Interfacial Water Structure Controls Protein Conformation

Specific anion effects on urease activity: A Hofmeister study

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

Effect of glycolysis on water holding capacity during postmortem aging of Jersey cattle-yak meat

BACKGROUND

Postmortem muscle moisture loss leads to a decrease in carcass weight and can adversely impact overall meat quality. Therefore, it is critical to investigate water holding capacity (WHC) to enhance meat quality. Current research has primarily focused on examining the correlation between signaling molecules and meat quality in relation to the glycolysis effect on muscle WHC. But there exists a significant knowledge gap regarding the mechanism of WHC in Jersey cattle-yak meat.

RESULTS

Jersey cattle-yak meat pH decreased and then increased during postmortem aging. Lactate content, cooking loss, pressing loss, drip loss and centrifuging loss of Jersey cattle-yak meat increased and then decreased during postmortem aging. The glycogen content of Jersey cattle-yak meat was significantly higher than that of yak meat at 6-120 h, being 8.40% higher than that of yak meat at 120 h. The activity of key glycolytic enzymes hexokinase (HK), pyruvate kinase (PK), phosphofructokinase (PFK) and lactate dehydrogenase (LDH) in Jersey cattle-yak meat was lower than that in yak meat. Correlation analysis showed that Jersey cattle-yak meat WHC was positively correlated with the activity of HK, PK, PFK and LDH.

CONCLUSIONS

The WHC of Jersey cattle-yak meat was higher than that of Gannan yak meat, and it was significantly positively correlated with the activity of key enzymes of the glycolytic signaling pathway. Therefore, the glycolysis rate can be reduced by inhibiting enzyme activity to improve Jersey cattle-yak meat WHC and meat quality. © 2023 Society of Chemical Industry.

Unified picture of vibrational relaxation of OH stretch at the air/water interface

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.

Application of Formulation Principles to Stability Issues Encountered During Processing, Manufacturing, and Storage of Drug Substance and Drug Product Protein Therapeutics

The field of formulation and stabilization of protein therapeutics has become rather extensive. However, most of the focus has been on stabilization of the final drug product. Yet, proteins experience stress and degradation through the manufacturing process, starting with fermentaition. This review describes how formulation principles can be applied to stabilize biopharmaceutical proteins during bioprocessing and manufacturing, considering each unit operation involved in prepration of the drug substance. In addition, the impact of the container on stabilty is discussed as well.

Photoactive Yellow Protein Adsorption at Hydrated Polyethyleneimine and Poly-l-Glutamic Acid Interfaces

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was performed in the 1400-1700 and 2800-3800 cm^-1 range to study the interfacial structure of photoactive yellow protein (PYP) adsorbed on polyethyleneimine (PEI) and poly-l-glutamic acid (PGA) surfaces. Nanometer-thick polyelectrolyte layers served as the substrate for PYP adsorption, with 6.5-pair layers providing the most homogeneous surfaces. When the topmost material was PGA, it acquired a random coil structure with a small number of β₂-fibrils. Upon adsorption on oppositely charged surfaces, PYP yielded similar achiral spectra. However, the VSFG signal intensity increased for PGA surfaces with a concomitant redshift of the chiral C^α-H and N-H stretching bands, suggesting increased adsorption for PGA compared to PEI. At low wavenumbers, both the backbone and the side chains of PYP induced drastic changes to all measured chiral and achiral VSFG spectra. Decreasing ambient humidity led to the loss of tertiary structure with a re-orientation of α-helixes, evidenced by a strongly blue-shifted chiral amide I band of the β-sheet structure with a shoulder at 1654 cm^-1. Our observations indicate that chiral VSFG spectroscopy is not only capable of determining the main type of secondary structure of PYP, i.e., β-scaffold, but is also sensitive to tertiary protein structure.

From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush

HYPOTHESIS

Specific ion effects govern myriad biological phenomena, including protein-ligand interactions and enzyme activity. Despite recent advances, detailed understanding of the role of ion hydrophobicity in specific ion effects, and the intersection with hydrotropic effects, remains elusive. Short chain fatty acid sodium salts are simple amphiphiles which play an integral role in our gastrointestinal health. We hypothesise that increasing a fatty acid's hydrophobicity will manifest stronger salting-out behaviour.

