An NH moiety is not required for anion binding to amides in aqueous solution

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.

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.

Solvent effects in anion recognition

Anion recognition is pertinent to a range of environmental, medicinal and industrial applications. Recent progress in the field has relied on advances in synthetic host design to afford a broad range of potent recognition motifs and novel supramolecular structures capable of effective binding both in solution and at derived molecular films. However, performance in aqueous media remains a critical challenge. Understanding the effects of bulk and local solvent on anion recognition by host scaffolds is imperative if effective and selective detection in real-world media is to be viable. This Review seeks to provide a framework within which these effects can be considered both experimentally and theoretically. We highlight proposed models for solvation effects on anion binding and discuss approaches to retain strong anion binding in highly competitive (polar) solvents. The synthetic design principles for exploiting the aforementioned solvent effects are explored.

Interactions between the protein barnase and co-solutes studied by NMR

Protein solubility and stability depend on the co-solutes present. There is little theoretical basis for selection of suitable co-solutes. Some guidance is provided by the Hofmeister series, an empirical ordering of anions according to their effect on solubility and stability; and by osmolytes, which are small organic molecules produced by cells to allow them to function in stressful environments. Here, NMR titrations of the protein barnase with Hofmeister anions and osmolytes are used to measure and locate binding, and thus to separate binding and bulk solvent effects. We describe a rationalisation of Hofmeister (and inverse Hofmeister) effects, which is similar to the traditional chaotrope/kosmotrope idea but based on solvent fluctuation rather than water withdrawal, and characterise how co-solutes affect protein stability and solubility, based on solvent fluctuations. This provides a coherent explanation for solute effects, and points towards a more rational basis for choice of excipients.

Ion-Specific Water-Macromolecule Interactions at the Air/Aqueous Interface: An Insight into Hofmeister Effect

The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH₂-groups, which follows the Hofmeister series: SO₄^2- > Cl^- > NO₃^- > Br^- > ClO₄^- > SCN^-. The minimal orientation angle of CH₂-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO₄^-, and SCN^-. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH₂-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.

Engineered Binding Microenvironments in Halogen Bonding Polymers for Enhanced Anion Sensing

Mimicking Nature's polymeric protein architectures by designing hosts with binding cavities screened from bulk solvent is a promising approach to achieving anion recognition in competitive media. Accomplishing this, however, can be synthetically demanding. Herein we present a synthetically tractable approach, by directly incorporating potent supramolecular anion-receptive motifs into a polymeric scaffold, tuneable through a judicious selection of the co-monomer. A comprehensive analysis of anion recognition and sensing is demonstrated with redox-active, halogen bonding polymeric hosts. Notably, the polymeric hosts consistently outperform their monomeric analogues, with especially large halide binding enhancements of ca. 50-fold observed in aqueous-organic solvent mixtures. These binding enhancements are rationalised by the generation and presentation of low dielectric constant binding microenvironments from which there is appreciable solvent exclusion.

Ion-specific binding of cations to the carboxylate and of anions to the amide of alanylalanine

Studies of ion-specific effects on oligopeptides have aided our understanding of Hofmeister effects on proteins, yet the use of different model peptides and different experimental sensitivities have led to conflicting conclusions. To resolve these controversies, we study a small model peptide, L-Alanyl-L-alanine (2Ala), carrying all fundamental chemical protein motifs: C-terminus, amide bond, and N-terminus. We elucidate the effect of GdmCl, LiCl, KCl, KI, and KSCN by combining dielectric relaxation, nuclear magnetic resonance (¹H-NMR), and (two-dimensional) infrared spectroscopy. Our dielectric results show that all ions reduce the rotational mobility of 2Ala, yet the magnitude of the reduction is larger for denaturing cations than for anions. The NMR chemical shifts of the amide group are particularly sensitive to denaturing anions, indicative of anion-amide interactions. Infrared experiments reveal that LiCl alters the spectral homogeneity and dynamics of the carboxylate, but not the amide group. Interaction of LiCl with the negatively charged pole of 2Ala, the COO^- group, can explain the marked cationic effect on dipolar rotation, while interaction of anions between the poles, at the amide, only weakly perturbs dipolar dynamics. As such, our results provide a unifying view on ions' preferential interaction sites at 2Ala and help rationalize Hofmeister effects on proteins.

