Role of Carboxylate Side Chains in the Cation Hofmeister Series

Gelation of gellan induced by trivalent cations and coexisting trivalent with monovalent cations studied by rheological and DSC measurements

The effect of trivalent cation Fe3+ on the gelation process of a sodium salt form of gellan (DG, deacylated gellan gum) was investigated by rheology and DSC studies. On addition of a fairly low concentration of Fe3+ (1 mM), both the complex modulus (G*) of a 1.0 % DG solution in gel state and the sol-gel transition temperature (Tgel) slightly decreased. At higher Fe3+ concentrations (2 and 3 mM), however, a slight increase in the G* and Tgel was observed. In the coexisting monovalent cation (K+) solutions, addition of Fe3+ always improved the G* in gel state and the Tgel in a concentration-dependent manner. Moreover, for all Fe3+ DG solutions, the ordered structure formation temperature (Torder) was always lower than Tgel and increased with increasing Fe3+ concentration. This finding indicates that the network formation in the DG solutions should occur in advance of the ordered structure formation of the DG chains and that the presence of Fe3+ unfavorably affected the conformational transition of DG. In coexisting cation solution, the presence of K+ ion made a favorable contribution to the binding of Fe3+ to the disordered DG chains and to the subsequent ordered structure formation of the DG chains.

Quantitative measurement of cation-mediated adhesion of DNA to anionic surfaces

Anionic polyelectrolytes, such as DNA, are attracted to anionic surfaces in the presence of multivalent cations. A major barrier toward molecular-level understanding of these attractive interactions is the paucity of measurements of the binding strength. Here, atomic force microscopy-based single molecule force spectroscopy was used to quantify the binding free energy of double-stranded DNA to an anionic surface, with complementary density functional theory calculations of the binding energies of metal ion-ligand complexes. The results support both electrostatic attraction and ion-specific binding. Our study suggests that the correlated interactions between counterions are responsible for attraction between DNA and an anionic surface, but the strength of this attraction is modulated by the identity of the metal ion. We propose a mechanism in which the strength of metal-ligand binding, as well as the preference for particular binding sites, influence both the concentration dependence and the strength of the DNA-surface interactions.

Greasy Cations Bind to Neutral Macromolecules in Aqueous Solution

Ions influence the solution properties of macromolecules. Although much is known about anions, cationic effects are considered mostly in terms of weak interactions or exclusion from neutral interfaces. Herein, we have systematically studied the effect of quaternary tetraalkylammonium cations (NH4+, NMe4+, NEt4+, NPr4+, NBu4+) on the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution. Solubility measurements were coupled to 1H NMR and ATR-FTIR spectroscopic measurements. The solubility and NMR measurements revealed a direct binding between the greasiest cations and the isopropyl group of the macromolecule, evidenced from the nonlinear, Langmuir-type chemical shift response only at the isopropyl NMR signals with increasing salt concentrations. The ATR-FTIR measurements focusing on the amide oxygen showed that it is not the main direct-binding site. Additionally, the salting-out effects of the greasier cations correlate with their hydration entropies. These results demonstrate that the most weakly hydrated cations can bind to macromolecules as strongly as the weakly hydrated Hofmeister anions.

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 10 000 cycles. This study realizes cation specificity in hydrogels and designs multifunctional hydro-aerogels for promising applications in various fields.

Assessing Weak Anion Binding to Small Peptides

Since Hofmeister's seminal studies in the late 19th century, it has been known that salts and buffers can drastically affect the properties of peptides and proteins. These Hofmeister effects can be conceived of in terms of three distinct phenomena/mechanisms: water-salt interactions that indirectly induce the salting-out of a protein by water sequestration by the salt, and direct salt-protein interactions that can either salt-in or salt-out the protein. Unfortunately, direct salt-protein interactions responsible for Hofmeister effects are weak and difficult to quantify. As such, they are frequently construed of as being nonspecific. Nevertheless, there has been considerable effort to better specify these interactions. Here, we use pentapeptides to demonstrate the utility of the H-dimension of nuclear magnetic resonance (NMR) spectroscopy to assess anion binding using N-H signal shifts. We qualify binding using these, demonstrating the upfield shifts induced by anion association and revealing how they are much larger than the corresponding downfield shifts induced by magnetic susceptibility and other ionic strength change effects. We also qualify binding in terms of how the pattern of signal shifts changes with point mutations. In general, we find that the observed upfield shifts are small compared with those induced by anion binding to amide-based hosts, and MD simulations suggest that this is so. Thus, charge-diffuse anions associate mostly with the nonpolar regions of the peptide rather than directly interacting with the amide N-H groups. These findings reveal the utility of 1H NMR spectroscopy for qualifying affinity to peptides─even when affinity constants are very low─and serve as a benchmark for using NMR spectroscopy to study anion binding to more complex systems.