EXPERIMENTS

Here we study the effect of these amphiphiles on an exemplar thermoresponsive polymer brush system, conserving the carboxylate anion identity while varying anion hydrophobicity via the carbon chain length. Ellipsometry and quartz crystal microbalance with dissipation monitoring were used to characterise the thermoresponse and viscoelasticity of the brush, respectively, whilst neutron reflectometry was used to reveal the internal structure of the brush. Diffusion-ordered nuclear magnetic resonance spectroscopy and computational investigations provide insight into polymer-ion interactions.

FINDINGS

Surface sensitive techniques unveiled a non-monotonic trend in salting-out ability with increasing anion hydrophobicity, revealing the bundle-like morphology of the ion-collapsed system. An intersection between ion-specific and hydrotropic effects was observed both experimentally and computationally; trending from good anti-hydrotrope towards hydrotropic behaviour with increasing anion hydrophobicity, accompanying a change in hydrophobic hydration.

A research note: Effect of Hofmeister salts on meat iridescence in cooked pork

The effect of Hofmeister salts (NaCl, NaSCN, Na₂SO₄, KCl, LiCl, CaCl₂) on surface iridescence in cooked pork was investigated. Strongest iridescence occurred in samples treated with NaSCN, NaCl and KCl. Control samples and LiCl, CaCl₂ and Na₂SO₄ treatments showed weaker iridescence. However, differences between KCl and LiCl, CaCl2 and Na₂SO₄ were not statistically significant (p > 0.05). Nevertheless, a tendency of chaotropic salts (NaSCN, NaCl, KCl) to cause stronger iridescence was noted that might be explained with a more effective solubilization of myofibrillar proteins (MPs), reducing incoherent scattering from the myofibrils and thus enhancing multilayer interference.

Understanding specific ion effects and the Hofmeister series

Specific ion effects (SIE), encompassing the Hofmeister Series, have been known for more than 130 years since Hofmeister and Lewith's foundational work. SIEs are ubiquitous and are observed across the medical, biological, chemical and industrial sciences. Nevertheless, no general predictive theory has yet been able to explain ion specificity across these fields; it remains impossible to predict when, how, and to what magnitude, a SIE will be observed. In part, this is due to the complexity of real systems in which ions, counterions, solvents and cosolutes all play varying roles, which give rise to anomalies and reversals in anticipated SIEs. Herein we review the historical explanations for SIE in water and the key ion properties that have been attributed to them. Systems where the Hofmeister series is perturbed or reversed are explored, as is the behaviour of ions at the liquid-vapour interface. We discuss SIEs in mixed electrolytes, nonaqueous solvents, and in highly concentrated electrolyte solutions - exciting frontiers in this field with particular relevance to biological and electrochemical applications. We conclude the perspective by summarising the challenges and opportunities facing this SIE research that highlight potential pathways towards a general predictive theory of SIE.

Ion-Mediated Morphological Diversity in Protein Amyloid Systems

Salt ions are considered among the major determinants ruling protein folding, stability, and self-assembly in the context of amyloid-related diseases, protein drug development, and functional biomaterials. Here, we report that Hofmeister ions not only determine the rate constants of the aggregation reaction for human insulin and hen egg white lysozyme but also control the generation of a plethora of amyloid-like morphologies ranging from the nanoscale to the microscale. We anticipate that the latter is a result of a balance between colloidal and conformational stability combined with an ion-specific effect and highlight the importance of salt ions in controlling the biological functions of protein aggregates.

Luminol–hydrogen peroxide–horseradish peroxidase chemiluminescence intensification by kosmotrope ammonium sulfate

The kosmotropic effect induced by ammonium sulfate (AS) at concentrations greater than approximately 2.8 M allows the marked intensification of chemiluminescence (CL) arising from a conventional luminol–hydrogen peroxide (H₂O₂)–horseradish peroxidase (HRP) reaction. Because of the kosmotropic effect, CL is intensified by at least three orders of magnitude than that from the conventional HRP-catalyzed luminol reaction with no AS; the linear relationship between the CL intensity and the HRP concentration is established over the range of 0.3 pM to several tens of pM. The novel CL intensification effect on the HRP-catalyzed luminol CL can be stably and reproducibly induced.

