Thermodynamic origin of hofmeister ion effects

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.

Hofmeister Anion-Induced Tunable Rheology of Self-Healing Supramolecular Hydrogels

ᅟ: Physical gelation behaviors of a series of D-gluconic acetal-based derivatives bearing fatty alkyl amine moieties have been investigated. One of these molecules exhibits excellent gelation behaviors in water, and the resultant hydrogels are found to display self-healing properties. Interestingly, the elasticity and strength of the resulting gel can be tuned by the addition of different kinds of Hofmeister salts. The gel formation mechanism was proposed based on the analysis of FT-IR,1HNMR, and XRD, indicating that the main driving force for the self-assembly was the π-π stacking of the benzene rings in the aqueous solution system. Overall, our research provides an efficient approach for facilely tuning the properties of the D-gluconic acetal-based hydrogel. ᅟ.

The nature of salt effect in enhancing the extraction of rare earths by non-functional ionic liquids: Synergism of salt anion complexation and Hofmeister bias

Separation and recycling of rare-earths using ionic liquids as extractant are becoming a promising approach to replace traditional volatile organic solvents in recent years. Generally, the addition of some special salts could improve the extraction efficiency of rare-earths by numerous non-functional ionic liquids. However, knowledge behind the nature of the salt effect is limited. Here, we found that the enhancement in the extraction of rare-earth ions, Pr³⁺ ions, using non-functional ionic liquid, [A336][NO₃] (Tricaprylmethylammonium nitrate) was driven by the synergism of Hofmeister bias and complexation behaviors of salt anions with Pr³⁺ ions. Molecular dynamic simulations offered a new insight into the interaction mechanism of the ionic liquid with Pr³⁺ ions at liquid/liquid interface. It was revealed that salt anions could perform as a bridge to connect Pr³⁺ ions and the ionic liquid, so that promoted the extraction of Pr³⁺ ions. Therefore, the strong complexation ability of salt anions with Pr³⁺ ions and poor hydration of salt anions faciliated the transport of Pr³⁺ ions across liquid/liquid interface.

Counteraction of denaturant-induced protein unfolding is a general property of stabilizing agents

DSC measurements on RNase A at neutral pH show that five stabilizing agents, namely trimethylamine N-oxide, glucose, sucrose, betaine and sodium sulfate, can counteract the destabilizing action of urea, sodium perchlorate, guanidinium chloride and guanidinium thiocyanate. This is an important finding inferring that counteraction has a common physical origin, regardless of the chemical differences among the stabilizing agents and among the destabilizing ones. A rationalization is provided grounded on the following line of reasoning: (a) the decrease in solvent-excluded volume effect is the main stabilizing contribution of the native state; (b) its magnitude increases on increasing the density of the aqueous solution; (c) the density increases significantly in the ternary solutions containing water, a stabilizing agent and a destabilizing one, as indicated by the present experimental data.

The Thermodynamics of Anion Complexation to Nonpolar Pockets

The interactions between nonpolar surfaces and polarizable anions lie in a gray area between the hydrophobic and Hofmeister effects. To assess the affinity of these interactions, NMR and ITC were used to probe the thermodynamics of eight anions binding to four different hosts whose pockets each consist primarily of hydrocarbon. Two classes of host were examined: cavitands and cyclodextrins. For all hosts, anion affinity was found to follow the Hofmeister series, with associations ranging from 1.6-5.7 kcal mol-1. Despite the fact that cavitand hosts 1 and 2 possess intrinsic negative electrostatic fields, it was determined that these more enveloping hosts generally bound anions more strongly. The observation that the four hosts each possess specific anion affinities that cannot be readily explained by their structures, points to the importance of counter cations and the solvation of the "empty" hosts, free guests, and host-guest complexes, in defining the affinity.

Retracted: Molecular characterization of excipients' preferential interactions with therapeutic monoclonal antibodies

Retraction: Molecular characterization of excipients' preferential interactions with therapeutic monoclonal antibodies by Jehoon Kim, Mark R. H. Krebs and Bernhardt L. Trout The above article from the Journal of Pharmacy and Pharmacology, first published online on 4 August 2017 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, the journal Editor-in-Chief, Professor David Jones, and John Wiley & Sons Ltd. The authors discovered that the analysis of simulations was faulty making the data incorrect. Reference Kim J et al. Molecular characterization of excipients' preferential interactions with therapeutic monoclonal antibodies. J Pharm Pharmacol 2017. https://doi.org/10.1111/jphp.12787.