Characterization of the Effects of Low-Sodium Salt Substitution on Sensory Quality, Protein Oxidation, and Hydrolysis of Air-Dried Chicken Meat and Its Molecular Mechanisms Based on Tandem Mass Tagging-Labeled Quantitative Proteomics

The effects of low-sodium salt mixture substitution on the sensory quality, protein oxidation, and hydrolysis of air-dried chicken and its molecular mechanisms were investigated based on tandem mass tagging (TMT) quantitative proteomics. The composite salt formulated with 1.6% KCl, 0.8% MgCl2, and 5.6% NaCl was found to improve the freshness and texture quality scores. Low-sodium salt mixture substitution significantly decreased the carbonyl content (1.52 nmol/mg), surface hydrophobicity (102.58 μg), and dimeric tyrosine content (2.69 A.U.), and significantly increased the sulfhydryl content (74.46 nmol/mg) and tryptophan fluorescence intensity, suggesting that protein oxidation was inhibited. Furthermore, low-sodium salt mixture substitution significantly increased the protein hydrolysis index (0.067), and cathepsin B and L activities (102.13 U/g and 349.25 U/g), suggesting that protein hydrolysis was facilitated. The correlation results showed that changes in the degree of protein hydrolysis and protein oxidation were closely related to sensory quality. TMT quantitative proteomics indicated that the degradation of myosin and titin as well as changes in the activities of the enzymes, CNDP2, DPP7, ABHD12B, FADH2A, and AASS, were responsible for the changes in the taste quality. In addition, CNDP2, ALDH1A1, and NMNAT1 are key enzymes that reduce protein oxidation. Overall, KCl and MgCl2 composite salt substitution is an effective method for producing low-sodium air-dried chicken.

Binding of carboxylate and water to monovalent cations

The interactions of carboxylate anions with water and cations are important for a wide variety of systems, both biological and synthetic. To gain insight on properties of the local complexes, we apply density functional theory, to treat the complex electrostatic interactions, and investigate mixtures with varied numbers of carboxylate anions (acetate) and waters binding to monovalent cations, Li+, Na+ and K+. The optimal structure with overall lowest free energy contains two acetates and two waters such that the cation is four-fold coordinated, similar to structures found earlier for pure water or pure carboxylate ligands. More generally, the complexes with two acetates have the lowest free energy. In transitioning from the overall optimal state, exchanging an acetate for water has a lower free energy barrier than exchanging water for an acetate. In most cases, the carboxylates are monodentate and in the first solvation shell. As water is added to the system, hydrogen bonding between waters and carboxylate O atoms further stabilizes monodentate structures. These structures, which have strong electrostatic interactions that involve hydrogen bonds of varying strength, are significantly polarized, with ChelpG partial charges that vary substantially as the bonding geometry varies. Overall, these results emphasize the increasing importance of water as a component of binding sites as the number of ligands increases, thus affecting the preferential solvation of specific metal ions and clarifying Hofmeister effects. Finally, structural analysis correlated with free energy analysis supports the idea that binding to more than the preferred number of carboxylates under architectural constraints are a key to ion transport.

Hofmeister Series: From Aqueous Solution of Biomolecules to Single Cells and Nanovesicles

Hofmeister effects play a critical role in numerous physicochemical and biological phenomena, including the solubility and/or accumulation of proteins, the activities of enzymes, ion transport in biochannels, the structure of lipid bilayers, and the dynamics of vesicle opening and exocytosis. This minireview focuses on how ionic specificity affects the physicochemical properties of biomolecules to regulate cellular exocytosis, vesicular content, and nanovesicle opening. We summarize recent progress in further understanding Hofmeister effects on biomacromolecules and their applications in biological systems. These important steps have increased our understanding of the Hofmeister effects on cellular exocytosis, vesicular content, and nanovesicle opening. Increasing evidence is firmly establishing that the ions along the Hofmeister series play an important role in living organisms that has often been ignored.

Effects of Salts and Surface Charge on the Biophysical Stability of a Low pI Monoclonal Antibody

The impact of five representative Hofmeister salts (NaCl, KCl, MgCl₂, Na₂SO₄, and NaSCN) on the thermal stability and aggregation kinetics of a slightly acidic monoclonal antibody (mAb) were investigated under different pH conditions. The thermal stability of the mAb was assessed by measuring the lowest unfolding transition temperature, T_m, with differential scanning fluorimetry. MgCl₂ and NaSCN significantly decreased T_m at all three charged states of the mAb, but to the greatest extent when the mAb surface charge was net positive. Non-native aggregation kinetics was monitored by measuring Rayleigh light scattering. When the mAb surface charge was net positive or net neutral, the nucleation rate increased non-monotonically with MgCl₂ and NaSCN but decreased monotonically with NaCl, KCl, and Na₂SO₄. By contrast, when the mAb surface was negatively charged, there were only minor changes in the nucleation rate with all salts tested. Furthermore, there was less structural perturbation and slower aggregation rates when the mAb was net negatively charged than when it was net neutrally or positively charged. The observed salt effects on thermal unfolding are consistent with ion-specific mechanisms dominated by short-range amide backbone binding. On the other hand, the salt effects on nucleation rates appear to be influenced by both amide backbone binding and long-range electrostatic binding of ions to charged amino acid side chains.

Thermodynamics of Aggregation between the Cationic Surfactant and Polymer Based on Biodegradable Poly(vinyl Alcohol)

The formation of aggregates between carboxylated (PVCOOH) or neutral hydrolyzed (PVOH) poly(vinyl alcohol) and hexadecylpyridinium chloride (C₁₆PyCl) was examined by conductimetry, turbidimetry, and isothermal titration calorimetry (ITC) in the presence of different NaCl concentrations. The interaction between the polymers and C₁₆PyCl in pure water showed a critical aggregation concentration (cac = 0.8 mmol L^-1) only for the neutral polymer. PVCOOH interacted with the surfactant through electrostatic attraction, forming macroscopic aggregates. Integral enthalpy changes for aggregate formation (ΔH_agg) obtained from ITC curves varied from -0.61 (for the PVOH system in pure water) to -4.14 kJ mol^-1 (for PVOH in the presence of 10.0 mmol L^-1 NaCl), indicating that the formation of the aggregates was enthalpically favored. However, hydrophobic interactions drove the process for low surfactant concentration for both polymers. Saturation concentrations (C2) obtained from conductimetry were smaller than those from ITC, revealing that the binding of C₁₆PyCl on the chain of the polymers at higher surfactant concentrations shows the same electric properties as that of free micelles on the solution. Increase of the ionic strength favored the aggregation and decreased the complexity of the ITC curves, suggesting that the reorganization of the surfactant monomers on the polymeric chain with the increase in their concentration was suppressed.

