The effect of Hofmeister anions on water structure at protein surfaces

Particle formation during peristaltic pumping of therapeutic proteins: Hofmeister anions effect

This study reveals specific ion effects on particle formation during peristaltic pumping of a monoclonal Antibody (Antibody A). For this purpose, three anions in the direct Hofmeister series were selected, ranging from the kosmotropic SO42- to the more neutral Cl- and the chaotropic SCN-. Protein particle formation during peristaltic pumping is described primarily as a surface-driven mechanism. Therefore, the effect of the anions was hypothesised to affect the particle formation with the smallest amount of protein adsorbing and the least particles formed in the presence of SCN-, followed by the highest in SO42-. The alternative hypothesis was that most protein particles would be formed in SCN- due to the lower intrinsic stability of Antibody A. On the other hand, if none of the factors dominates the particle formation, it would not necessarily follow the Hofmeister series linearly. This was shown to be the case as significantly more particles were formed in the presence of NaCl, which could be explained by the interplay of the protein's intrinsic, colloidal, and interfacial stability. Antibody A had the highest protein adsorption in NaCl and the lowest colloidal stability compared to Na2SO4 or NaSCN, which led to the highest amount of subvisual particles formed during pumping.

Ion-specific ice provides a facile approach for reducing ice friction

HYPOTHESIS

Ice friction plays a crucial role in both basic study and practical use. Various strategies for controlling ice friction have been developed. However, one unsolved puzzle regarding ice friction is the effect of ion-ice interplay on its tribological properties.

EXPERIMENTS AND SIMULATIONS

Here, we conducted ice friction experiments and summarized the specific effects of hydrated ions on ice friction. By selecting cations and anions, the coefficient of ice friction can be reduced by more than 70 percent. Experimental spectra, low-field nuclear magnetic resonance (LF-NMR), density functional theory (DFT) calculations, and Molecular dynamics (MD) simulations demonstrated that the addition of ions could break the H-bonds in water.

FINDINGS

The link between the charge density of ions and the coefficients of ice friction was revealed. A part of the ice structure was changed from an ice-like to a liquid-like interfacial water structure with the addition of ions. Lower charge density ions led to weaker ionic forces with the water molecules in the immobilized water layer, resulting in free water molecules increasing in the lubricating layer. This study provides guidance for preparing ice-making solutions with low friction coefficients and a fuller understanding of the interfacial water structure at low temperatures.

Probing the adsorption configuration of methanol at a charged air/aqueous interface using nonlinear spectroscopy

Investigating the effects of electrolyte ions on the adsorption configuration of methanol at a charged interface is important for studying the interface structure of electrolyte solutions and the oxidation mechanism of methanol in fuel cells. This study uses sum frequency generation (SFG) and heterodyne-detected second harmonic generation (HD-SHG) to investigate the adsorption configuration of methanol at the air/aqueous interface of 0.1 M NaClO4 solution, 0.1 M HClO4 solution and pure water. The results elucidate that the ion effect in the electrolyte solution affects the interface's charged state and the methanol's adsorption conformation at the interface. The negatively charged surface of the 0.1 M NaClO4 solution and the positively charged surface of the 0.1 M HClO4 solution arise from the corresponding specific ionic effects of the electrolyte solution. The orientation angle of methyl with respect to the surface normal is 43.4° ± 0.1° at the 0.1 M NaClO4 solution surface and 21.5° ± 0.2° at the 0.1 M HClO4 solution surface. Examining these adsorption configurations in detail, we find that at the negatively charged surface the inclined orientation angle (43.4°) of methanol favors the hydroxymethyl production by breaking the C-H bond, while at the positively charged surface the upright orientation angle (21.5°) of methanol promotes the methoxy formation by breaking the O-H bond. These findings not only illuminate the intricate ion effects on small organic molecules but also contribute to a molecular-level comprehension of the oxidation mechanism of methanol at electrode interfaces.