How does solvation in the cell affect protein folding and binding?

The cellular environment is highly diverse and capable of rapid changes in solute composition and concentrations. Decades of protein studies have highlighted their sensitivity to solute environment, yet these studies were rarely performed in situ. Recently, new techniques capable of monitoring proteins in their natural context within a live cell have emerged. A recurring theme of these investigations is the importance of the often-neglected cellular solvation environment to protein function. An emerging consensus is that protein processes in the cell are affected by a combination of steric and non-steric interactions with this solution. Here we explain how protein surface area and volume changes control these two interaction types, and give recent examples that highlight how even mild environmental changes can alter cellular processes.

Trehalose acts as a uridine 5'-diphosphoglucose-competitive inhibitor of trehalose 6-phosphate synthase in Corynebacterium glutamicum

Trehalose is a compatible solute widely distributed in nature. The most prevalent pathway for its synthesis starts from condensation of glucose 6-phosphate (Glc6P) and uridine 5'-diphosphoglucose (UDP-Glc) catalyzed by trehalose 6-phosphate synthase (TPS). A previous laboratory evolution experiment with the bacterium Corynebacterium glutamicum generated strains adapted to supraoptimal temperatures, and the R328H substitution of the TPS encoded by otsA was shown to be associated with thermotolerance acquired by the evolved strains. In this study, we found that the OtsA:R328H substitution promotes both intra- and extracellular trehalose accumulation and demonstrated that build-up of intracellular trehalose accounts for the OtsA^R328H -dependent thermotolerance, using the mycobacterial trehalose-specific transporter. Counterintuitively, characterization of the recombinant OtsA proteins revealed that the mutation downshifts the temperature optimum of OtsA. A search for the molecular basis of OtsA^R328H -dependent enhancement of trehalose synthesis led to the unexpected findings that trehalose is an effective inhibitor of OtsA and that OtsA^R328H is highly tolerant to the trehalose-mediated inhibition. The only available report on such feedback regulation of TPS is for the silk moth from over 50 years ago [Murphy TA and Wyatt GR (1965) J Biol Chem 240, 1500-1508]. While trehalose acts as a Glc6P-competitive inhibitor in the silk moth, the disaccharide was found to inhibit OtsA in a UDP-Glc-competitive manner in C. glutamicum, suggesting independent origins of the negative feedback regulations found for the two species. We showed that overexpression of the wild-type OtsA counteracts the trehalose-dependent regulation and restores the evolved strain-like phenotype to the isogenic wild-type otsA revertant, demonstrating that thermotolerance conferred by OtsA^R328H is attributable to its feedback-resistant property.

How Water's Properties Are Encoded in Its Molecular Structure and Energies

How are water's material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth's living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies-water's solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions-hydroxide and protons-diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water's molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water's orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties.

Specific cation effects at aqueous solution-vapor interfaces: Surfactant-like behavior of Li+ revealed by experiments and simulations

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K⁺ and Li⁺ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li⁺ adsorbs to the aqueous solution-vapor interface, while K⁺ does not. Thus, we provide experimental validation of the "surfactant-like" behavior of Li⁺ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K⁺ and Li⁺ ions to a difference in the resilience of their hydration shells.

What is the fundamental ion-specific series for anions and cations? Ion specificity in standard partial molar volumes of electrolytes and electrostriction in water and non-aqueous solvents

The importance of electrolyte solutions cannot be overstated. Beyond the ionic strength of electrolyte solutions the specific nature of the ions present is vital in controlling a host of properties. Therefore ion specificity is fundamentally important in physical chemistry, engineering and biology. The observation that the strengths of the effect of ions often follows well established series suggests that a single predictive and quantitative description of specific-ion effects covering a wide range of systems is possible. Such a theory would revolutionise applications of physical chemistry from polymer precipitation to drug design. Current approaches to understanding specific-ion effects involve consideration of the ions themselves, the solvent and relevant interfaces and the interactions between them. Here we investigate the specific-ion effects trends of standard partial molar volumes and electrostrictive volumes of electrolytes in water and eleven non-aqueous solvents. We choose these measures as they relate to bulk properties at infinite dilution, therefore they are the simplest electrolyte systems. This is done to test the hypothesis that the ions alone exhibit a specific-ion effect series that is independent of the solvent and unrelated to surface properties. The specific-ion effects trends of standard partial molar volumes and normalised electrostrictive volumes examined in this work show a fundamental ion-specific series that is reproduced across the solvents, which is the Hofmeister series for anions and the reverse lyotropic series for cations, supporting the hypothesis. This outcome is important in demonstrating that ion specificity is observed at infinite dilution and demonstrates that the complexity observed in the manifestation of specific-ion effects in a very wide range of systems is due to perturbations of solvent, surfaces and concentration on the underlying fundamental series. This knowledge will guide a general understanding of specific-ion effects and assist in the development of a quantitative predictive theory of ion specificity.

