Specific Anion Effects Research Articles

Elucidating the effects of hofmeister salts on the formation mechanism and biocompatibility of lysozyme-hyaluronic acid complexes

This study aimed to investigate the morphologies, biocompatibility, and formation mechanism of lysozyme-hyaluronic acid complexes in the presence of various Hofmeister salts. During the complexation of lysozyme (Lys) and hyaluronic acid (HA), salts can control the formation of colloidal nanoparticles, amyloid-like aggregates, and amorphous aggregates. Circular dichroism spectra revealed that the α-helix content of Lys involved in complexation significantly increased from 21.40 % to 34.19 %, whereas the β-sheet content significantly decreased from 38.65 % to 24.42 % with increasing salt concentration. The fluorescence spectra indicated that the number of binding sites for HA and Lys decreased from 2.19 to 0.63 as the salt concentration increased from 0 to 300 mM, which was consistent with the different anion-specific effects (NaCl < NaBr < NaI). Interestingly, in vitro experiment results demonstrated that colloidal nanoparticles and amorphous aggregates have good biocompatibility, with NCM460 cell viability exceeding 85.92 %, whereas amyloid like aggregates exhibit certain cytotoxicity, with cell viability significantly reduced to 50.47 %. Overall, these findings provide a better understanding of the conformational changes of Lys involved in complexation with HA in the presence of salts, expanding its application in the food and pharmaceutical industries.

Tough Hydro-Aerogels with Cation Specificity Enabled Ultra-High Stability for Multifunctional Sensing and Quasi-Solid-State Electrolyte Applications.

The anion-specific effects of the salting-in and salting-out phenomena are extensively observed in hydrogels, whereas the cation specificity of hydrogels is rarely reported. Herein, a multi-step strategy including borax pre-gelation, saline soaking, freeze-drying, and rehydrating is developed to fabricate polyvinyl alcohol gels with cation specificity, exhibiting the specific ordering of effects on the mechanical properties of gels as Ca2+ > Li+ > Mg2+ >> Fe3+ > Cu2+ >> Co2+ ≈ Ni2+ ≈ Zn2+. The multiple effects of the fabrication strategy, including the electrostatic repulsion among cations, skeleton support function of graphene oxide nanosheets, and water absorption and retention of ions, endow the gels with the dual characteristics of hydrogels and aerogels (i.e., hydro-aerogels). The hydro-aerogels prepared with the cationic salting-out effect display attractive pressure sensing performance with excellent stability over 90 days and enable continuous monitoring of ambient humidity in real-time and effective work in seawater to detect various parameters (e.g., depth, salinity, and temperature). The hydro-aerogels prepared without borax pretreatment or using the cationic salting-in effect can serve as quasi-solid-state electrolytes in supercapacitors, with 99.59% capacitance retention after 10000 cycles. This study realizes cation specificity in hydrogels and designs multifunctional hydro-aerogels for promising applications in various fields.

Specific anion effects on urease activity: A Hofmeister study

The effects of a range of electrolytes on the hydrolysis of urea by the enzyme urease is explored. The autocatalytic behavior of urease in unbuffered solutions and its pH clock reactions are studied. The concentration dependence of the experimental variables is analyzed in terms of specific ion-enzyme interactions and hydration. The results offer insights into the molecular mechanisms of the enzyme, and on the nature of its interactions with the electrolytes. We found that urease can tolerate mild electrolytes in its environment, while it is strongly inhibited by both strong kosmotropic and strong chaotropic anions. This study may cast light on an alternative therapy for Helicobacter pylori infections and contribute to the design of innovative materials and provide new approaches for the modulation of the enzymatic activity.

Anion-Specific Adsorption of Carboxymethyl Cellulose on Cellulose.