Life in Multi-Extreme Environments: Brines, Osmotic and Hydrostatic Pressure─A Physicochemical View

Elucidating the details of the formation, stability, interactions, and reactivity of biomolecular systems under extreme environmental conditions, including high salt concentrations in brines and high osmotic and high hydrostatic pressures, is of fundamental biological, astrobiological, and biotechnological importance. Bacteria and archaea are able to survive in the deep ocean or subsurface of Earth, where pressures of up to 1 kbar are reached. The deep subsurface of Mars may host high concentrations of ions in brines, such as perchlorates, but we know little about how these conditions and the resulting osmotic stress conditions would affect the habitability of such environments for cellular life. We discuss the combined effects of osmotic (salts, organic cosolvents) and hydrostatic pressures on the structure, stability, and reactivity of biomolecular systems, including membranes, proteins, and nucleic acids. To this end, a variety of biophysical techniques have been applied, including calorimetry, UV/vis, FTIR and fluorescence spectroscopy, and neutron and X-ray scattering, in conjunction with high pressure techniques. Knowledge of these effects is essential to our understanding of life exposed to such harsh conditions, and of the physical limits of life in general. Finally, we discuss strategies that not only help us understand the adaptive mechanisms of organisms that thrive in such harsh geological settings but could also have important ramifications in biotechnological and pharmaceutical applications.

Total free energy analysis of fully hydrated proteins

The total free energy of a hydrated biomolecule and its corresponding decomposition of energy and entropy provides detailed information about regions of thermodynamic stability or instability. The free energies of four hydrated globular proteins with different net charges are calculated from a molecular dynamics simulation, with the energy coming from the system Hamiltonian and entropy using multiscale cell correlation. Water is found to be most stable around anionic residues, intermediate around cationic and polar residues, and least stable near hydrophobic residues, especially when more buried, with stability displaying moderate entropy-enthalpy compensation. Conversely, anionic residues in the proteins are energetically destabilized relative to singly solvated amino acids, while trends for other residues are less clear-cut. Almost all residues lose intraresidue entropy when in the protein, enthalpy changes are negative on average but may be positive or negative, and the resulting overall stability is moderate for some proteins and negligible for others. The free energy of water around single amino acids is found to closely match existing hydrophobicity scales. Regarding the effect of secondary structure, water is slightly more stable around loops, of intermediate stability around β strands and turns, and least stable around helices. An interesting asymmetry observed is that cationic residues stabilize a residue when bonded to its N-terminal side but destabilize it when on the C-terminal side, with a weaker reversed trend for anionic residues.

Water does not dance as ions sing: A new approach in elucidation of ion-invariant water fluctuations

Aqueous solutions of salts composed from monovalent ions are explored using temperature-dependent FT-IR spectroscopy in transmission. Water combination band, being extremely sensitive to the network of hydrogen bonds due to the contribution of water librations (ρ_LH₂O), is analyzed in uni- and multivariate fashion. Univariate analysis of the combination band maximum (ν_max) reveals that perturbation of water hydrogen bond network by ions is primary driven by electrostatic interactions between water and ions. Using multivariate curve resolution with alternating least squares and evolving factor analysis this band is separated into two components that represent low- and high-density water. The observed asymmetry in their behavior is interpreted in terms of fluctuations of a hydrogen bond network of two water components. The significance of the found phenomenon is unambiguously confirmed by performing analogous analysis in the spectral range that contains partial signature of water linear bending (δHOH) and is free from ρ_LH₂O, in which the asymmetry is absent. Additionally, we show that this phenomenon, namely ion-invariant behavior of water fluctuations, persists even in the regime of high ionic strengths. Although ions indeed participate in shaping of water hydrogen bond network, this straightforward approach shows that its temperature-dependent fluctuations are ion-independent.

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.

Oligomers, fibrils and aggregates formed by alpha-synuclein: role of solution conditions

The classical Hofmeister series orders ions into kosmotropes and chaotropes, based on their interaction with the solvent, water. The role of protein is mostly ignored probably because most of the proteins studied are natively folded and broadly follow this classification pattern. Recent reports suggest that the interaction of ions is different with solvent molecules of proximal layer and bulk. Intrinsically disordered proteins (IDPs) differ from globular proteins in the fraction of polar vis-à-vis hydrophobic amino acids and the absence of distinct secondary and tertiary structures. The kosmotrope, ammonium sulphate, increases the compactness of the polypeptide conformation, with differing effects for globular proteins and IDPs. For globular proteins, lowered flexibility corresponds to a more stable native structure. Using oligomer-specific and aggregation-specific antibodies and comparing with fibrillation results, we show for alpha-synuclein, an IDP, ammonium sulphate-induced compaction results in the formation of the aggregation-prone hydrophobic core, which combines with other similar moieties to form the fibrillar 'seed'. SEC-HPLC and SAXS analysis show the presence of the threshold oligomers. In the presence of the aggregation suppressor, arginine too, an oligomer is formed. This oligomer, however, is 'dead', and does not move further along the aggregation pathway. Thus, alpha-synuclein undergoes compaction in the presence of protein stabilisers, with differing consequences. In case of the chaotropes, KSCN and urea, aggregation of alpha-synuclein is partially inhibited. However, the amounts and types of aggregates formed are different in the two cases. Thus, the classical catalogue of molecules into protein stabilisers and destabilisers requires a relook for IDPs.Communicated by Ramaswamy H. Sarma.