Influence of Divalent Metal Ions on the Precipitation of the Plasma Protein Fibrinogen

Fibrinogen nanofibers are very attractive biomaterials to mimic the native blood clot architecture. Previously, we reported the self-assembly of fibrinogen nanofibers in the presence of monovalent salts and have now studied how divalent salts influence fibrinogen precipitation. Although the secondary fibrinogen structure was significantly altered with divalent metal ions, morphological analysis revealed exclusively smooth fibrinogen precipitates. In situ monitoring of the surface roughness facilitated predicting the tendency of various salts to form fibrinogen fibers or smooth films. Analysis of the chemical composition revealed that divalent salts were removed from smooth fibrinogen films upon rinsing while monovalent Na⁺ species were still present in fibrinogen fibers. Therefore, we assume that the decisive factor controlling the morphology of fibrinogen precipitates is direct ion-protein contact, which requires disruption of the ion-surrounding hydration shells. We conclude that in fibrinogen aggregates, this mechanism is effective only for monovalent ions, whereas divalent ions are limited to indirect fibrinogen adsorption.

Anion-Specific Effects on the Alkaline State of Cytochrome c

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

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

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

Glycocalyx regulates the strength and kinetics of cancer cell adhesion revealed by biophysical models based on high resolution label-free optical data

The glycocalyx is thought to perform a potent, but not yet defined function in cellular adhesion and signaling. Since 95% of cancer cells have altered glycocalyx structure, this role can be especially important in cancer development and metastasis. The glycocalyx layer of cancer cells directly influences cancer progression, involving the complicated kinetic process of cellular adhesion at various levels. In the present work, we investigated the effect of enzymatic digestion of specific glycocalyx components on cancer cell adhesion to RGD (arginine-glycine-aspartic acid) peptide motif displaying surfaces. High resolution kinetic data of cell adhesion was recorded by the surface sensitive label-free resonant waveguide grating (RWG) biosensor, supported by fluorescent staining of the cells and cell surface charge measurements. We found that intense removal of chondroitin sulfate (CS) and dermatan sulfate chains by chondroitinase ABC reduced the speed and decreased the strength of adhesion of HeLa cells. In contrast, mild digestion of glycocalyx resulted in faster and stronger adhesion. Control experiments on a healthy and another cancer cell line were also conducted, and the discrepancies were analysed. We developed a biophysical model which was fitted to the kinetic data of HeLa cells. Our analysis suggests that the rate of integrin receptor transport to the adhesion zone and integrin-RGD binding is strongly influenced by the presence of glycocalyx components, but the integrin-RGD dissociation is not. Moreover, based on the kinetic data we calculated the dependence of the dissociation constant of integrin-RGD binding on the enzyme concentration. We also determined the dissociation constant using a 2D receptor binding model based on saturation level static data recorded at surfaces with tuned RGD densities. We analyzed the discrepancies of the kinetic and static dissociation constants, further illuminating the role of cancer cell glycocalyx during the adhesion process. Altogether, our experimental results and modelling demonstrated that the chondroitin sulfate and dermatan sulfate chains of glycocalyx have an important regulatory function during the cellular adhesion process, mainly controlling the kinetics of integrin transport and integrin assembly into mature adhesion sites. Our results potentially open the way for novel type of cancer treatments affecting these regulatory mechanisms of cellular glycocalyx.

A Computational Model of Protein Induced Membrane Morphology with Geodesic Curvature Driven Protein-Membrane Interface

Continuum or hybrid modeling of bilayer membrane morphological dynamics induced by embedded proteins necessitates the identification of protein-membrane interfaces and coupling of deformations of two surfaces. In this article we developed (i) a minimal total geodesic curvature model to describe these interfaces, and (ii) a numerical one-one mapping between two surface through a conformal mapping of each surface to the common middle annulus. Our work provides the first computational tractable approach for determining the interfaces between bilayer and embedded proteins. The one-one mapping allows a convenient coupling of the morphology of two surfaces. We integrated these two new developments into the energetic model of protein-membrane interactions, and developed the full set of numerical methods for the coupled system. Numerical examples are presented to demonstrate (1) the efficiency and robustness of our methods in locating the curves with minimal total geodesic curvature on highly complicated protein surfaces, (2) the usefulness of these interfaces as interior boundaries for membrane deformation, and (3) the rich morphology of bilayer surfaces for different protein-membrane interfaces.

