 3 h prior to and up to 7 h following exposure to NH 4 + . Upon first exposure to 50 or 100 μM NH 4 + , NO 3 - , absorption remained constant; but during the subsequent 6 to 7h, NO 3 - , absorption continually increased. Net K + absorption decreased immediately following its first exposure to NH 4 + , but gradually recovered during the 7 h following first exposure. Changes in K + absorption were not correlated with changes in NO 3 - absorption. Proton efflux gradually increased under NH 4 + exposure and was significantly correlated with the observed increase in NO 3 - absorption. When roots absorbing NO 3 - were exposed to 5000 μM NH 4 + , NO 3 - absorption declined throughout the entire observation period.