High pressures increase α-chymotrypsin enzyme activity under perchlorate stress

Deep subsurface environments can harbour high concentrations of dissolved ions, yet we know little about how this shapes the conditions for life. We know even less about how the combined effects of high pressure influence the way in which ions constrain the possibilities for life. One such ion is perchlorate, which is found in extreme environments on Earth and pervasively on Mars. We investigated the interactions of high pressure and high perchlorate concentrations on enzymatic activity. We demonstrate that high pressures increase α-chymotrypsin enzyme activity even in the presence of high perchlorate concentrations. Perchlorate salts were shown to shift the folded α-chymotrypsin phase space to lower temperatures and pressures. The results presented here may suggest that high pressures increase the habitability of environments under perchlorate stress. Therefore, deep subsurface environments that combine these stressors, potentially including the subsurface of Mars, may be more habitable than previously thought.

Gault et al. show that high pressures increase α-chymotrypsin enzyme activity in the presence of high perchlorate concentrations. These perchlorate salts shift the folded enzyme phase space to lower temperatures and pressure and may move the optimum enzyme activity towards lower temperatures in addition to higher pressures, which has implications for Martian habitability.

Trehalose Effect on the Aggregation of Model Proteins into Amyloid Fibrils

Protein aggregation into amyloid fibrils is a phenomenon that attracts attention from a wide and composite part of the scientific community. Indeed, the presence of mature fibrils is associated with several neurodegenerative diseases, and in addition these supramolecular aggregates are considered promising self-assembling nanomaterials. In this framework, investigation on the effect of cosolutes on protein propensity to aggregate into fibrils is receiving growing interest, and new insights on this aspect might represent valuable steps towards comprehension of highly complex biological processes. In this work we studied the influence exerted by the osmolyte trehalose on fibrillation of two model proteins, that is, lysozyme and insulin, investigated during concomitant variation of the solution ionic strength due to NaCl. In order to monitor both secondary structures and the overall tridimensional conformations, we have performed UV spectroscopy measurements with Congo Red, Circular Dichroism, and synchrotron Small Angle X-ray Scattering. For both proteins we describe the effect of trehalose in changing the fibrillation pattern and, as main result, we observe that ionic strength in solution is a key factor in determining trehalose efficiency in slowing down or blocking protein fibrillation. Ionic strength reveals to be a competitive element with respect to trehalose, being able to counteract its inhibiting effects toward amyloidogenesis. Reported data highlight the importance of combining studies carried out on cosolutes with valuation of other physiological parameters that may affect the aggregation process. Also, the obtained experimental results allow to hypothesize a plausible mechanism adopted by the osmolyte to preserve protein surface and prevent protein fibrillation.

Hofmeister Series: Insights of Ion Specificity from Amphiphilic Assembly and Interface Property

Hofmeister series (HS), ion specific effect, or lyotropic sequence acts as a pivotal part in a number of biological and physicochemical phenomena, e.g., changing the solubility of hydrophobic solutes, the cloud points of polymers and nonionic surfactants, the activities of various enzymes, the action of ions on an ion-channel, and the surface tension of electrolyte solutions, etc. This review focused on how ion specificity influences the critical micelle concentration (CMC) and how the thermoresponsive behavior of surfactants, and the dynamic transition of the aggregate, controls the aggregate transition and gel formation and tunes the properties of air/water interfaces (Langmuir monolayer and interfacial free energy). Recent progress of the ion specific effect in bulk phase and at interfaces in amphiphilic systems and gels is summarized. Applications and a molecular level theoretical explanation of HS are discussed comprehensively. This review is aimed to supply a fresh and comprehensive understanding of Hofmiester phenomena in surfactants, polymers, colloids, and interface science and to provide a guideline to design the microstructures and templates for preparation of nanomaterials.