Competitive specific ion effects in mixed salt solutions on a thermoresponsive polymer brush

HYPOTHESIS

Grafted poly(ethylene glycol) methyl ether methacrylate (POEGMA) copolymer brushes change conformation in response to temperature ('thermoresponse'). In the presence of different ions the thermoresponse of these coatings is dramatically altered. These effects are complex and poorly understood with no all-inclusive predictive theory of specific ion effects. As natural environments are composed of mixed electrolytes, it is imperative we understand the interplay of different ions for future applications. We hypothesise anion mixtures from the same end of the Hofmeister series (same-type anions) will exhibit non-additive and competitive behaviour.

EXPERIMENTS

The behaviour of POEGMA brushes, synthesised via surface-initiated ARGET-ATRP, in both single and mixed aqueous electrolyte solutions was characterised with ellipsometry and neutron reflectometry as a function of temperature.

FINDINGS

In mixed fluoride and chloride aqueous electrolytes (salting-out ions), or mixed thiocyanate and iodide aqueous electrolytes (salting-in ions), a non-monotonic concentration-dependent influence of the two anions on the thermoresponse of the brush was observed. A new term, δ, has been defined to quantitively describe synergistic or antagonistic behaviour. This study determined the specific ion effects imparted by salting-out ions are dependent on available solvent molecules, whereas the influence of salting-in ions is dependent on the interactions of the anions and polymer chains.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

FTIR Spectroscopy Study of the Secondary Structure Changes in Human Serum Albumin and Trypsin under Neutral Salts

The effect of neutral salts on protein conformation was first analyzed by Hofmeister in 1888, however, even today this phenomenon is not completely understood. To clarify this effect, we studied changes in the secondary structure of two proteins: human serum albumin with predominantly α-helical structure and porcine pancreas β-trypsin with the typical β-structural arrangement in aqueous solutions of neutral salts (KSCN, KCl, (NH₄)₂SO₄). The changes in the secondary structure were studied at 23 °C and 80 °C by using the second derivative deconvolution method of the IR spectra. Our results demonstrated that the ability of the salts to stabilize/destabilize these two proteins correlates with the Hofmeister series of ions. At the same time, some exceptions were also observed. The destabilization of the native structures of both α-helical albumin and β-structural trypsin upon interaction with neutral salts leads to the formation of intermolecular β-sheets typical for amyloid fibrils or amorphous aggregates. Thus, our quantitative FTIR-spectroscopy analysis allowed us to further clarify the mechanisms and complexity of the neutral salt actions on protein structures which may lead to strategies preventing unwelcome misfolding of proteins.

Role of Protein-Water Interface in the Stacking Interactions of Granum Thylakoid Membranes-As Revealed by the Effects of Hofmeister Salts

The thylakoid membranes of vascular plants are differentiated into stacked granum and unstacked stroma regions. The formation of grana is triggered by the macrodomain formation of photosystem II and light-harvesting complex II (PSII-LHCII) and thus their lateral segregation from the photosystem I-light-harvesting complex I (PSI-LHCI) super-complexes and the ATP-synthase; which is then stabilized by stacking interactions of the adjacent PSII-LHCII enriched regions of the thylakoid membranes. The self-assembly and dynamics of this highly organized membrane system and the nature of forces acting between the PSII-LHCII macrodomains are not well understood. By using circular dichroism (CD) spectroscopy, small-angle neutron scattering (SANS) and transmission electron microscopy (TEM), we investigated the effects of Hofmeister salts on the organization of pigment-protein complexes and on the ultrastructure of thylakoid membranes. We found that the kosmotropic agent (NH₄)₂SO₄ and the Hofmeister-neutral NaCl, up to 2 M concentrations, hardly affected the macro-organization of the protein complexes and the membrane ultrastructure. In contrast, chaotropic salts, NaClO₄, and NaSCN destroyed the mesoscopic structures, the multilamellar organization of the thylakoid membranes and the chiral macrodomains of the protein complexes but without noticeably affecting the short-range, pigment-pigment excitonic interactions. Comparison of the concentration- and time-dependences of SANS, TEM and CD parameters revealed the main steps of the disassembly of grana in the presence of chaotropes. It begins with a rapid diminishment of the long-range periodic order of the grana membranes, apparently due to an increased stacking disorder of the thylakoid membranes, as reflected by SANS experiments. SANS measurements also allowed discrimination between the cationic and anionic effects-in stacking and disorder, respectively. This step is followed by a somewhat slower disorganization of the TEM ultrastructure, due to the gradual loss of stacked membrane pairs. Occurring last is the stepwise decrease and disappearance of the long-range chiral order of the protein complexes, the rate of which was faster in LHCII-deficient membranes. These data are interpreted in terms of a theory, from our laboratory, according to which Hofmeister salts primarily affect the hydrophylic-hydrophobic interactions of proteins, and the stroma-exposed regions of the intrinsic membrane proteins, in particular-pointing to the role of protein-water interface in the stacking interactions of granum thylakoid membranes.