Integration of fiber modification step with a modern pulp mill is a resource efficient way to produce functional fibers. Motivated by the need to integrate polymer adsorption with the current pulping system, anion-specific effects in carboxymethylcellulose (CMC) adsorption have been studied. The QCM-D adsorption experiments revealed that CMC adsorption to the cellulose model surface is prone to anion-specific effects. A correlation was observed between the adsorbed CMC and the degree of hydration of the co-ions present in the magnesium salts. The presence of a chaotropic co-ion such as nitrate increased the adsorption of CMC on cellulose compared to the presence of the kosmotropic sulfate co-ion. However, anion-specificity was not significant in the case of salts containing zinc cations. The hydration of anions determines the distribution of the ions at the interface. Chaotropic ions, such as nitrates, are likely to be distributed near the chaotropic cellulose surface, causing changes in the ordering of water molecules and resulting in greater entropy gain once released from the surface, thus increasing CMC adsorption.

Tailoring the humidity response of cellulose nanocrystal-based films by specific ion effects

HypothesisThe optical properties and humidity response of iridescent films made of cellulose nanocrystal (CNC) and polyethylene glycol (PEG) can be tailored by the incorporation of electrolytes chosen based on specific ion effects (SIE). ExperimentsA series of inorganic salts comprising five different cations and five anions based on the Hofmeister series were mixed with CNC/PEG suspensions, followed by an air-dried process into iridescent solid films. These films were tested in changing relative humidity (RH) environments from 30% to 90% and their photonic properties and mass change monitored. The underlying structures and the mechanism of their formation were quantified in terms of interparticle distance derived from small angle X-ray scattering experiment and pitch size quantified by scanning electron microscope (SEM). FindingsThe specific color and color range of CNC/PEG based films are controlled by a specific anion effect achieved by selection of the salt while the specific cation effect is negligible. The salting-in type anions with the same valency result in a red-shift color when films are in the dried state. The salting-in type leads to a greater color changing range during RH changes than the salting-out type. The resultant mass gain/loss trend is consistent with the color change. In contrast, cations do not show any relationships between salting-in effect and the measured properties as observed for anions. The observed SIE can be used to engineer CNC/polymer-based humidity and bio-diagnostic colorimetric indicator devices.

Biomolecular Complexation on the "Wrong Side": A Case Study of the Influence of Salts and Sugars on the Interactions between Bovine Serum Albumin and Sodium Polystyrene Sulfonate.

In the protein purification, drug delivery, food industry, and biotechnological applications involving protein–polyelectrolyte complexation, proper selection of co-solutes and solution conditions plays a crucial role. The onset of (bio)macromolecular complexation occurs even on the so-called “wrong side” of the protein isoionic point where both the protein and the polyelectrolyte are net like-charged. To gain mechanistic insights into the modulatory role of salts (NaCl, NaBr, and NaI) and sugars (sucrose and sucralose) in protein–polyelectrolyte complexation under such conditions, interaction between bovine serum albumin (BSA) and sodium polystyrene sulfonate (NaPSS) at pH = 8.0 was studied by a combination of isothermal titration calorimetry, fluorescence spectroscopy, circular dichroism, and thermodynamic modeling. The BSA–NaPSS complexation proceeds by two binding processes (first, formation of intrapolymer complexes and then formation of interpolymer complexes), both driven by favorable electrostatic interactions between the negatively charged sulfonic groups (−SO3–) of NaPSS and positively charged patches on the BSA surface. Two such positive patches were identified, each responsible for one of the two binding processes. The presence of salts screened both short-range attractive and long-range repulsive electrostatic interactions between both macromolecules, resulting in a nonmonotonic dependence of the binding affinity on the total ionic strength for both binding processes. In addition, distinct anion-specific effects were observed (NaCl < NaBr < NaI). The effect of sugars was less pronounced: sucrose had no effect on the complexation, but its chlorinated analogue, sucralose, promoted it slightly due to the screening of long-range repulsive electrostatic interactions between BSA and NaPSS. Although short-range non-electrostatic interactions are frequently mentioned in the literature in relation to BSA or NaPSS, we found that the main driving force of complexation on the “wrong side” are electrostatic interactions.