A Polyanionic Tartrate-containing Temperature-responsive Hydrogel

Thermogels, a class of hydrogels which show spontaneous sol-gel phase transition when warmed, are an important class of soft biomaterials. To date, however, most amphiphilic polymers that are able to form thermogels in aqueous solution are uncharged, and the influence of ionisable groups on thermogelation are largely unknown. Herein, we report the first example of a polyanionic amphiphilic multi-block copolymer, containing multiple pendant carboxylate groups, that can form transparent thermogels spontaneously when warmed up to physiological temperature. We demonstrate that introducing negative charges onto thermogelling polymers could significantly alter the properties of the micelles and thermogels formed. Furthermore, the polymer's polyanionic character provides new options for modulating the gel rheological properties, such as stiffness and gelation temperatures, through electrostatic interactions with different cations. We also demonstrated the polyanionic thermogel allowed slower sustained release of a cationic model drug compound compared to an anionic one over 2 weeks. The findings from our study demonstrate exciting new possibilities for advanced biomedical applications using charged polyelectrolyte thermogel materials.

Protein Vesicles with pH-Responsive Disassembly

Protein biomaterials offer several advantages over those made from other components because their amino acid sequence can be precisely controlled with genetic engineering to produce a diverse set of material building blocks. In this work, three different elastin-like polypeptide (ELP) sequences were designed to synthesize pH-responsive protein vesicles. ELPs undergo a thermally induced hydrophobic transition that enables self-assembly of different kinds of protein biomaterials. The transition can be tuned by the composition of the guest residue, X, within the ELP pentapeptide repeat unit, VPGXG. When the guest residue is substituted with an ionizable amino acid, such as histidine, the ELP undergoes a pH-dependent hydrophobic phase transition. We used pH-responsive ELPs with different levels of histidine substitution, in combination with leucine zippers and globular, functional proteins, to fabricate protein vesicles. We demonstrate pH-dependent self-assembly, diameter, and disassembly of the vesicles using a combination of turbidimetry, dynamic light scattering, microscopy, and small angle X-ray scattering. As the ELP transition is dependent on the sequence, the vesicle properties also depend on the histidine content in the ELP building blocks. These results demonstrate the tunability of protein vesicles endowed with pH responsiveness, which expands their potential in drug-delivery applications.

Hofmeister Effects of Group II Cations as Seen in the Unfolding of Ribonuclease A

This work studies the effects of alkaline-earth cation addition upon the unfolding free energy of a model protein, pancreatic Ribonuclease A (RNase A) by DSC analysis.  RNase A was chosen because it: a) does not specifically bind Mg 2+ , Ca 2+ and Sr 2+ cations and b) maintains its structural integrity throughout a large pH range. We have measured and compared the effects of NaCl, MgCl 2 , CaCl 2 and SrCl 2 addition on the melting point of RNase A.  Our results show that even though the addition of group II cations to aqueous solvent reduces the solubility of nonpolar residues (and enhances the hydrophobic effect), their interactions with the amide moieties are strong enough to "salt-them-in" the solvent, thereby causing an overall reduction in protein stability. We demonstrate that amide-cation interactions are a major contributor to the observed "Hofmeister Effects" of group II cations in protein folding. Our analysis suggests that protein folding "Hofmeister Effects" of group II cations, are mostly the aggregate sum of how cation addition simultaneously salts-out hydrophobic moieties through increasing the cavitation free energy, while promoting the salting-in of amide moieties through contact pair formation.

Supramolecular Thixotropic Ionogel Electrolyte for Sodium Batteries

Owing to the potential of sodium as an alternative to lithium as charge carrier, increasing attention has been focused on the development of high-performance electrolytes for Na batteries in recent years. In this regard, gel-type electrolytes, which combine the outstanding ionic conductivity of liquid electrolytes and the safety of solid electrolytes, demonstrate immense application prospects. However, most gel electrolytes not only need a number of specific techniques for molding, but also typically suffer from breakage, leading to a short service life and severe safety issues. In this study, a supramolecular thixotropic ionogel electrolyte is proposed to address these problems. This thixotropic electrolyte is formed by the supramolecular self-assembly of D-gluconic acetal-based gelator (B8) in an ionic liquid solution of a Na salt, which exhibits moldability, a high ionic conductivity, and a rapid self-healing property. The ionogel electrolyte is chemically stable to Na and exhibits a good Na⁺ transference number. In addition, the self-assembly mechanism of B8 and thixotropic mechanism of ionogel are investigated. The safe, low-cost and multifunctional ionogel electrolyte developed herein supports the development of future high-performance Na batteries.

Anion-cation contrast of small molecule solvation in salt solutions

The contributions from anions and cations from salt are inseparable in their perturbation of molecular systems by experimental and computational methods, rendering it difficult to dissect the effects exerted by the anions and cations individually. Here we investigate the solvation of a small molecule, caffeine, and its perturbation by monovalent salts from various parts of the Hofmeister series. Using molecular dynamics and the energy-representation theory of solvation, we estimate the solvation free energy of caffeine and decompose it into the contributions from anions, cations, and water. We also decompose the contributions arising from the solute-solvent and solute-ions interactions and that from excluded volume, enabling us to pin-point the mechanism of salt. Anions and cations revealed high contrast in their perturbation of caffeine solvation, with the cations salting-in caffeine via binding to the polar ketone groups, while the anions were found to be salting-out via perturbations of water. In agreement with previous findings, the perturbation by salt is mostly anion dependent, with the magnitude of the excluded-volume effect found to be the governing mechanism. The free-energy decomposition as conducted in the present work can be useful to understand ion-specific effects and the associated Hofmeister series.

Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO₄) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K⁺ to the fatty sulfate interface showed 10 times greater affinity than for Li⁺ and approximately 3 times greater than for Na⁺. All three cations formed solvent shared ion pairs when the mole fraction of ESO₄ was 0.33 or lower. Above this threshold charge density, Li⁺ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K⁺ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na⁺ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.

Conformation and Metal Cation Binding of Zwitterionic Alanine Tripeptide in Saline Solutions by Infrared Vibrational Spectroscopy and Molecular Dynamics Simulations

In this work, linear infrared (IR) spectroscopy and molecular dynamics (MD) simulations were used to examine the interaction of different metal cations (Na⁺, Ca²⁺, Mg²⁺, and Zn²⁺) with backbone (amide C═O) and C-terminal carboxylate (COO^-) groups in zwitterionic alanine tripeptide (Ala3) in aqueous solutions with varying saline concentrations. Circular dichroism spectra and MD results suggest that Ala3 is predominantly in polyproline-II (PP_II) conformation, whose amide-I and asymmetric carboxylate stretching IR vibration signatures are also supported by quantum-chemistry calculations. The zwitterionic form of Ala3 separates the two amide-I modes in frequency, which are weakly coupled modes, as revealed by two-dimensional IR measurement, and can be used to probe backbone-cation interactions at different scenarios (near charged or neutral chemical groups respectively). Cation concentration-dependent IR frequency red shifts in the amide-I mode are seen for both amide-I modes, whereas blue shifts are also seen in the amide-I mode far from the NH₃⁺ group. The observed spectral changes are discussed from the perspective of the salting-in and salting-out abilities of the cations. In addition, all the metal cations studied here (except Zn²⁺) can specifically coordinate to the COO^- group in bidentate and pseudo-bridging forms simultaneously. For Zn²⁺, only the pseudo-bridging form exists. Our results shed light on the macroscopic protein salting-in and salting-out phenomena from the perspective of key chemical bonds in peptides.

Membrane protein extraction and purification using partially-esterified SMA polymers

Styrene maleic acid (SMA) polymers have proven to be very successful for the extraction of membrane proteins, forming SMA lipid particles (SMALPs), which maintain a lipid bilayer around the membrane protein. SMALP-encapsulated membrane proteins can be used for functional and structural studies. The SMALP approach allows retention of important protein-annular lipid interactions, exerts lateral pressure, and offers greater stability than traditional detergent solubilisation. However, SMA polymer does have some limitations, including a sensitivity to divalent cations and low pH, an absorbance spectrum that overlaps with many proteins, and possible restrictions on protein conformational change. Various modified polymers have been developed to try to overcome these challenges, but no clear solution has been found. A series of partially-esterified variants of SMA (SMA 2625, SMA 1440 and SMA 17352) has previously been shown to be highly effective for solubilisation of plant and cyanobacterial thylakoid membranes. It was hypothesised that the partial esterification of maleic acid groups would increase tolerance to divalent cations. Therefore, these partially-esterified polymers were tested for the solubilisation of lipids and membrane proteins, and their tolerance to magnesium ions. It was found that all partially esterified polymers were capable of solubilising and purifying a range of membrane proteins, but the yield of protein was lower with SMA 1440, and the degree of purity was lower for both SMA 1440 and SMA 17352. SMA 2625 performed comparably to SMA 2000. SMA 1440 also showed an increased sensitivity to divalent cations. Thus, it appears the interactions between SMA and divalent cations are more complex than proposed and require further investigation.

Hofmeister effects on protein stability are dependent on the nature of the unfolded state

The interpretation of a salt's effect on protein stability traditionally discriminates low concentration regimes (<0.3 M), dominated by electrostatic forces, and high concentration regimes, generally described by ion-specific Hofmeister effects. However, increased theoretical and experimental studies have highlighted observations of the Hofmeister phenomena at concentration ranges as low as 0.001 M. Reasonable quantitative predictions of such observations have been successfully achieved throughout the inclusion of ion dispersion forces in classical electrostatic theories. This molecular description is also on the basis of quantitative estimates obtained resorting to surface/bulk solvent partition models developed for ion-specific Hofmeister effects. However, the latter are limited by the availability of reliable structures representative of the unfolded state. Here, we use myoglobin as a model to explore how ion-dependency on the nature of the unfolded state affects protein stability, combining spectroscopic techniques with molecular dynamic simulations. To this end, the thermal and chemical stability of myoglobin was assessed in the presence of three different salts (NaCl, (NH₄)₂SO₄ and Na₂SO₄), at physiologically relevant concentrations (0-0.3 M). We observed mild destabilization of the native state induced by each ion, attributed to unfavorable neutralization and hydrogen-bonding with the protein side-chains. Both effects, combined with binding of Na⁺, Cl^- and SO₄^2- to the thermally unfolded state, resulted in an overall destabilization of the protein. Contrastingly, ion binding was hindered in the chemically unfolded conformation, due to occupation of the binding sites by urea molecules. Such mechanistic action led to a lower degree of destabilization, promoting surface tension effects that stabilized myoglobin according to the Hofmeister series. Therefore, we demonstrate that Hofmeister effects on protein stability are modulated by the heterogeneous physico-chemical nature of the unfolded state. Altogether, our findings evidence the need to characterize the structure of the unfolded state when attempting to dissect the molecular mechanisms underlying the effects of salts on protein stability.