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.

Role of Mono- and Divalent Surface Cations on the Structure and Adsorption Behavior of Water on Mica Surface

Understanding solid-water(vapor) interfacial systems is relevant for both industrial and academic scenarios for their presence in wide areas ranging from tribology to geochemistry. Using grand canonical Monte Carlo simulations, we have investigated the role of monovalent (lithium, Li⁺; sodium, Na⁺; and potassium, K⁺) and divalent (magnesium, Mg²⁺; calcium, Ca²⁺) cations on the structure and adsorption behavior of water on mica surface. The water density adjacent to the surface exhibits (a) oscillations due to hydration of surface cations (interfacial layer), (b) followed by a thick liquidlike layer. The thickness of the interfacial layer is strongly dependent on the hydration shell size and hydration energy of surface ions. Water molecules immediately next to the surface (contact layers) adsorb on ditrigonal (hexagonal) cavities of mica surface and form an ordered structure. The Li⁺, Na⁺, Mg²⁺, and Ca²⁺ surface ions are coadsorbed with water molecules on the ditrigonal cavities due to their smaller hydration shell. Majority of water molecules in the second contact layer hydrate the surface ions and, together with the rest of the water molecules, form hydrogen bonds among themselves. The structure of the water molecules in the third and subsequent layer is random and more bulk liquidlike, except those molecules that hydrate the surface ions. The adsorption isotherm of water on all ion-exposed mica surface exhibits three regimes: (a) an initial rapid increase in water loading for relative vapor pressure ( p/ p₀) ≤0.2 due to hydration of surface ions; (b) followed by a linear increase between p/ p₀ = 0.2 and 0.7, where the hydrogen bond formation between the water molecules of the interfacial layer occurs; and (c) exponential growth beyond p/ p₀ = 0.7 due to thickening of the liquidlike layer. The water loading on divalent-ion-exposed mica surface is higher compared to the monovalent ions case. Although the divalent ions have higher hydration energy, the fraction of water molecules hydrating the surface ions is less compared to nonhydrating water molecules. We found that ion hydration energy and size of hydration shell play a crucial role in their structure adjacent to mica surface. At lower water loadings, the surface ions form a hydration shell with surface bridging oxygens, whereas at higher water content, the hydration preference is shifted toward mobile water molecules. The detailed understanding obtained from this work will be useful in identifying the role of ions in cloud formation, nanotribological studies, and activities of biological molecules and catalysts.

Effect of ionic strength and salt identity on poly(N-isopropylacrylamide) brush modified colloidal silica particles

HYPOTHESIS

The thermoresponse of poly(N-isopropylacrylamide) stabilised particles is influenced by the presence of salt and is dependent on the concentration, and ions present. The conformation and electrophoretic mobility of core/shell PNIPAM brush modified silica particles is expected to vary as a function of these specific ion effects.

EXPERIMENTS

The thermoresponse of PNIPAM brush modified silica particles was investigated via dynamic light scattering and electrophoretic mobility measurements between 5 and 45 °C in the presence of 11 different salt solutions.