Specific anion effects on the interfacial properties and aggregation of alkylphenol ethoxylate surfactants

The effect of Hofmeister anions on aggregation of nonionic surfactants and the formation of microemulsion systems was evaluated. Added salt effects on the interfacial properties and aggregation of ethylene oxide condensates of nonylphenol were characterized by cloud point measurements, surface tension, fluorescence spectroscopy, light scattering, and formulation scans. The specific effects of anions on the cloud point followed the trend predicted by the Hofmeister series: H2PO4− < Cl− < Br− < NO3− < I− < SCN−, with the H2PO4− being the most kosmotropic anion and generating the most significant cloud point reduction. The specific effects of anions on the critical micelle concentration (CMC) also followed the Hofmeister series, with kosmotropic anions causing a decrease and chaotropic anions generating an increase in the CMC. Chaotropic anions decreased the size of the aggregates, while kosmotropic anions increased it, as evaluated by dynamic light scattering. The effect of Hofmeister anions on microemulsion formation in nonionic surfactant-oil–water (SOW) systems was assessed by formulation scans. Although the kosmotropicity of the anion did not influence the optimum formulation value, it did affect the phase behavior around the optimum formulation, pointing to the possibility of tuning microemulsion formulations via the choice of added electrolyte.

Modulation of global stability, ligand binding and catalytic properties of trypsin by anions

Specific salts effect is well-known on stability and solubility of proteins, however, relatively limited knowledge is known regarding the effect on catalytic properties of enzymes. Here, we examined the effect of four sodium anions on thermal stability and catalytic properties of trypsin and binding of the fluorescent probe, p-aminobenzamidine (PAB), to the enzyme. We show that the specific anions effect on trypsin properties agrees with the localization of the anions in the Hofmeister series. Thermal stability of trypsin, Tm, the affinity of the fluorescent probe to the binding site, Kd, and the rate constant, kcat, of trypsin-catalyzed hydrolysis of the substrate N-benzoyl-L-arginine ethyl ester (BAEE) increase with increasing kosmotropic character of anions in the order: perchlorate<bromide<chloride<sulfate, while the value of Michaelis constant, KM, decreases. Correlations between the values of Tm, Kd for PAB, kcat, and KM for BAEE in the presence of 1 M studied salts suggest interrelation among these parameters of the enzyme. Global stabilization as well as increased rigidity of trypsin is accompanied by strengthening of interaction with fluorescent probe PAB and in accordance with decreasing values of KM for the substrate BAEE. Strong correlations between parameters characterizing the trypsin properties with the charge densities of anions clearly indicate direct electrostatic interaction as a basis of the specific anion effect on the conformational and functional properties of the enzyme.

Specific anion effect on properties of HRV 3C protease

Specific salts effect is intensively studied from the prospective of modification of different physico-chemical properties of biomacromolecules. Limited knowledge of the specific salts effect on enzymes led us to address the influence of five sodium anions: sulfate, phosphate, chloride, bromide, and perchlorate, on catalytic and conformational properties of human rhinovirus-14 (HRV) 3C protease. The enzyme conformation was monitored by circular dichroism spectrum (CD) and by tyrosines fluorescence. Stability and flexibility of the enzyme have been analyzed by CD in the far-UV region, differential scanning calorimetry and molecular dynamics simulations, respectively. We showed significant influence of the anions on the enzyme properties in accordance with the Hofmeister effect. The HRV 3C protease in the presence of kosmotropic anions, in contrast with chaotropic anions, exhibits increased stability, rigidity. Correlations of stabilization effect of anions on the enzyme with their charge density and the rate constant of the enzyme with the viscosity B-coefficients of anions suggest direct interaction of the anions with HRV 3C protease. The role of stabilization and decreased fluctuation of the polypeptide chain of HRV 3C protease on its activation in the presence of kosmotropic anions is discussed within the frame of the macromolecular rate theory.

Anion Specific Effects at Negatively Charged Interfaces: Influence of Cl-, Br-, I-, and SCN- on the Interactions of Na+ with the Carboxylic Acid Moiety.

Unlike counterion interactions with charged interfaces, the influence of co-ions is only scarcely reported in the literature. In this work, the effect of SCN– and the halide co-ions in the interactions of Na+ with carboxylic acid Langmuir monolayers is investigated by using vibrational sum frequency spectroscopy. At 1 M concentrations in the subphase, the identity of the anion is shown to have a remarkable influence on the charging behavior and degree of deprotonation of the monolayer, with ions ordering in the sequence I– > SCN– > Cl– ≈ Br–. The same trend is observed at both pH 6 and pH 9 when the monolayer is intrinsically more charged. Spectroscopic evidence is found for both the presence of I– and SCN– in the interfacial region at levels close to their detection limits. The results contradict electrostatic theories on charged interfaces where co-ions are not expected to play any significant role. The higher propensity for the large polarizable anions to deprotonate the monolayer is explained in terms of their ability to modify the cations affinity toward the carboxylic acid groups present at the surface.