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.

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.

Fluorescence enhancement of a ligand-activated fluorescent protein induced by collective noncovalent interactions

Fluorescent proteins contain an internal chromophore constituted of amino acids or an external chromophore covalently bonded to the protein. To increase their fluorescence intensities, many research groups have attempted to mutate amino acids within or near the chromophore. Recently, a new type of fluorescent protein, called UnaG, in which the ligand binds to the protein through many noncovalent interactions was discovered. Later, a series of mutants of the UnaG protein were introduced, which include eUnaG with valine 2 mutated to leucine emitting significantly stronger fluorescence than the wild type and V2T mutant, in which valine 2 is mutated to threonine, emitting weaker fluorescence than the wild type. Interestingly, the single mutation sites of both eUnaG and V2T mutants are distant from the fluorophore, bilirubin, which renders the mechanism of such fluorescence enhancement or reduction unclear. To elucidate the origin of fluorescence intensity changes induced by the single mutations, we carried out extensive analyses on MD simulations for the original UnaG, eUnaG and V2T, and found that the bilirubin ligand bound to eUnaG is conformationally more rigid than the wild-type, particularly in the skeletal dihedral angles, possibly resulting in the increase of quantum yield through a reduction of non-radiative decay. On the other hand, the bilirubin bound to the V2T appears to be flexible than that in the UnaG. Furthermore, examining the structural correlations between the ligand and proteins, we found evidence that the bilirubin ligand is encapsulated in different environments composed of protein residues and water molecules that increase or decrease the stability of the ligand. The changed protein stability affects the mobility and confinement of water molecules captured between bilirubin and the protein. Since the flexible ligand contains multiple hydrogen bond (H-bond) donors and acceptors, the H-bonding structure and dynamics of bound water molecules are highly correlated with the rigidity of the bound ligand. Our results suggest that, to understand the fluorescence properties of protein mutants, especially the ones with noncovalently bound fluorophores with internal rotations, the interaction network among protein residues, ligand, and water molecules within the binding cavity should be investigated rather than focusing on the local structure near the fluorescing moiety. Our in-depth simulation study may offer a foundation for the design principles for engineering this new class of fluorescent proteins.

Specific Salt Effect on the Interaction between Rare Earth Ions and Trioctylphosphine Oxide Molecules at the Organic-Aqueous Two-Phase Interface: Experiments and Molecular Dynamics Simulations

Understanding the essence of the specific salt effect on the enhancement of the transport of metal ions across the liquid/liquid interface during the process of solvent extraction is of crucial importance for the development of a new approach to extract and selectively separate various valuable metals from complex aqueous solutions. However, some abnormal experimental phenomena involved in the salt effect on the liquid/liquid solvent extraction could not be understood only from the conventional interpretation based on the salting-out ability of salt ions. The knowledge into the microscopic mechanism behind the specific salt effect was urgent. Herein, as an example, the effect of adding various salts on the extraction performance of rare earth ion Pr³⁺ using trioctylphosphine oxide (TOPO) as the organic extractant was investigated. It was revealed that the difference in the interface propensity of different salt anions enriched at the organic-aqueous two-phase interface played a crucial role in promoting the interaction of TOPO molecules with Pr³⁺ ions, despite the occurrence of the salting-out effect of those salt anions with the increase of their concentrations in aqueous solutions. The interfacial interaction mechanism obtained by molecular dynamics simulations revealed that both the interface propensity and the salting-out ability of the coexisting salt anions contributed to the enhancement in the extraction of Pr³⁺ into the TOPO organic phase. However, when the concentrations of coexisting salts in aqueous solutions were low enough, the extraction of Pr³⁺ was mainly dominated by the interface propensity of those added salt anions but not their salting-out ability. With the increase in the concentration of salts, the salting-out effect gradually became significant and, therefore, began to join with the interface propensity of salt anions to co-dominate the  transport of Pr³⁺ ions across the liquid/liquid interface. The present study highlights the microscopic nature of the salt effect on promoting the extraction of rare earth ions and suggests that the interaction of organic extractant molecules with rare earth ions at liquid/liquid interface was dependent not only upon the salting-out ability of the coexisting salt ions but also their interface propensity.