FINDINGS

Specific ion effects were observed in the presence of salt solutions for concentrations between 50 and 1000 mM. The temperature response could be mapped to a master curve unlike PNIPAM brush behaviour on planar substrates. The magnitude of brush layer lower critical solution temperature reduction was found to follow the order F^- > CH₃CO₂^- > Cl^- > NO₃^- ∼ Br^- > I^- > SCN^- for the potassium series and Na⁺ > K⁺ > Cs⁺ > Li⁺ ∼ NH₄⁺ for the chloride salts. The electrophoretic mobility of the modified particles in the presence of 100 mM potassium salts increased in magnitude as the brush layer collapsed and also with the chaotropic nature of the anion.

Ion-induced alterations of the local hydration environment elucidate Hofmeister effect in a simple classical model of Trp-cage miniprotein

Protein stability is known to be influenced by the presence of Hofmeister active ions in the solution. In addition to direct ion-protein interactions, this influence manifests through the local alterations of the interfacial water structure induced by the anions and cations present in this region. In our earlier works it was pointed out that the effects of Hofmeister active salts on the stability of Trp-cage miniprotein can be modeled qualitatively using non-polarizable force fields. These simulations reproduced the structure-stabilization and structure-destabilization effects of selected kosmotropic and chaotropic salts, respectively. In the present study we use the same model system to elucidate atomic processes behind the chaotropic destabilization and kosmotropic stabilization of the miniprotein. We focus on changes of the local hydration environment of the miniprotein upon addition of NaClO₄ and NaF salts to the solution. The process is separated into two parts. In the first, 'promotion' phase, the protein structure is fixed, and the local hydration properties induced by the simultaneous presence of protein and ions are investigated, with a special focus on the interaction of Hofmeister active anions with the charged and polar sites. In the second, 'rearrangement' phase we follow changes of the hydration of ions and the protein, accompanying the conformational relaxation of the protein. We identify significant factors of an enthalpic and entropic nature behind the ion-induced free energy changes of the protein-water system, and also propose a possible atomic mechanism consistent with the Collins's rule, for the chaotropic destabilization and kosmotropic stabilization of protein conformation.

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.

Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes

The influence of specific anions on the equilibrium thermoresponse of poly(N-isopropylacrylamide) (pNIPAM) brushes has been studied using in situ ellipsometry, quartz crystal microbalance with dissipation (QCM-D) and static contact angle measurements between 20 and 45 °C in the presence of up to 250 mM acetate and thiocyanate anions in water. The thickness and changes in dissipation exhibited a broad swelling transition spanning approximately 15 °C from collapsed (high temperatures) to swollen conformation (low temperatures) while the brush surface wettability changed over approximately 2 °C. In the presence of the kosmotropic acetate anions, the measured lower critical solution temperature (LCST) by the three techniques was very similar and decreased linearly as a function of ionic strength. Conversely, increasing the concentration of the chaotropic thiocyanate anions raised the LCST of the pNIPAM brushes with variation in the measured LCST between the three techniques increasing with ionic strength. The thickness of the pNIPAM brush was seen to progressively increase with increasing thiocyanate concentration at all temperatures. It is proposed that specific ion binding of the chaotropic thiocyanate anion with pNIPAM amide moieties increases the electrostatic intra- and intermolecular repulsion within and between pNIPAM chains. This allows the brush to begin to swell at higher temperatures and to an overall greater extent.

Protein-surface interactions on stimuli-responsive polymeric biomaterials

Responsive surfaces: a review of the dependence of protein adsorption on the reversible volume phase transition in stimuli-responsive polymers. Specifically addressed are a widely studied subset: thermoresponsive polymers. Findings are also generalizable to other materials which undergo a similarly reversible volume phase transition. As of 2015, over 100,000 articles have been published on stimuli-responsive polymers and many more on protein-biomaterial interactions. Significantly, fewer than 100 of these have focused specifically on protein interactions with stimuli-responsive polymers. These report a clear trend of increased protein adsorption in the collapsed state compared to the swollen state. This control over protein interactions makes stimuli-responsive polymers highly useful in biomedical applications such as wound repair scaffolds, on-demand drug delivery, and antifouling surfaces. Outstanding questions are whether the protein adsorption is reversible with the volume phase transition and whether there is a time-dependence. A clear understanding of protein interactions with stimuli-responsive polymers will advance theoretical models, experimental results, and biomedical applications.