Anion Specific Effects Drive the Formation of Li-Salt Based Aqueous Biphasic Systems.

Aqueous biphasic systems (ABSs) can form when mixing water with two compounds such as polymers, ionic liquids, or simple salts. While this phenomenon has been known for decades and found applications in various fields such as biology, recycling, or even more recently electrochemistry, the physics behind the formation of ABSs remains ill-understood. It was recently demonstrated that ABSs can be composed of two salts sharing the same cation (Li+) but different anions (sulfonamide and halide). Interestingly, their formation could not be explained by the position of the anions within the chaotropic/kosmotropic series and was rather proposed to originate from an anion size mismatch, albeit the size for these anions was never measured yet owing to the lack of a proper experimental methodology. Here, we combine experimental techniques and molecular simulations to assess the specific effects (size, shape, hydrophobic/hydrophilic character) of a series of anions and correlate them with the formation of ABSs. We demonstrate that while the anion size mismatch is a prerequisite for the formation of Li-salts based ABSs, their shape can also play an important role, providing general guidelines for forming new ABSs with potential future applications.

"Breaking" and "Making" of Water Structure at the Air/Water-Electrolyte (NaXO3; X = Cl, Br, I) Interface.

The prevalence of ions at the aqueous interface has been widely recognized, but their effect on the structure of interfacial water (e.g., hydrogen (H)-bonding) remains enigmatic. Using heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman difference spectroscopy with simultaneous curve fitting (DS-SCF) analysis, we show that the ion-induced perturbations of H-bonding at the air/water interface and in the bulk water are strongly correlated. Specifically, the structure-breaking anions such as ClO3- decrease the average H-bonding of water at the air/water interface, as it does to the water in its hydration shell in the bulk. The structure-making anion of the same series (IO3-) does exactly the reverse. None of the electrolytes (NaXO3; X = Cl, Br, I) form well-defined electric double layers that significantly increase or reverse the hydrogen-down (H-down) orientation of water at the air/water interface. These results provide a unified picture of specific anion effect at the air/water interface and in the bulk water.

Specific Anion Effects on Charged-Neutral Random Copolymers: Interplay between Different Anion-Polymer Interactions.

The study of ion specificities of charged-neutral random copolymers is of great importance for understanding specific ion effects on natural macromolecules. In the present work, we have investigated the specific anion effects on the thermoresponsive behavior of poly([2-(methacryloyloxy)ethyl trimethylammonium chloride]-co-N-isopropylacrylamide) [P(METAC-co-NIPAM)] random copolymers. Our study demonstrates that the anion specificities of the P(METAC-co-NIPAM) copolymers are dependent on their chemical compositions. The specific anion effects on the copolymers with high mole fractions of poly(N-isopropylacrylamide) (PNIPAM) are similar to those on the PNIPAM homopolymer. As the mole fraction of PNIPAM decreases to a certain value, a V-shaped anion series can be observed in terms of the anion-specific cloud point temperature of the copolymer, as induced by the interplay between different anion-polymer interactions. Our study also suggests that both the direct and the indirect anion-polymer interactions contribute to the anion specificities of the copolymers. This work would improve our understanding of the relationship between the ion specificities and the ion-macromolecule interactions for naturally occurring macromolecules.

Bulk Phase Behavior vs Interface Adsorption: Specific Multivalent Cation and Anion Effects on BSA Interactions.