The Influences of Sulphation, Salt Type, and Salt Concentration on the Structural Heterogeneity of Glycosaminoglycans

The increasing recognition of the biochemical importance of glycosaminoglycans (GAGs) has in recent times made them the center of attention of recent research investigations. It became evident that subtle conformational factors play an important role in determining the relationship between the chemical composition of GAGs and their activity. Therefore, a thorough understanding of their structural flexibility is needed, which is addressed in this work by means of all-atom molecular dynamics (MD) simulations. Four major GAGs with different substitution patterns, namely hyaluronic acid as unsulphated GAG, heparan-6-sulphate, chondroitin-4-sulphate, and chondroitin-6-sulphate, were investigated to elucidate the influence of sulphation on the dynamical features of GAGs. Moreover, the effects of increasing NaCl and KCl concentrations were studied as well. Different structural parameters were determined from the MD simulations, in combination with a presentation of the free energy landscape of the GAG conformations, which allowed us to unravel the conformational fingerprints unique to each GAG. The largest effects on the GAG structures were found for sulphation at position 6, as well as binding of the metal ions in the absence of chloride ions to the carboxylate and sulphate groups, which both increase the GAG conformational flexibility.

Plant Nanomaterials and Inspiration from Nature: Water Interactions and Hierarchically Structured Hydrogels

Recent developments in the area of plant-based hydrogels are introduced, especially those derived from wood as a widely available, multiscale, and hierarchical source of nanomaterials, as well as other cell wall elements. With water being fundamental in a hydrogel, water interactions, hydration, and swelling, all critically important in designing, processing, and achieving the desired properties of sustainable and functional hydrogels, are highlighted. A plant, by itself, is a form of a hydrogel, at least at given states of development, and for this reason phenomena such as fluid transport, diffusion, capillarity, and ionic effects are examined. These aspects are highly relevant not only to plants, especially lignified tissues, but also to the porous structures produced after removal of water (foams, sponges, cryogels, xerogels, and aerogels). Thus, a useful source of critical and comprehensive information is provided regarding the synthesis of hydrogels from plant materials (and especially wood nanostructures), and about the role of water, not only for processing but for developing hydrogel properties and uses.

Binary structure and dynamics of the hydrogen bonds in the hydration shells of ions

Ion-specific effects of cations (Li+, Na+, K+, Mg2+, Ca2+) and anions (F-, Cl-) on the hydrogen bond structure and dynamics of the coordination waters in the hydration shells have been studied using molecular dynamics simulations. Our simulations indicate that the hydrogen bonds between the first and second hydration shell waters show binary structural and dynamic properties. The hydrogen bond with a first shell water as the donor (HD) is strengthened, while those with a first shell water as the acceptor (HA) are weakened. For a hydrated anion, this binary effect reverses, but is less significant. This ion-specific binary effect correlates with the size and the valence of the ion, and is more significant for the strong kosmotropic ions of high charge density.

Contact Ion Pairs in the Bulk Affect Anion Interactions with Poly(N-isopropylacrylamide)

Salt effects on the solubility of uncharged polymers in aqueous solutions are usually dominated by anions, while the role of the cation with which they are paired is often ignored. In this study, we examine the influence of three aqueous metal iodide salt solutions (LiI, NaI, and CsI) on the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) by measuring the turbidity change of the solutions. Weakly hydrated anions, such as iodide, are known to interact with the polymer and thereby lead to salting-in behavior at low salt concentration followed by salting-out behavior at higher salt concentration. When varying the cation type, an unexpected salting-out trend is observed at higher salt concentrations, Cs⁺ > Na⁺ > Li⁺. Using molecular dynamics simulations, it is demonstrated that this originates from contact ion pair formation in the bulk solution, which introduces a competition for iodide ions between the polymer and cations. The weakly hydrated cation, Cs⁺, forms contact ion pairs with I^- in the bulk solution, leading to depletion of CsI from the polymer-water interface. Microscopically, this is correlated with the repulsion of iodide ions from the amide moiety.

Spectroscopic Evidence of the Salt-Induced Conformational Change around the Localized Electric Charges on the Protein Surface of Fibronectin Type III

The effect of salt on the electrostatic interaction of a protein is an important issue, because addition of salt affects protein stability and association/aggregation. Although adding salt is a generally recognized strategy to improve protein stability, this improvement does not necessarily occur. The lack of an effect upon the addition of salt was previously confirmed for the tenth fibronectin type III domain from human fibronectin (FN3) by thermal stability analysis. However, the detailed molecular mechanism is unknown. In the present study, by employing the negatively charged carboxyl triad on the surface of FN3 as a case study, the molecular mechanism of the inefficient NaCl effect on protein stability was experimentally addressed using spectroscopic methods. Complementary analysis using Raman spectroscopy and 8-anilino-1-naphthalenesulfonic acid fluorescence revealed the three-phase behavior of the salt-protein interaction between NaCl and FN3 over a wide salt concentration range from 100 mM to 4.0 M, suggesting that the Na⁺-specific binding to the negatively charged carboxyl triad causes a local conformational change around the binding site with an accompanying structural change in the overall protein, which contributes to the protein's structural destabilization. This spectroscopic evidence clarifies the molecular understanding of the inefficiency of salt to improve protein stability. The findings will inform the optimization of formulation conditions.