Distinctly different solvation behaviors of poly(N,N-diethylacrylamide) gels in water/acetone and water/DMSO mixtures

The solvation behaviors and intermolecular interactions of a poly(N,N-diethylacrylamide) (PDEA) gel network in water/DMSO and water/acetone mixtures have been investigated by variable-temperature high-resolution ¹H MAS NMR. Unlike decreasing volume phase transition temperature (VPTT) of the typical thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel induced by both acetone and DMSO in a water-rich region, distinct phase transition behaviors are revealed for the PDEA gel. That is, acetone is found to increase the VPTT of PDEA directly in the water-rich region while DMSO is also found to increase the VPTT of PDEA at a very low concentration but then decrease the VPTT as the concentration further increases. The above distinctly different VPTT shifts of PDEA are attributed to the different polymer-cosolvent interactions in water/acetone and water/DMSO systems. DMSO molecules with a strong water affinity are preferentially excluded by the PDEA gel network, and can promote the volume phase transition by favoring the collapse of the PDEA gel network, while acetone molecules preferentially adsorbed on the polymer interact with PDEA via non-specific van der Waals interaction, which favors the swollen state of the PDEA gel.

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.

A Protein Data Bank survey of multimodal binding of thiocyanate to proteins: Evidence for thiocyanate promiscuity

Over the last one and half century, a myriad of studies has demonstrated that Hofmeister ions have a major impact on protein stability and solubility. Nevertheless, the definition of the physico-chemical basis of their activity has proved to be highly challenging and controversial. Here, by exploiting the enormous information content of the Protein Data Bank, we explored the binding to proteins of thiocyanate, the anion of the series exerting the highest solubilization/destabilization effects. The survey, which led to the identification and characterization of 712 thiocyanate binding sites, provides a comprehensive and atomic-level view of the varied interactions that the ion forms with proteins. The inspection of these sites highlights a limited tendency of thiocyanate to interact with structured water molecules, in line with the reported poor hydration of the ion. On the other hand, the thiocyanate makes interactions with protein nonpolar moieties, especially with the backbone C^α atom. In as many as 104 cases, the ion exclusively makes nonpolar contacts. In conclusion, these findings suggest that the ability of thiocyanate to bind all types of protein exposed patches may lead to the formation of a negatively charged electrostatic barrier that could prevent protein-protein aggregation and promote protein solubility. Moreover, the denaturing action of thiocyanate may be ascribed to its ability to establish multiple attractive interactions with protein surfaces.

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.

Weakly hydrated anions bind to polymers but not monomers in aqueous solutions

Weakly hydrated anions help to solubilize hydrophobic macromolecules in aqueous solutions, but small molecules comprising the same chemical constituents precipitate out when exposed to these ions. Here, this apparent contradiction is resolved by systematically investigating the interactions of NaSCN with polyethylene oxide oligomers and polymers of varying molecular weight. A combination of spectroscopic and computational results reveals that SCN^- accumulates near the surface of polymers, but is excluded from monomers. This occurs because SCN^- preferentially binds to the centre of macromolecular chains, where the local water hydrogen-bonding network is disrupted. These findings suggest a link between ion-specific effects and theories addressing how hydrophobic hydration is modulated by the size and shape of a hydrophobic entity.

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.

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.

Thermoresponsive Polymers of Poly(2-(N-alkylacrylamide)ethyl acetate)s

Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including poly(2-(N-methylacrylamide) ethyl acetate) (PNMAAEA), poly(2-(N-ethylacrylamide) ethyl acetate) (PNEAAEA), and poly(2-(N-propylacrylamide) ethyl acetate) (PNPAAEA), as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that there are induction periods in the RAFT polymerization of these monomers, and the induction time correlates with the length of the N-alkyl groups in the monomers and follows the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s in water is also firmly dependent on the length of the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water and undergo the reversible soluble-to-insoluble transition at a critical solution temperature. The cloud point temperature (Tcp) of the thermoresponsive polymers is in the order of PNEAAEA < PNAEAA < PNMAAEA. The parameters affecting the Tcp of thermoresponsive polymers, e.g., degree of polymerization (DP), polymer concentration, salt, urea, and phenol, are investigated. Thermoresponsive PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions are synthesized, and their thermoresponse is checked.