Proteins are ubiquitous and play a critical role in many areas from living organisms to protein microchips. In humans, serum albumin has a prominent role in the foreign body response since it is the first protein which will interact with, e.g., an implant or stent. In this study, we focused on the influence of salts (i.e., different cations (Y3+, La3+) and anions (Cl-, I-) on bovine serum albumin (BSA) in terms of its bulk behavior as well as the role of charges for protein adsorption at the solid-liquid interface in order to understand and control the underlying molecular mechanisms and interactions. This is part of our group's effort to gain a deeper understanding of protein-protein and protein-surface interactions in the presence of multivalent ions. In the bulk, we established two new phase diagrams and found not only multivalent cation-triggered phase transitions, but also a dependence of the protein behavior on the type of anion. The attractive interactions between proteins were observed to increase from Cl- < NO3- < I-, resulting in iodide preventing re-entrant condensation and promoting liquid-liquid phase separation in bulk. Using ellipsometry and a quartz-crystal microbalance with dissipation (QCM-D), we obtained insight into the growth of the protein adsorption layer. Importantly, we found that phase transitions at the substrate can be triggered by certain interface properties, whether they exist in the bulk solution or not. Through the use of a hydrophilic, negatively charged surface (native silica), the direct binding of anions to the interface was prevented. Interestingly, this led to re-entrant adsorption even in the absence of re-entrant condensation in bulk. However, the overall amount of adsorbed protein was enhanced through stronger attractive protein-protein interactions in the presence of iodide salts. These findings illustrate how carefully chosen surface properties and salts can directly steer the binding of anions and cations, which guide protein behavior, thus paving the way for specific/triggered protein-protein, protein-salt, and protein-surface interactions.

Anion-Specific Effects on the Alkaline State of Cytochrome c.

Specific effects of anions on the structure, thermal stability, and peroxidase activity of native (state III) and alkaline (state IV) cytochrome c (cyt c) have been studied by the UV-VIS absorbance spectroscopy, intrinsic tryptophan fluorescence, and circular dichroism. Thermal and isothermal denaturation monitored by the tryptophan fluorescence and circular dichroism, respectively, implied lower stability of cyt c state IV in comparison with the state III. The pKa value of alkaline isomerization of cyt c depended on the present salts, i.e., kosmotropic anions increased and chaotropic anions decreased pKa (Hofmeister effect on protein stability). The peroxidase activity of cyt c in the state III, measured by oxidation of guaiacol, showed clear dependence on the salt position in the Hofmeister series, while cyt c in the alkaline state lacked the peroxidase activity regardless of the type of anions present in the solution. The alkaline isomerization of cyt c in the presence of 8 M urea, measured by Trp59 fluorescence, implied an existence of a high-affinity non-native ligand for the heme iron even in a partially denatured protein conformation. The conformation of the cyt c alkaline state in 8 M urea was considerably modulated by the specific effect of anions. Based on the Trp59 fluorescence quenching upon titration to alkaline pH in 8 M urea and molecular dynamics simulation, we hypothesize that the Lys79 conformer is most likely the predominant alkaline conformer of cyt c. The high affinity of the sixth ligand for the heme iron is likely a reason of the lack of peroxidase activity of cyt c in the alkaline state.

Specific Anion Effects on Lipase Adsorption and Enzymatic Synthesis of Biodiesel in Nonaqueous Media.

Pseudomonas fluorescens lipase (Pfl) was adsorbed on macroporous polypropylene to obtain a heterogeneous biocatalyst. The effect of NaCl concentration and of different 100 mm sodium salts on the Pfl loading and catalytic performance toward biodiesel synthesis via the solvent-free methanolysis of triglycerides was studied. Although lipase adsorption onto polypropylene is governed by hydrophobic interactions, both salt concentration and anion type affect lipase loading. Protein loading decreased along the series: Cl- > SO42- ≈ no salt > Br- > I- > SCN- > F- > AcO-. This nonmonotonic ion-specific trend can be the result of opposite mechanisms occurring during the adsorption step. A similar trend is observed also for triglyceride conversion and biodiesel yield. It is likely that ions affect the microenvironment of the adsorbed lipase by interacting specifically with the hydration water and polypeptide chains, thus affecting enzyme catalysis.