Specific and non-specific interactions between metal cations and zwitterionic alanine tripeptide in saline solutions reported by the symmetric carboxylate stretching and amide-II vibrations

The "specific" interaction between metal cations (Na⁺, Ca²⁺, Mg²⁺, and Zn²⁺) and the charged COO^- group, and the "non-specific" interaction between these cations and the peptide backbone of a zwitterionic trialanine (Ala3) in aqueous solutions were examined in detail, using linear infrared (IR) absorptions of the COO^- symmetric stretching and the amide-II (mainly the C-N stretching) modes as IR probes. Different IR spectral changes in peak positions and intensities of the two IR probes clearly demonstrate their sensitivities to nearby cation distributions in distance and population. Quantum chemistry calculations and molecular dynamics simulations were used to describe the cation-peptide interaction picture. These combined results suggest that Na⁺ and Ca²⁺ tend to bind to the COO^- group in the bidentate form, while Mg²⁺ and Zn²⁺ tend to bind to the COO^- group in the pseudo-bridging form. The results also show that while all three divalent cations indirectly interact with the peptide backbone with large population, Ca²⁺ and Mg²⁺ can be sometimes distributed very close to the backbone. Such a non-specific cation interaction can be moderately sensed by the C-N stretching of the amide-II mode when cations approach the polar amide C[double bond, length as m-dash]O group, and is also influenced by the NH₃⁺ charge group located at the N-terminus. The results suggest that the experimentally observed complication of the Hofmeister cation series shall be understood as a combined specific and non-specific cation-peptide interactions.

Molecular Mechanism for the Interactions of Hofmeister Cations with Macromolecules in Aqueous Solution

Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.

Cation Specific Effects on the Domain-Domain Interaction of Heterogeneous Dimeric Protein Revealed by FRET Analysis

Specific monovalent cation effects on the domain-domain interaction of heterogeneous dimeric protein were investigated using green fluorescent protein (GFP)-glutathione-s-transferase (GST) fusion protein as a model protein. Conjugating N-terminal of GST domain with a fluorescence probe Cyanine3, complementary increase and decrease of fluorescence intensities of Cyanine3 and GFP were recognized on the exclusive excitation of GFP and further the fluorescence decay of GFP was remarkably accelerated to show that an excellent Förster type of resonance excitation energy transfer (FRET) pair was constructed between GFP- and GST-domain. The spectral overlap integral and critical distance of the FRET pair were estimated to be 5.96×10¹³ M^-1cm³ and 62.5 Å, respectively. The FRET rate and efficiency evaluated by fluorescence lifetime of the energy donor, GFP, were influenced by the monovalent cations included in the buffer solution to suggest that the domain-domain interactions of GFP-GST fusion protein would be susceptible to cation species and their concentrations. The order affecting the domain-domain interaction was estimated to be Li⁺>NH₄⁺ >Na⁺>K⁺>Cs⁺, almost corresponding to the reverse Hofmeister series.

Divalent cations promote TALE DNA-binding specificity

Recent advances in gene editing have been enabled by programmable nucleases such as transcription activator-like effector nucleases (TALENs) and CRISPR–Cas9. However, several open questions remain regarding the molecular machinery in these systems, including fundamental search and binding behavior as well as role of off-target binding and specificity. In order to achieve efficient and specific cleavage at target sites, a high degree of target site discrimination must be demonstrated for gene editing applications. In this work, we studied the binding affinity and specificity for a series of TALE proteins under a variety of solution conditions using in vitro fluorescence methods and molecular dynamics (MD) simulations. Remarkably, we identified that TALEs demonstrate high sequence specificity only upon addition of small amounts of certain divalent cations (Mg²⁺, Ca²⁺). However, under purely monovalent salt conditions (K⁺, Na⁺), TALEs bind to specific and non-specific DNA with nearly equal affinity. Divalent cations preferentially bind to DNA over monovalent cations, which attenuates non-specific interactions between TALEs and DNA and further stabilizes specific interactions. Overall, these results uncover new mechanistic insights into the binding action of TALEs and further provide potential avenues for engineering and application of TALE- or TALEN-based systems for genome editing and regulation.

Specificity of protein-DNA interactions in hypersaline environment: structural studies on complexes of Halobacterium salinarum oxidative stress-dependent protein hsRosR

Interactions between proteins and DNA are crucial for all biological systems. Many studies have shown the dependence of protein–DNA interactions on the surrounding salt concentration. How these interactions are maintained in the hypersaline environments that halophiles inhabit remains puzzling. Towards solving this enigma, we identified the DNA motif recognized by the Halobactrium salinarum ROS-dependent transcription factor (hsRosR), determined the structure of several hsRosR–DNA complexes and investigated the DNA-binding process under extreme high-salt conditions. The picture that emerges from this work contributes to our understanding of the principles underlying the interplay between electrostatic interactions and salt-mediated protein–DNA interactions in an ionic environment characterized by molar salt concentrations.

Incorporation of short, charged peptide tags affects the temperature responsiveness of positively-charged elastin-like polypeptides

Elastin-like polypeptides (ELPs) are recombinant protein domains exhibiting lower critical solution temperature (LCST) behavior. This LCST behavior is controlled not only by intrinsic factors including amino acid composition and polypeptide chain length but also by non-ELP fusion domains. Here, we report that the presence of a composite non-ELP sequence that includes both His and T7 tags or a short Ser-Lys-Gly-Pro-Gly (SKGPG) sequence can dramatically change the LCST behavior of a positively-charged ELP domain. Both the His and T7 tags have been widely used in recombinant protein design to enable affinity chromatography and serve as epitopes for protein detection. The SKGPG sequence has been used to improve the expression of ELPs. Both the composite tag and the SKGPG sequence are <15% of the total length of the ELP fusion proteins. Despite the small size of the composite tag, its incorporation imparted pH-sensitive LCST behavior to the positively-charged ELP fusion protein. This pH sensitivity was not observed with the incorporation of the SKGPG sequence. The pH sensitivity results from both electrostatic and hydrophobic interactions between the composite tag and the positively-charged ELP domain. The hydrophobicity of the composite tag also alters the ELP interaction with Hofmeister salts by changing the overall hydrophobicity of the fusion protein. Our results suggest that incorporation of short tag sequences should be considered when designing temperature-responsive ELPs and provide insights into utilizing both electrostatic and hydrophobic interactions to design temperature-responsive recombinant proteins as well as synthetic polymers.