Dynamic mechanism of halide salts on the phase transition of protein models, poly(N-isopropylacrylamide) and poly(N,N-diethylacrylamide)

The effects of salts on protein systems are not yet fully understood. We investigated the ionic dynamics of three halide salts (NaI, NaBr, and NaCl) with two protein models, namely poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEA), using multinuclear NMR, dispersion corrected density functional theory (DFT-D) calculations and dynamic light scattering (DLS) methods. The variation in ionic line-widths and chemical shifts induced by the polymers clearly illustrates that anions rather than cations interact directly with the polymers. From the variable temperature measurements of the NMR transverse relaxation rates of anions, which characterize the polymer-anion interaction intensities, the evolution behaviors of Cl^-/Br^-/I^- during phase transitions are similar in each polymer system but differ between the two polymer systems. The NMR transverse relaxation rates of anions change synchronously with the phase transition of PNIPAM upon heating, but they drop rapidly and vanish about 3-4.5 °C before the phase transition of PDEA. By combining the DFT-D and DLS data, the relaxation results imply that anions escape from the interacting sites with PDEA prior to full polymer dehydration or collapse, which can be attributed to the lack of anion-NH interactions. The different dynamic evolutions of the anions in the PNIPAM and PDEA systems give us an important clue for understanding the micro-mechanism of protein folding in a complex salt aqueous solvent.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Salt Effects on Hydrophobic Solvation: Is the Observed Salt Specificity the Result of Excluded Volume Effects or Water Mediated Ion-Hydrophobe Association?

The solubility of hydrophobic molecules in water is sensitive to salt addition in an ion-specific manner. Such "salting-out" and "salting-in" properties have been shown to be a major contributor to the measured ion-specific Hofmeister effects that are observed in many biophysical phenomena. Various theoretical models have suggested a number of disparate mechanisms for salting-out (salting-in) of hydrophobic moieties, the most popular of which include preferential interaction, water-mediated association, and electrostriction models. However, a complete molecular level description of this ion-specificity is not yet available. This work investigates the ion-specific nature of hydrophobic solvation by studying how sodium and chloride salts affect the thermodynamics of 1,2-hexanediol micellization. The results of this study are analyzed in terms of scaled-particle theory and we show that salt addition can affect hydrophobic solvation in two modalities: salt addition changes the cavitation free energy; salt addition also influences the solvent-solute interaction energy by changing the hydration of the hydrophobic solute. These two effects are salt specific in nature and we suggest that for small hydrophobic solutes these effects are the main cause of salt-specific Hofmeister effects on their solubility.

Expansion of Ion Effects on Water Induced by a High Hydrophilic Surface of a Polymer Network

The spatial extent and anion-cation cooperativity of the ion effect on the structure and dynamics of water have long been debated but are still controversial. Previously, we experimentally demonstrated the extensive and cooperative effect of ions on water in a polyamide network by measuring the reflection wavelength (λ) on the ion sensor of poly(N-isopropylacrylamide) (PNIPAAm) hydrogel-immobilized photonic crystals. In the present study, we investigated the influence of the polymer surface on the ion effect by adopting a highly hydrophilic poly(N-isopropylacrylamide-co-N-acryloylaza-18-crown-6) hydrogel as a sensor matrix. In alkaline earth metal salt solutions, the copolymer hydrogel membrane sensor showed the redshift of λ for the specific combination of cations and anions, that is, Ca²⁺/Cl^- and Sr²⁺/NO₃^-, which resulted from the concerted binding of ion pairs to the copolymer receptor. In alkali metal salt solutions, the ion sensor showed the blueshift of λ originating from the osmotic dehydration suppressed by the salts. The strength of the ion effect was evaluated by the average osmotic pressure (Π_A) required for the salt-inhibited dehydration in the early stage of hydrogel contraction. From the calculation results of Π_A for the copolymer and PNIPAAm hydrogels, it was found that the high hydrophilic copolymer surface more significantly enhanced the ion effect of structure-making cations (i.e., Li⁺) compared with borderline (Na⁺) and structure-breaking (K⁺ and Cs⁺) cations. Furthermore, the ion effect exhibited the higher ion cooperativity in combination with chloride anions than with nitrate anions. The enhancement of the long-range cooperative ion effect is derived from the expansion of the interactions between ions, water molecules, and the hydrophilic polymer network.

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.