Anion-Specific Effects on Activity Coefficients in Aqueous Solutions of Sodium Salts: Modeling with the Extended Debye–Hückel Theory

The extended Debye–Huckel theory, which allows for concentration variation of electrolyte solution static permittivity, is employed to predict activity coefficients in aqueous solutions of sodium salts with various univalent anions (NaCl, NaBr, NaI, NaNO3, NaClO4 and NaSCN) at ambient conditions. Calculations without empirical adjustments reproduced the activity coefficients for NaI in the concentration range up to 6 mol·kg−1 and for NaSCN up to 2 mol·kg−1. In the case of other solutions, calculations underestimate water activity coefficients and overestimate mean ionic activity coefficients at concentrations beyond 0.5 mol·kg−1. In order to improve the representations, the model was extended to include ion pairing, which resulted in a better agreement between calculated activity coefficients and experimental data, especially for NaNO3. The ion pairing equilibrium constants were estimated and compared with available literature values. The extent of ion pairing was found to increase in the sequence NaI < NaSCN < NaBr < NaCl < NaClO4 < NaNO3, with violation of the Collins rule in the case of polyatomic oxygen-containing anions.

Combined Experimental and Theoretical Study of Weak Polyelectrolyte Brushes in Salt Mixtures.

The swelling behavior of a hydrophobic poly(2diisopropylamino)ethyl methacrylate (PDPA) brush immersed in aqueous solutions of single and mixed salts has been investigated using ellipsometry and numerical self-consistent field (nSCF) theory. As a function of solution ionic strength, the osmotic and salted brush regimes of weak polyelectrolyte brushes as well as substantial specific anion effects in the presence of K+ salts of Cl–, NO3–, and SCN– are found. For solutions containing mixtures of NO3– and Cl–, the brush swelling is the same as one would expect on the basis of the concentration-weighted average of the brush behavior in the single salt solutions. However, in mixtures of SCN– and Cl–, the swelling response is more complicated and substantial divergence from ideal behavior is observed. Mean-field theory shows excellent qualitative agreement with the ellipsometry findings. nSCF reveals that for the SCN–/Cl– cases the swelling behavior of the PDPA brush most likely arises from the predominant localization of the weakly hydrated SCN– within the brush compared to the more strongly hydrated Cl–.

Specific anion effects on the hydration and tribological properties of zwitterionic phosphorylcholine-based brushes

Polymer brushes are known to form highly efficient lubricating layers for a broad range of contacting surfaces in water. Here we focus on brushes of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) polymers that are end-anchored to the substrate at a high density. We show that the friction coefficient, which is measured upon sliding atomic force microscopy colloid probes on PMPC brushes at different normal loads, decreases when salts are added (1 M sodium fluoride (NaF), sodium chloride (NaCl), sodium iodide (NaI), cesium chloride (CsCl) and cesium iodide (CsI)). Interestingly, we observe that the reduction in the friction coefficient increases with increasing anionic size in the solution, while the size of the cation has no measureable effect. We relate the reduction in the friction coefficient to a decrease in the compliance of the brush. In pure water the brush is swollen and soft, resulting in a large contact area between colloids and brushes and, therefore, a relatively “high” friction coefficient. In salt solutions, the stiffness of the brush increases with increasing anionic size resulting in a lower contact area and, therefore, lower friction coefficient.

Tuning the pH Response of Weak Polyelectrolyte Brushes with Specific Anion Effects

The positively charged poly( N, N'-dimethylaminoethyl methacrylate) (PDMAEMA) brushes have been employed as model weak polyelectrolyte brushes (WPBs) to demonstrate the tuning of the pH response of WPBs with specific anion effects. The charge density of PDMAEMA brushes can be modulated by specific ion-pairing interactions between counterions and the protonated dimethylamino group; as a result, the strength of the pH response of PDMAEMA brushes can be tuned by specific anion effects. A more chaotropic counterion can more strongly interact with the protonated dimethylamino group, thereby more effectively neutralizing the positively charged group associated with the grafted weak polyelectrolyte chains and more remarkably suppressing the pH response of PDMAEMA brushes. Although the pH response of PDMAEMA brushes is insensitive to the anion identity at a low salt concentration, it can be tuned by specific anion effects at relatively high salt concentrations. Our study demonstrates that the pH-responsive properties of PDMAEMA brushes including hydration, conformation, oil wettability, and adhesion can be tuned by specific anion effects. The work presented here provides a method to tune the pH response of WPBs by the anion identity.