The Hofmeister series: Specific ion effects in aqueous polymer solutions

Specific ion effects in aqueous polymer solutions have been under active investigation over the past few decades. The current state-of-the-art research is primarily focused on the understanding of the mechanisms through which ions interact with macromolecules and affect their solution stability. Hence, we herein first present the current opinion on the sources of ion-specific effects and review the relevant studies. This includes a summary of the molecular mechanisms through which ions can interact with polymers, quantification of the affinity of ions for the polymer surface, a thermodynamic description of the effects of salts on polymer stability, as well as a discussion on the different forces that contribute to ion-polymer interplay. Finally, we also highlight future research issues that call for further scrutiny. These include fundamental questions on the mechanisms of ion-specific effects and their correlation with polymer properties as well as a discussion on the specific ion effects in more complex systems such as mixed electrolyte solutions.

Do soft anions promote protein denaturation through binding interactions? A case study using ribonuclease A

It has long been known that large soft anions like bromide, iodide and thiocyanate are protein denaturing agents, but their mechanism of action is still unclear. In this work we have investigated the protein denaturing properties of these anions using Ribonuclease A (RNase A) as a model protein system. Salt-induced perturbations to the protein folding free energy were determined using differential scanning calorimetry and the results demonstrate that the addition of sodium iodide and sodium thiocyanate significantly decreases the melting temperature of the protein. In order to account for this reduction in protein stability, we show that the introduction of salts that contain soft anions to the aqueous solvent perturbs the protein unfolding free energy through three mechanisms: (a) screening Coulomb interactions that exist between charged protein residues, (b) Hofmeister effects, and (c) specific anion binding to CH and CH2 moieties in the protein polypeptide backbone. Using the micellization of 1,2-hexanediol as a ruler for hydrophobicity, we have devised a practical methodology that separates the Coulomb and Hofmeister contributions of salts to the protein unfolding free energy. This allowing us to isolate the contribution of soft anion binding interactions to the unfolding process. The analysis shows that binding contributions have the largest magnitude, confirming that it is the binding of soft anions to the polypeptide backbone that is the main promoter of protein unfolding.

Alkali ion influence on structure and stability of fibrillar amyloid-β oligomers

Alzheimer's disease is characterized by the aggregation of Amyloid-β (Aβ) peptide into oligomers, fibrils and plaques. Many factors influencing this process as well as the stability of the various Aβ aggregates are known to date, and include the concentration and type of metal ions. Most experimental and theoretical studies have concentrated on heavy metal ions, like Fe²⁺, Zn²⁺, or Cu²⁺, while the smaller alkali ions Li⁺, Na⁺, and K⁺ have not gained much attention notwithstanding their role and ubiquity in physiological environments. In this work, we applied atomistic molecular dynamics simulations to investigate the potential role of these alkali ions in stabilizing fibrillar Aβ oligomers of different size and topology, i.e., single and double filament systems comprising 3-24 peptide chains per filament. We find a pronounced difference on the molecular level in the interaction behavior with free carboxylate groups of the Aβ oligomer: Li⁺ forms stable bridged interactions, whereas K⁺ interacts more transiently and lacks bridging. The behavior of Na⁺ is in between, so that this ion-protein interaction obeys the renowned Hofmeister series. These differences are also reflected in the ability of the alkali ions to stabilize the oligomer secondary structure. The stabilizing effect is most pronounced for the smaller fibrillar oligomers, suggesting that the type of alkali ion critically affects the initial stages of fibril formation. Our findings thus offer a molecular explanation for the observation that the polymorphisms of Aβ fibril structures are caused by differences in the surrounding ionic environment. Graphical abstract Influence of alkali ions on the structure and stability of fibrillar amyloid-β oligomers.

Tuning the collapse transition of weakly charged polymers by ion-specific screening and adsorption

The experimentally observed swelling and collapse response of weakly charged polymers to the addition of specific salts displays quite convoluted behavior that is not easy to categorize. Here we use a minimalistic implicit-solvent/explicit-salt simulation model with a focus on ion-specific interactions between ions and a single weakly charged polyelectrolyte to qualitatively explain the observed effects. In particular, we demonstrate ion-specific screening and bridging effects cause collapse at low salt concentrations whereas the same strong ion-specific direct interactions drive re-entrant swelling at high concentrations. Consistently with experiments, a distinct salt concentration at which the salting-out power of anions inverts from the reverse to direct Hofmeister series is observed. At this so called isospheric point, the ion-specific effects vanish. Furthermore, with additional simplifying assumptions, an ion-specific mean-field model is developed for the collapse transition which quantitatively agrees with the simulations. Our work demonstrates the sensitivity of the structural behavior of charged polymers to the addition of specific salt beyond simple screening and shall be useful for further guidance of experiments.