Role of Macromolecular Crowding on Stability and Iron Release Kinetics of Serum Transferrin

The macromolecular crowding influences the structural stability and functional properties of transferrin (Tf). The equilibrium as well as kinetic studies of Tf at different concentrations of crowding agents (dextran 40, dextran 70, and ficoll 70) and at a fixed concentration of dextran 40 under different concentrations of NaCl at pH 7.4 and 5.6 (±1) revealed that (i) the crowder environment increases the diferric-Tf (Fe₂Tf) stability against iron loss and overall denaturation of the protein, (ii) both in the absence and presence of crowder, the presence of salt promotes the loss of iron and overall denaturation of Fe₂Tf which is due to ionic screening of electrostatic interactions, (iii) the crowder environment retards iron release from monoferric N-lobe of Tf (Fe_NTf) by increasing enthalpic barrier, (iv) the retardation of iron release by crowding is enthalpically dominated than the entropic one, (v) both in the absence and presence of crowder, the presence of salt accelerates the iron release from Fe_NTf due to ionic screening of electrostatic interactions and anion binding to KISAB sites, and (vi) the crowders environment is unable to diminish (a) the salt-induced destabilization of Fe₂Tf against the loss of iron and overall denaturation and (b) the anion effect and ionic screening of diffusive counterions responsible to promote iron release from Fe_NTf.

Globular Protein Folding In Vitro and In Vivo

In vitro, computational, and theoretical studies of protein folding have converged to paint a rich and complex energy landscape. This landscape is sensitively modulated by environmental conditions and subject to evolutionary pressure on protein function. Of these environments, none is more complex than the cell itself, where proteins function in the cytosol, in membranes, and in different compartments. A wide variety of kinetic and thermodynamics experiments, ranging from single-molecule studies to jump kinetics and from nuclear magnetic resonance to imaging on the microscope, have elucidated how protein energy landscapes facilitate folding and how they are subject to evolutionary constraints and environmental perturbation. Here we review some recent developments in the field and refer the reader to some original work and additional reviews that cover this broad topic in protein science.

Experimental Atom-by-Atom Dissection of Amide-Amide and Amide-Hydrocarbon Interactions in H2O

Quantitative information about amide interactions in water is needed to understand their contributions to protein folding and amide effects on aqueous processes and to compare with computer simulations. Here we quantify interactions of urea, alkylated ureas, and other amides by osmometry and amide-aromatic hydrocarbon interactions by solubility. Analysis of these data yields strengths of interaction of ureas and naphthalene with amide sp2O, amide sp2N, aliphatic sp3C, and amide and aromatic sp2C unified atoms in water. Interactions of amide sp2O with urea and naphthalene are favorable, while amide sp2O-alkylurea interactions are unfavorable, becoming more unfavorable with increasing alkylation. Hence, amide sp2O-amide sp2N interactions (proposed n-σ* hydrogen bond) and amide sp2O-aromatic sp2C (proposed n-π*) interactions are favorable in water, while amide sp2O-sp3C interactions are unfavorable. Interactions of all ureas with sp3C and amide sp2N are favorable and increase in strength with increasing alkylation, indicating favorable sp3C-amide sp2N and sp3C-sp3C interactions. Naphthalene results show that aromatic sp2C-amide sp2N interactions in water are unfavorable while sp2C-sp3C interactions are favorable. These results allow interactions of amide and hydrocarbon moieties and effects of urea and alkylureas on aqueous processes to be predicted or interpreted in terms of structural information. We predict strengths of favorable urea-benzene and N-methylacetamide interactions from experimental information to compare with simulations and indicate how amounts of hydrocarbon and amide surfaces buried in protein folding and other biopolymer processes and transition states can be determined from analysis of urea and diethylurea effects on equilibrium and rate constants.