Quantitative Evaluation of Long-Range and Cooperative Ion Effect on Water in Polyamide Network

Despite long-standing research efforts to elucidate the specific ion effect on the structure and dynamics of water, the spatial extent affected by ions and the cooperativity of ions and counterions are still controversial. Here, we demonstrate an undoubtable evidence of long-range and cooperative ion effect on water molecules in a polyamide network by using a precision ion sensor of photonic crystal hydrogel membrane. The ion effect was quantitatively evaluated by means of the osmotic work per unit cell volume change of photonic crystal, W_unit, required for the ion-inhibited dehydration, which means a suppressed migration of water molecules by the extensive effect of ions beyond their immediate hydration shells. It was found that  W_unit required for 14 vol % contraction of the membrane sensor in LiCl aqueous solutions was 7.7 times larger than that in Sr(NO₃)₂ solutions. The combination of structure-making Ca²⁺ and Sr²⁺ with nitrate anions lowered the ion effect than the chloride salts of borderline Na⁺ and Ba²⁺. Furthermore, the nitrate salt of Sr²⁺ exhibited a lower ion effect than the chloride salts of structure-breaking K⁺ and Cs⁺. These results have revealed that the ion effect acts to water extensively, which is modulated by cooperative interactions of ions and counterions.

The behaviour of hyaluronan solutions in the presence of Hofmeister ions: A light scattering, viscometry and surface tension study

Dynamic light scattering (DLS), viscosity and surface tension (SFT) measurements were used to characterize influence of salts containing ions of Hofmeister series (Na₂SO₄, (NH₄)₂SO₄, NaSCN, NH₄SCN and NaCl) on the behaviour of hyaluronan in diluted solutions at a temperature range of 15-45 °C. The results of the study showed that chaotropic and kosmotropic ions notably influenced the folding and unfolding of hyaluronan coils due to interactions between a respective ion and hydrophilic or hydrophobic patches present in the backbone of the polymer chains. This was mainly proved by viscosity and light scattering measurements. The temperature dependence of the hydrodynamic diameter of the hyaluronan coil determined by DLS demonstrated that combinations of chaotropic and kosmotropic ions in one salt (NaCl, NaSCN and (HN₄)₂SO₄) can stabilize the size of the coil in a wide range of temperatures. Tensiometry measurements indicated that certain types of ions present in the solution caused an unfolding of the hyaluronan coils, leading to a decrease of SFT.

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.

Specific Ion Effects on an Oligopeptide: Bidentate Binding Matters for the Guanidinium Cation

Ion-protein interactions are important for protein function, yet challenging to rationalize owing to the multitude of possible ion-protein interactions. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy shows that salts affect the peptide's reorientational dynamics, with a more pronounced effect for denaturing cations (Li⁺ , guanidinium (Gdm⁺ )) and anions (I^- , SCN^- ) than for weakly denaturing ones (K⁺ , Cl^- ). The effects of Gdm⁺ and Li⁺ were found to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm⁺ and Li⁺ to triglycine, yet with a different binding geometry: While Li⁺ predominantly binds to the C-terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm⁺ . This bidentate binding markedly affects peptide conformation, and may help to explain the high denaturation activity of Gdm⁺ salts.

Temperature dependent specific ion effects in mixed salt environments on a thermoresponsive poly(oligoethylene glycol methacrylate) brush

The temperature induced swelling/collapse transition of poly(oligoethylene glycol methacrylate) (POEGMA) brushes has been investigated in electrolyte solutions comprised of multiple anions.

Dynamics of the Phase Separation in a Thermoresponsive Polymer: Accelerated Phase Separation of Stereocontrolled Poly( N, N-diethylacrylamide) in Water

We studied the dependence on tacticity of the dynamic phase separation behavior of thermoresponsive poly( N, N-diethylacrylamide) (PDEA) in an aqueous solution. Using a laser temperature-jump technique combined with transient photometry, we determined the time constants of the phase separation and found that both atactic and isotactic-rich PDEAs had fast and slow phase separation processes (τ_fast and τ_slow). The fast process (τ_fast) was independent of the tacticity, irrespective of the concentration. On the other hand, the slow process had a strong dependence on the tacticity. We found the slow phase separation process got considerably faster with increasing isotacticity in dilute solutions. This effect due to the tacticity of the PDEA is totally different from that of poly( N-isopropylacrylamide) and can be explained on the basis of the difference between the hydrophobicity of atactic PDEA and that of isotactic-rich PDEA.