Metal Ion Interactions with Crude Oil Components: Specificity of Ca2+ Binding to Naphthenic Acid at an Oil/Water Interface

On the basis of dynamic interfacial tension measurements, Ca2+ has been shown specifically to interact with naphthenic acid (NA) at the n-heptane/water interface, consistent with NA adsorption followed by interfacial complexation and formation of a more ordered interfacial film. Optimum concentrations of Ca2+ and NA have been found to yield lower, time-dependent interfacial tensions, not evident for Mg2+ and Sr2+ or for several alkali metal ions studied. The results reflect the specific hydration and coordination chemistry of Ca2+ seen in biology. Owing to the ubiquitous presence of Ca2+ in oilfield waters, this finding has potential relevance to the surface chemistry underlying crude oil recovery. For example, “locking” acidic components at water/oil interfaces may be important for crude oil emulsion stability, or in bonding bulk oil to mineral surfaces through an aqueous phase, potentially relevant for carbonate reservoirs. The relevance of the present results to low salinity waterflooding as an enhanced crude oil recovery technique is also discussed.

Multistep Interactions between Ibuprofen and Lipid Membranes

Ibuprofen (IBU) interacts with phosphatidylcholine membranes in three distinct steps as a function of concentration. In a first step (<10 μM), IBU electrostatically adsorbs to the lipid headgroups and gradually decreases the interfacial potential. This first step helps to facilitate the second step (10-300 μM), in which hydrophobic insertion of the drug occurs. The second step disrupts the packing of the lipid acyl chains and expands the area per lipid. In a final step, above 300 μM IBU, the lipid membrane begins to solubilize, resulting in a detergent-like effect. The results described herein were obtained by a combination of fluorescence binding assays, vibrational sum frequency spectroscopy, and Langmuir monolayer compression experiments. By introducing trimethylammonium-propane, phosphatidylglycerol, and phosphatidylethanolamine lipids as well as cholesterol, we demonstrated that both the chemistry of the lipid headgroups and the packing of lipid acyl chains can substantially influence the interactions between IBU and the membranes. Moreover, different membrane chemistries can alter particular steps in the binding interaction.

Exploring Physicochemical Interactions of Different Salts with Sodium N-Dodecanoyl Sarcosinate in Aqueous Solution

Amino acid-based surfactants are used in academics and industry. Sodium N-dodecanoyl sarcosinate (SDDS) is such an amino acid-based surfactant having applications in pharmaceutical, food, and cosmetic formulations. Although the surface properties of this surfactant have been studied in the presence of univalent cationic and anionic salts, there is no report on such solution in the presence of higher valencies. In this experiment, critical micelle concentration (CMC) of SDDS from tensiometry, conductometry, and fluorimetry has been determined. In each case, CMC decreases with increasing salt concentration. Counterion binding of micelles (β), diffusion coefficient (D ₀), and surface properties, e.g., Gibbs free energy for micellization (ΔG _m ⁰), Gibbs surface excess (Γ_max), area of exclusion per surfactant monomer (A _min), surface pressure at CMC (π_cmc), etc., have been evaluated using methods such as tensiometry, conductometry, and fluorimetry. The hydrodynamic radius of SDDS in the presence of different salts was measured by the light scattering method. Aggregation number and shape of micelle have been determined by small-angle neutron scattering experiment. The nature of amphiphilic packing and the aggregation numbers of the assemblies have also been explored. The results from different experiments have been rationalized and represented systematically.

Dynamic light scattering studies of the effects of salts on the diffusivity of cationic and anionic cavitands

Although alkali halide salts play key roles in all living systems, the physical models used to describe the properties of aqueous solutions of salts do not take into account specific ion–ion interactions. To identify specific ion–ion interactions possibly contributing to the aggregation of proteins, we have used dynamic light scattering (DLS) to probe the aggregation of charged cavitands. DLS measurements of negatively charged 1 in the presence of a range of alkali metal halides reveal no significant aggregation of host 1 as a function of the nature of the cation of the added salt. Only at high concentrations could trace amounts of aggregation be detected by ¹H NMR spectroscopy. Contrarily, 1 was readily aggregated and precipitated by ZnCl₂. In contrast, although fluoride and chloride did not induce aggregation of positively charged host 2, this cavitand exhibited marked aggregation as a function of bromide and iodide concentration. Specifically, bromide induced small but significant amounts of dimerization, whilst iodide induced extreme aggregation. Moreover, in these cases aggregation of host 2 also exhibited a cationic dependence, with an observed trend Na⁺ > Li⁺ > K⁺ ≈ Cs⁺. In combination, these results reveal new details of specific ion pairings in aqueous solution and how this can influence the properties of dissolved organics.

Many-body effect determines the selectivity for Ca2+ and Mg2+ in proteins

Calcium ion is a versatile messenger in many cell-signaling processes. To achieve their functions, calcium-binding proteins selectively bind Ca2+ against a background of competing ions such as Mg2+ The high specificity of calcium-binding proteins has been intriguing since Mg2+ has a higher charge density than Ca2+ and is expected to bind more tightly to the carboxylate groups in calcium-binding pockets. Here, we showed that the specificity for Ca2+ is dictated by the many-body polarization effect, which is an energetic cost arising from the dense packing of multiple residues around the metal ion. Since polarization has stronger distance dependence compared with permanent electrostatics, the cost associated with the smaller Mg2+ is much higher than that with Ca2+ and outweighs the electrostatic attraction favorable for Mg2+ With the AMOEBA (atomic multipole optimized energetics for biomolecular simulation) polarizable force field, our simulations captured the relative binding free energy between Ca2+ and Mg2+ for proteins with various types of binding pockets and explained the nonmonotonic size dependence of the binding free energy in EF-hand proteins. Without electronic polarization, the smaller ions are always favored over larger ions and the relative binding free energy is roughly proportional to the net charge of the pocket. The many-body effect depends on both the number and the arrangement of charged residues. Fine-tuning of the ion selectivity could be achieved by combining the many-body effect and geometric constraint.