Hofmeister Ion-Induced Changes in Water Structure Correlate with Changes in Solvation of an Aggregated Protein Complex

RecA is a naturally aggregating Escherichia coli protein that catalyzes the strand exchange reaction utilized in DNA repair. Previous studies have shown that the presence of salts influence RecA activity, aggregation, and stability and that salts stabilize RecA in an inverse-anionic Hofmeister series. Here we utilized attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and circular dichroism (CD) to investigate how various Hofmeister salts alter the water structure and RecA solvation and aggregation. Spectroscopic studies performed in water and deuterium oxide suggest that salts alter water O-(1)H and O-(2)H stretch and bend vibrations as well as protein amide I (or I') and amide II (or II') vibrations. Anions have a much larger influence on water vibrations than cations. Water studies also show increased water-water and/or water-ion interactions in the presence of strongly hydrated SO4(2-) salts and evidence for decreased interactions with weakly hydrated Cl(-) and ClO4(-) salts. Salt-water difference infrared spectra show that kosmotropic salts are more hydrated than chaotropic salts. Interestingly, this is the opposite trend to the changes in protein solvation. Infrared spectra of RecA show that vibrations associated with protein desolvation were observed in the presence of SO4(2-) salts. Conversely, vibrations associated with protein solvation were observed in the presence of Cl(-) and ClO4(-) salts. Difference infrared studies on the dehydration of model proteins aided in identifying changes in RecA-solvent interactions. This study provides evidence that salt-induced changes in water vibrations correlate to changes in protein solvent interactions and thermal stability.

Protein Stabilization and Enzyme Activation in Ionic Liquids: Specific Ion Effects

There are still debates on whether the hydration of ions perturbs the water structure, and what is the degree of such disturbance; therefore, the origin of Hofmeister effect on protein stabilization continues being questioned. For this reason, it is suggested to use the 'specific ion effect' instead of other misleading terms such as Hofmeister effect, Hofmeister series, lyotropic effect, and lyotropic series. In this review, we firstly discuss the controversial aspect of inorganic ion effects on water structures, and several possible contributors to the specific ion effect of protein stability. Due to recent overwhelming attraction of ionic liquids (ILs) as benign solvents in many enzymatic reactions, we further evaluate the structural properties and molecular-level interactions in neat ILs and their aqueous solutions. Next, we systematically compare the specific ion effects of ILs on enzyme stability and activity, and conclude that (a) the specificity of many enzymatic systems in diluted aqueous IL solutions is roughly in line with the traditional Hofmeister series albeit some exceptions; (b) however, the specificity follows a different track in concentrated or neat ILs because other factors (such as hydrogen-bond basicity, nucelophilicity, and hydrophobicity, etc) are playing leading roles. In addition, we demonstrate some examples of biocatalytic reactions in IL systems that are guided by the empirical specificity rule.

Regulation of the growth, morphology, mechanical properties and biocompatibility of natural polysaccharide-based multilayers by Hofmeister anions

Herein the optimization of the physicochemical properties and surface biocompatibility of polyelectrolyte multilayers of the natural, biocompatible and biodegradable, linear polysaccharides hyaluronan and chitosan by Hofmeister anions was systematically investigated.

Specific anion effects in Artemia salina

The specific anion effect on the vitality of Artemia salina was investigated by measuring the Lethal Time LT50 of the crustaceans in the presence of different sodium salts solutions at room temperature and at the same ionic strength as natural seawater. Fluoride, thiocyanate and perchlorate are the most toxic agents, while chloride, bromide and sulfate are well tolerated. The rates of oxygen consumption of brine shrimps were recorded in mixed NaCl+NaF or NaCl+NaSCN solutions as a function of time. The results are discussed in terms of the Hofmeister series, and suggest that, besides the biochemical processes that involve F(-), SCN(-) and ClO4(-), the different physico-chemical properties of the strong kosmotropic and chaotropic anions may contribute in determining their strong toxicity for A. salina.