Effect of organic and inorganic ions on the lower critical solution transition and aggregation of PNIPAM

We have studied the effect of different ions belonging to the extended Hofmeister series on the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM), by combining Differential Scanning Calorimetry (DSC) and Dynamic Light Scattering (DLS). The variations in the lower critical solution temperature (TLCS) and enthalpy change during PNIPAM phase separation evidence the importance of considering both hydration and hydrophobicity to explain the interaction of ions with interfaces. The results obtained in the presence of inorganic ions can be explained by the tendency of water molecules to preferentially hydrate the PNIPAM chains or the ions, depending on the kosmotropic (highly hydrated) or chaotropic (poorly hydrated) character of the ions. On the contrary, tetraphenyl organic ions (Ph4B- and Ph4As+) interact with the hydrophobic moieties of PNIPAM chains, inducing a significant reduction of the TLCS. DLS results show that the aggregation state of PNIPAM above the TLCS is also strongly influenced by the presence of ions. While macroscopic phase separation (formation of a polymer-rich phase insoluble in water) was apparent in the presence of inorganic ions, we observed the formation of submicron PNIPAM aggregates at temperatures above the TLCS in the presence of the hydrophobic ions. Kinetically arrested monodisperse PNIPAM nanoparticles were formed in the presence of the Ph4B- anion, while a rather polydisperse distribution of particle sizes was observed in the presence of Ph4As+. These results show that ionic specificity influences both the static (thermodynamic) and dynamic (kinetically controlled aggregation) states of PNIPAM in an aqueous environment.

Interrogating the Osmotic Pressure of Self-Crowded Bovine Serum Albumin Solutions: Implications of Specific Monovalent Anion Effects Relative to the Hofmeister Series

The free-solvent-based (FSB) model and osmotic pressure were used to probe the ion binding and protein hydration for self-crowded bovine serum albumin in 0.15 M NaF, NaCl, NaI, and NaSCN solutions. All experiments were conducted with solutions at pH 7.4. The regressed results of the FSB model behavior to the measured osmotic pressure were excellent, albeit, the osmotic pressure data for NaSCN were noisy. The resulting ion binding and hydration were realistic values and the covariance of the two parameters was exceptionally low, providing substantial credibility to the FSB model. The results showed that the kosmotropic F^- and neutral Cl^- solutions generated significantly higher ion binding and protein hydration than the chaotropic solutions of I^- and SCN^-. Further, the ionic strength ratio and resulting hydration implied that the chaotropic solutions had substantially higher aggregation than the other salts investigated. Overall, the FSB model provides an additional, complementary tool to contribute to the analysis of crowded protein solutions relative to anions in the Hofmeister series as it can interrogate crowded solutions directly; something that is not possible with many measurement techniques.

Hofmeister Effect on PNIPAM in Bulk and at an Interface: Surface Partitioning of Weakly Hydrated Anions

The effect of sodium fluoride, sodium trichloroacetate, and sodium thiocyanate on the stability and conformation of poly(N-isopropylacrylamide), in bulk solution and at the gold-aqueous interface, is investigated by differential scanning calorimetry, dynamic light scattering, quartz crystal microbalance, and atomic force microscopy. The results indicate a surface partitioning of the weakly hydrated anions, i.e., thiocyanate and trichloroacetate, and the findings are discussed in terms of anion-induced electrostatic stabilization. Although attractive polymer-ion interactions are suggested for thiocyanate and trichloroacetate, a salting-out effect is found for sodium trichloroacetate. This apparent contradiction is explained by a combination of previously suggested mechanisms for the salting-out effect by weakly hydrated anions.

Beyond the Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions

Ions differ in their ability to salt out proteins from solution as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting out/salting in idea to include enzyme activities and the crystallization of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its molecular origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion-specific behavior in various environments, rather than to reach a molecular level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solutions. In this Feature Article we show that the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in solution. At the same time, we demonstrate the limitations of separating Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, we outline the route beyond Hofmeister chemistry in the direction of understanding specific roles of ions in various biological functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.

Guanidinium can both Cause and Prevent the Hydrophobic Collapse of Biomacromolecules