Basis of Protein Stabilization by K Glutamate: Unfavorable Interactions with Carbon, Oxygen Groups

Potassium glutamate (KGlu) is the primary Escherichia coli cytoplasmic salt. After sudden osmotic upshift, cytoplasmic KGlu concentration increases, initially because of water efflux and subsequently by K⁺ transport and Glu^- synthesis, allowing water uptake and resumption of growth at high osmolality. In vitro, KGlu ranks with Hofmeister salts KF and K₂SO₄ in driving protein folding and assembly. Replacement of KCl by KGlu stabilizes protein-nucleic acid complexes. To interpret and predict KGlu effects on protein processes, preferential interactions of KGlu with 15 model compounds displaying six protein functional groups-sp³ (aliphatic) C; sp² (aromatic, amide, carboxylate) C; amide and anionic (carboxylate) O; and amide and cationic N-were determined by osmometry or solubility assays. Analysis of these data yields interaction potentials (α-values) quantifying non-Coulombic chemical interactions of KGlu with unit area of these six groups. Interactions of KGlu with the 15 model compounds predicted from these six α-values agree well with experimental data. KGlu interactions with all carbon groups and with anionic (carboxylate) and amide oxygen are unfavorable, while KGlu interactions with cationic and amide nitrogen are favorable. These α-values, together with surface area information, provide quantitative predictions of why KGlu is an effective E. coli cytoplasmic osmolyte (because of the dominant effect of unfavorable interactions of KGlu with anionic and amide oxygens and hydrocarbon groups on the water-accessible surface of cytoplasmic biopolymers) and why KGlu is a strong stabilizer of folded proteins (because of the dominant effect of unfavorable interactions of KGlu with hydrocarbon groups and amide oxygens exposed in unfolding).

TMAO-Protein Preferential Interaction Profile Determines TMAO's Conditional In Vivo Compatibility

Trimethylamine N-oxide (TMAO) exemplifies how Nature uses the solute effect as a simple chemical strategy to cope with hydrodynamic pressure or urea stress to maintain proteostasis. It is a gut-microbe-generated metabolite that strongly promotes the development of atherosclerosis. It remains unclear how TMAO exerts its effects. In this study, we experimentally characterized the profile of the preferential interaction potential of TMAO with proteins, a thermodynamic key to understanding the effects of TMAO on protein processes and the distinction of TMAO among osmolytes. TMAO is thus found to be highly preferentially excluded from most types of protein surface, which explains why TMAO is a strong globular protein stabilizer and identifies the dominant stabilizing factor as the unfavorable interaction of TMAO with the hydrophobic surface exposed upon unfolding. We dissected the mechanism of the counteracting effects of TMAO and urea: the contrary feature of the interaction profiles of the two solutes maximizes the possibility for them to offset each other's perturbing effect on protein processes. The interaction profile also predicts that TMAO promotes aggregation of amyloidogenic intrinsically disordered peptide, as demonstrated here in Aβ42, and that TMAO has a strong potential to impact protein processes in the absence of stressors. Our data suggest that although TMAO is an evolutionally selected chemical chaperone for some organisms or organs, its compatibility in vivo is conditional and determined by its interaction profile with biopolymers and the nature of the essential biopolymer processes. Our thermodynamic framework plus the TMAO-protein interaction profile provides a basis for exploring the broad biological significance of TMAO, including its pathological impact in the absence of stressors. We argue for the general importance of controlling in vivo background solutes and the pathological significance of a control failure.

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.

Investigations of the Hofmeister series and other specific ion effects using lipid model systems

From the ion point-of-view specific ion effects (SIE) arise as an interplay of ionic size and shape and charge distribution. However in aqueous systems SIE invariably involve water, and at surfaces they involve both interacting surface groups and local fields emanating from the surface. In this review we highlight the fundamental importance of ionic size and hydration on SIE, properties which encompass all types of interacting forces and ion-pairing phenomena and make the Hofmeister or lyotropic series of ions pertinent to a broad range of systems and phenomena. On the other hand ionic hydrophobicity and complexation capacity also determine ionic behavior in a variety of contexts. Over the years we have carried out carefully designed experiments on a few selected soft matter model systems, most involving zwitterionic phospholipids, to assess the importance of fundamental ionic and interfacial properties on ion specific effects. By tuning down direct Coulomb interactions, working with different interfacial geometries, and carefully tuning ion-lipid headgroup interactions it is possible to assess the importance of different parameters contributing to ion specific behavior. We argue that the majority of specific ion effects involving relatively simple soft matter systems can be at least qualitatively understood and demystified.