Effect of Hofmeister cosolutes on the photocycle of photoactive yellow protein at moderately alkaline pH

The photocycle of photoactive yellow protein was studied by kinetic absorption spectroscopy from below 100ns to seconds, at moderately alkaline pH, in the presence of high concentrations of various salts. Chemometric analysis combined with multiexponential fit of the flash-induced difference spectra provided evidence for five intermediates, including a spectrally silent form before the final recovery of the parent state, but only three with significantly distinct spectra. The calculated intermediate spectra constituted the input for the following spectrotemporal model fit using a sufficiently complex photocycle scheme with reversible transitions. This yielded the rate coefficients of the molecular transitions, the final spectra and the kinetics of the intermediates. Except for the transition between the two red shifted (early) intermediates (pR1 and pR2) and the final photocycle step, all reactions appeared to be reversible. Kosmotropic and chaotropic cosolutes had a systematic effect on the molecular rate coefficients. The largest effect, associated presumably with the exposure of the hydrophobic interior of the protein, accompanies the transition between the second red-shifted and the first blue-shifted intermediate (pR2 and pB1, respectively), i.e. it coincides with the chromophore protonation. The dependence of the rate coefficients on the Hofmeister cosolutes suggests that the conformational change of photoactive yellow protein leading eventually to the most unfolded signaling state takes place in several steps, and starts already with the relaxation after the chromophore isomerization in the microsecond time domain.

Hofmeister effect on the interfacial dynamics of single polymer molecules

The Hofmeister effect on interfacial dynamics has been discovered for single charged polymer molecules (sodium polystyrene sulfonate) adsorbed on a hydrophobic surface from an aqueous solution. The presence of ions in the aqueous solution affects the surface diffusivity, and its amplitudes and the surface friction follow the Hofmeister series-the kosmotropic ions slowed down the surface diffusivity and the chaotropic ions speeded it up. The amplitude of the surface friction exhibits a good correlation with the surface tension increment, indicating the interfacial feature of the Hofmeister effect.

Correlated structural kinetics and retarded solvent dynamics at the metalloprotease active site

Solvent dynamics can play a major role in enzyme activity, but obtaining an accurate, quantitative picture of solvent activity during catalysis is quite challenging. Here, we combine terahertz spectroscopy and X-ray absorption analyses to measure changes in the coupled water-protein motions during peptide hydrolysis by a zinc-dependent human metalloprotease. These changes were tightly correlated with rearrangements at the active site during the formation of productive enzyme-substrate intermediates and were different from those in an enzyme-inhibitor complex. Molecular dynamics simulations showed a steep gradient of fast-to-slow coupled protein-water motions around the protein, active site and substrate. Our results show that water retardation occurs before formation of the functional Michaelis complex. We propose that the observed gradient of coupled protein-water motions may assist enzyme-substrate interactions through water-polarizing mechanisms that are remotely mediated by the catalytic metal ion and the enzyme active site.

Effects of Hofmeister anions on the aggregation behavior of PEO-PPO-PEO triblock copolymers

The effects of a series of Hofmeister anions on the phase behaviors of a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer were investigated with an automated melting point system. Well hydrated anions and poorly hydrated anions interacted with the polymer differently and further affected the phase transition of the polymer. Poorly hydrated anions worked through changing the interfacial tension at the polymer/aqueous interface and in enhancing the polymer hydration by ion binding. The phase transition of the polymer in the presence of well hydrated anions correlated directly to the hydration entropy of the anions. As a consequence, the polymer showed a two-step phase transition in solutions containing poorly hydrated anions while displayed a single-step phase transition in the presence of well hydrated anions. The mechanisms of how ions interact with the polymer and further modulate its phase behaviors were discussed.

Salting-in characteristics of globular proteins

Protein solubility, and the formation of various solid phases, is of interest in both bioprocessing and the study of protein condensation diseases. Here we examine the the phase behavior of three proteins (chymosin B, β-lactoglobulin B, and pumpkin seed globulin) previously known to display salting-in behavior, and measure their solubility as a function of pH, ionic strength, and salt type. Although the phase behavior of the three proteins is quantitatively different, general trends emerge. Stable crystal nucleation does not occur within the salting-in region for the proteins examined, despite the crystal being observed as the most stable solid phase. Instead, two types of amorphous phases were found within the salting-in region; additionally, an analog to the instantaneous clouding curve was observed within the salting-in region for chymosin B. Also, protein solutions containing sulfate salts resulted in different crystal morphologies depending on whether Li(2)SO(4) or (NH(4))(2)SO(4) was used.