A combination of Fourier transform infrared and phase transition measurements as well as molecular computer simulations, and thermodynamic modeling were performed to probe the mechanisms by which guanidinium (Gnd⁺) salts influence the stability of the collapsed versus uncollapsed state of an elastin-like polypeptide (ELP), an uncharged thermoresponsive polymer. We found that the cation’s action was highly dependent upon the counteranion with which it was paired. Specifically, Gnd⁺ was depleted from the ELP/water interface and was found to stabilize the collapsed state of the macromolecule when paired with well-hydrated anions such as SO₄^2–. Stabilization in this case occurred via an excluded volume (or depletion) effect, whereby SO₄^2– was strongly partitioned away from the ELP/water interface. Intriguingly, at low salt concentrations, Gnd⁺ was also found to stabilize the collapsed state of the ELP when paired with SCN^–, which is a strong binder for the ELP. In this case, the anion and cation were both found to be enriched in the collapsed state of the polymer. The collapsed state was favored because the Gnd⁺ cross-linked the polymer chains together. Moreover, the anion helped partition Gnd⁺ to the polymer surface. At higher salt concentrations (>1.5 M), GndSCN switched to stabilizing the uncollapsed state because a sufficient amount of Gnd⁺ and SCN^– partitioned to the polymer surface to prevent cross-linking from occurring. Finally, in a third case, it was found that salts which interacted in an intermediate fashion with the polymer (e.g., GndCl) favored the uncollapsed conformation at all salt concentrations. These results provide a detailed, molecular-level, mechanistic picture of how Gnd⁺ influences the stability of polypeptides in three distinct physical regimes by varying the anion. It also helps explain the circumstances under which guanidinium salts can act as powerful and versatile protein denaturants.

A new thermoresponsive polymer of poly(N-acryloylsarcosine methyl ester) with a tunable LCST

A thermoresponsive polymer of the tertiary amide-based polyacrylamide, PNASME, was synthesized and its tunable thermoresponse was investigated.

Ion specificities of artificial macromolecules

Artificial macromolecules are well-defined synthetic polymers, with a relatively simple structure as compared to naturally occurring macromolecules. This review focuses on the ion specificities of artifical macromolecules. Ion specificities are influenced by solvent-mediated indirect ion-macromolecule interactions and also by direct ion-macromolecule interactions. In aqueous solutions, the role of water-mediated indirect ion-macromolecule interactions will be discussed. The addition of organic solvents to aqueous solutions significantly changes the ion specificities due to the formation of water-organic solvent complexes. For direct ion-macromolecule interactions, we will discuss specific ion-pairing interactions for charged macromolecules and specific ion-neutral site interactions for uncharged macromolecules. When the medium conditions change from dilute solutions to crowded environments, the ion specificities can be modified by either the volume exclusion effect, the variation of dielectric constant, or the interactions between ions, macromolecules, and crowding agents.

Molecular Mechanism for the Hofmeister Effect Derived from NMR and DSC Measurements on Barnase

The effects of sodium thiocyanate, sodium chloride, and sodium sulfate on the ribonuclease barnase were studied using differential scanning calorimetry (DSC) and NMR. Both measurements reveal specific and saturable binding at low anion concentrations (up to 250 mM), which produces localized conformational and energetic effects that are unrelated to the Hofmeister series. The binding of sulfate slows intramolecular motions, as revealed by peak broadening in ¹³C heteronuclear single quantum coherence spectroscopy. None of the anions shows significant binding to hydrophobic groups. Above 250 mM, the DSC results are consistent with the expected Hofmeister effects in that the chaotropic anion thiocyanate destabilizes barnase. In this higher concentration range, the anions have approximately linear effects on protein NMR chemical shifts, with no evidence for direct interaction of the anions with the protein surface. We conclude that the effects of the anions on barnase are mediated by solvent interactions. The results are not consistent with the predictions of the preferential interaction, preferential hydration, and excluded volume models commonly used to describe Hofmeister effects. Instead, they suggest that the Hofmeister anion effects on both stability and solubility of barnase are due to the way in which the protein interacts with water molecules, and in particular with water dipoles, which are more ordered around sulfate anions and less ordered around thiocyanate anions.

Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective

The recognition of anions in water remains a key challenge in modern supramolecular chemistry, and is essential if proposed applications in biological, medical, and environmental arenas that typically require aqueous conditions are to be achieved. However, synthetic anion receptors that operate in water have, in general, been the exception rather than the norm to date. Nevertheless, a significant step change towards routinely conducting anion recognition in water has been achieved in the past few years, and this Review highlights these approaches, with particular focus on controlling and using the hydrophobic effect, as well as more exotic interactions such as C-H hydrogen bonding and halogen bonding. We also look beyond the field of small-molecule recognition into the macromolecular domain, covering recent advances in anion recognition based on biomolecules, polymers, and nanoparticles.

Craig VS, Zhang G, Liu G. Mimicking enzymatic systems: modulation of the performance of polymeric organocatalysts by ion-specific effects

The performance of polymeric organocatalysts can be modulated by ion-specific effects based on the lessons learned from natural enzymatic systems.