Molecular-Level Insight of the Effect of Hofmeister Anions on the Interfacial Surface Tension of a Model Protein

The effect of the Hofmeister anions on the precipitation of proteins is often discussed using liquid-vapor coexisting systems with the assumption that the liquid-vapor interface mimics the liquid-protein interface. Solvated proteins, however, have both hydrophobic and hydrophilic regions on their surfaces rather than just a pure hydrophobic one. Using a solvated parallel β-sheet layer consisting of both hydrophobic and positively charged hydrophilic surfaces, we investigated the adsorption of kosmotropic (SO₄^2-) and chaotropic (ClO₄^-) anions toward the protein's hydrophobic and hydrophilic surfaces via Born-Oppenheimer molecular dynamics simulations using the BLYP density functional theory. It was found that both anions prefer to reside on the hydrophilic surface. Furthermore, kosmotropic anions, like SO₄^2-, enhance the interfacial surface tension of the protein and stabilize the protein, whereas, in contrast, chaotropic anions, like ClO₄^-, weaken the interfacial surface tension of the protein and allow water molecules to penetrate toward the peptide bonds to form water-peptide hydrogen bonds, thus destabilizing the protein.

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.

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.

Pair correlations that link the hydrophobic and Hofmeister effects

The Hofmeister effect describes how different ions make solutes more or less hydrophobic. The effect is thought to occur due to structural changes in the solvent induced by the ion's presence, particularly in water. In this study, the structural changes in water due to the presence of ions are investigated by molecular dynamics simulations of various monatomic ions in the SPC/E water model. Structural analyses reveal specific orientations of solvating waters around each of the ions studied. Using a new method, these orientations are quantified by a set of pair correlation functions that describe dipole-ion correlations in structure. These correlations are shown to contribute to the potential of mean force between waters and the ion of interest, and therefore to the free energy of the system. The magnitude of this free energy is found to result in a Hofmeister series for the various ions studied, therefore demonstrating a Hofmeister effect with respect to water's structure that is quantified by pair correlation functions. Most crucially, the pair correlations that lead to this Hofmeister effect also contribute to the hydrophobic effect (the entropy of hydrophobic solvation) [Liu et al., J. Chem. Phys., 2015, 142, 114117], and those which dominate the hydrophobic effect are modulated by an ion's presence, therefore demonstrating a mechanistic link between the two effects.

Positioning the Intracellular Salt Potassium Glutamate in the Hofmeister Series by Chemical Unfolding Studies of NTL9

In vitro, replacing KCl with potassium glutamate (KGlu), the Escherichia coli cytoplasmic salt and osmolyte, stabilizes folded proteins and protein-nucleic acid complexes. To understand the chemical basis for these effects and rank Glu- in the Hofmeister anion series for protein unfolding, we quantify and interpret the strong stabilizing effect of KGlu on the ribosomal protein domain NTL9, relative to the effects of other stabilizers (KCl, KF, and K2SO4) and destabilizers (GuHCl and GuHSCN). GuHSCN titrations at 20 ° C, performed as a function of the concentration of KGlu or another salt and monitored by NTL9 fluorescence, are analyzed to obtain R-values quantifying the Hofmeister salt concentration (m3) dependence of the unfolding equilibrium constant K(obs) [r-value = −d ln K(obs)/dm3 = (1/RT) dΔG(obs) ° /dm3 = m-value/RT]. r-Values for both stabilizing K+ salts and destabilizing GuH+ salts are compared with predictions from model compound data. For two-salt mixtures, we find that contributions of stabilizing and destabilizing salts to observed r-values are additive and independent. At 20 ° C, we determine a KGlu r-value of 3.22 m(−1) and K2SO4, KF, KCl, GuHCl, and GuHSCN r-values of 5.38, 1.05, 0.64, −1.38, and −3.00 m(−1), respectively. The KGlu r-value represents a 25-fold (1.9 kcal) stabilization per molal KGlu added. KGlu is much more stabilizing than KF, and the stabilizing effect of KGlu is larger in magnitude than the destabilizing effect of GuHSCN. Interpretation of the data reveals good agreement between predicted and observed relative r-values and indicates the presence of significant residual structure in GuHSCN-unfolded NTL9 at 20 ° C.

Tuning protein-protein interactions using cosolvents: specific effects of ionic and non-ionic additives on protein phase behavior

Cosolvents are routinely used to modulate the (thermal) stability of proteins and, hence, their interactions with proteins have been studied intensely. However, less is known about their specific effects on protein-protein interactions, which we characterize in terms of the protein phase behavior. We analyze the phase behavior of lysozyme solutions in the presence of sodium chloride (NaCl), guanidine hydrochloride (GuHCl), glycerol, and dimethyl sulfoxide (DMSO). We experimentally determined the crystallization boundary (XB) and, in combination with data on the cloud-point temperatures (CPTs), the crystallization gap. In agreement with other studies, our data indicate that the additives might affect the protein phase behavior through electrostatic screening and additive-specific contributions. At high salt concentrations, where electrostatic interactions are screened, both the CPT and the XB are found to be linear functions of the additive concentration. Their slopes quantify the additive-specific changes of the phase behavior and thus of the protein-protein interactions. While the specific effect of NaCl is to induce attractions between proteins, DMSO, glycerol and GuHCl (with increasing strength) weaken attractions and/or induce repulsions. Except for DMSO, changes of the CPT are stronger than those of the XB. Furthermore, the crystallization gap widens in the case of GuHCl and glycerol and narrows in the case of NaCl. We relate these changes to colloidal interaction models, namely square-well and patchy interactions.

Effects of alkali and ammonium ions in the detection of poly(ethyleneglycol) by alpha-hemolysin nanopore sensor

Cations influence the sensitivity of the sensor formed by alpha-hemolysin nanopore.

Hofmeister effect on thermo-responsive poly(propylene oxide) in H2O and D2O

The Hofmeister effect of NaSCN, NaCl and NaF on poly(propylene oxide) solutions in H₂O and D₂O.

Hofmeister effect on thermo-responsive poly(propylene oxide): Role of polymer molecular weight and concentration

Although a vast amount of research has been dedicated to investigate the Hofmeister effect on the stability of polymer solutions, a clear understanding of the role of polymer properties in this phenomenon is still missing. Here, the Hofmeister effect of NaCl (destabilizing) and NaSCN (stabilizing) salts on aqueous solutions of poly(propylene oxide) (PPO) is studied. Four different molecular weights of PPO were investigated, to determine how the variation in the polymer coil size affects the Hofmeister effect. The investigation was further conducted for different PPO concentrations, in order to understand the effect of inter-chain interactions on the response to addition of salt. The temperature-driven phase separation of the solutions was monitored by differential scanning calorimetry, which provides the precise value of the phase separation temperature, as well as the enthalpy change accompanied with the transition. It was observed that increasing the molecular weight weakens the effect of the both salts, which is interpreted in terms of a scaling law between the molecular weight and the accessible surface area of the polymers. Increasing the PPO concentration further diminished the NaCl effect, but amplified the NaSCN effect. This difference is attributed to an electrostatic stabilization mechanism in the case of NaSCN.

Its preferential interactions with biopolymers account for diverse observed effects of trehalose

Biopolymer homeostasis underlies the health of organisms, and protective osmolytes have emerged as one strategy used by Nature to preserve biopolymer homeostasis. However, a great deal remains unknown about the mechanism of action of osmolytes. Trehalose, as a prominent example, stabilizes proteins against denaturation by extreme temperature and denaturants, preserves membrane integrity upon freezing or in dry conditions, inhibits polyQ-mediated protein aggregation, and suppresses the aggregation of denatured proteins. The underlying thermodynamic mechanisms of such diverse effects of trehalose remain unclear or controversial. In this study, we applied the surface-additive method developed in the Record laboratory to attack this issue. We characterized the key features of trehalose-biopolymer preferential interactions and found that trehalose has strong unfavorable interactions with aliphatic carbon and significant favorable interactions with amide/anionic oxygen. This dissection has allowed us to elucidate the diverse effects of trehalose and to identify the crucial functional group(s) responsible for its effects. With (semi)quantitative thermodynamic analysis, we discovered that 1) the unfavorable interaction of trehalose with hydrophobic surfaces is the dominant factor in its effect on protein stability, 2) the favorable interaction of trehalose with polar amides enables it to inhibit polyQ-mediated protein aggregation and the aggregation of denatured protein in general, and 3) the favorable interaction of trehalose with phosphate oxygens, together with its unfavorable interaction with aliphatic carbons, enables trehalose to preserve membrane integrity in aqueous solution. These results provide a basis for a full understanding of the role of trehalose in biopolymer homeostasis and the reason behind its evolutionary selection as an osmolyte, as well as for a better application of trehalose as a chemical chaperone.