Chemistry of Hofmeister anions and osmolytes

Destabilization of Surfactant-Dispersed Carbon Nanotubes by Anions

The colloidal stability of surfactant-dispersed single-wall carbon nanotubes (SWCNTs) is determined by microscopic physicochemical processes, such as association, partitioning, and adsorption propensities. These processes can be controlled by the addition of solutes. While the effects of cations on the colloidal stability of SWCNTs are relatively well understood, little is known about the effects of anions. In this study, we examined the effects of anions on the stability of SWCNTs dispersed by sodium dodecyl sulfate (SDS) using sodium salts, such as NaCl and NaSCN. We observed that the intensity of the radial breathing mode Raman peaks rapidly decreased as the salts were added, even at concentrations less than 25 mM, indicating the association of SWCNTs. The effect was stronger with NaSCN than NaCl. We propose that the association of SWCNTs was caused by thermodynamic destabilization of SDS assemblies on SWCNT surfaces by these salts, which was confirmed through SWCNT separation experiments using aqueous two-phase extraction and gel chromatography. These results demonstrate that neutral salts can be used to control the colloidal stability of surfactant-dispersed SWCNTs.

Incorporation of ion and solvent structure into mean-field modeling of the electric double layer

An electric double layer forms when the small mobile ions of an electrolyte interact with an extended charged object, a macroion. The competition between electrostatic attraction and translational entropy loss of the small ions results in a diffuse layer of partially immobilized ions in the vicinity of the macroion. Modeling structure and energy of the electric double layer has a long history that has lead to the classical Poisson-Boltzmann theory and numerous extensions that account for ion-ion correlations and structural ion and solvent properties. The present review focuses on approaches that instead of going beyond the mean-field character of Poisson-Boltzmann theory introduce structural details of the ions and the solvent into the Poisson-Boltzmann modeling framework. The former include not only excluded volume effects but also the presence of charge distributions on individual ions, spatially extended ions, and internal ionic degrees of freedom. The latter treat the solvent either explicitly as interacting Langevin dipoles or in the form of effective non-electrostatic interactions, in particular Yukawa interactions, that are added to the Coulomb potential. We discuss how various theoretical models predict structural properties of the electric double layer such as the differential capacitance and compare some of these predictions with computer simulations.

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.

Using chirality to probe the conformational dynamics and assembly of intrinsically disordered amyloid proteins

Intrinsically disordered protein (IDP) conformers occupy large regions of conformational space and display relatively flat energy surfaces. Amyloid-forming IDPs, unlike natively folded proteins, have folding trajectories that frequently involve movements up shallow energy gradients prior to the “downhill” folding leading to fibril formation. We suggest that structural perturbations caused by chiral inversions of amino acid side-chains may be especially valuable in elucidating these pathways of IDP folding. Chiral inversions are subtle in that they do not change side-chain size, flexibility, hydropathy, charge, or polarizability. They allow focus to be placed solely on the question of how changes in amino acid side-chain orientation, and the resultant alterations in peptide backbone structure, affect a peptide’s conformational landscape (Ramachandran space). If specific inversions affect folding and assembly, then the sites involved likely are important in mediating these processes. We suggest here a “focused chiral mutant library” approach for the unbiased study of amyloid-forming IDPs.

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.

Affinity of IDPs to their targets is modulated by ion-specific changes in kinetics and residual structure

Intrinsically disordered proteins (IDPs) are characterized by a lack of defined structure. Instead, they populate ensembles of rapidly interconverting conformations with marginal structural stabilities. Changes in solution conditions such as temperature and crowding agents consequently affect IDPs more than their folded counterparts. Here we reveal that the residual structure content of IDPs is modulated both by ionic strength and by the type of ions present in solution. We show that these ion-specific structural changes result in binding affinity shifts of up to sixfold, which happen through alteration of both association and dissociation rates. These effects follow the Hofmeister series, but unlike the well-established effects on the stability of folded proteins, they already occur at low, hypotonic concentrations of salt. We attribute this sensitivity to the marginal stability of IDPs, which could have physiological implications given the role of IDPs in signaling, the asymmetric ion profiles of different cellular compartments, and the role of ions in biology.

Aqueous ionic liquids and their effects on protein structures: an overview on recent theoretical and experimental results

Ionic liquids (ILs) are used in a variety of technological and biological applications. Recent experimental and simulation results reveal the influence of aqueous ionic liquids on the stability of protein and enzyme structures. Depending on different parameters like the concentration and the ion composition, one can observe distinct stabilization or denaturation mechanisms for various ILs. In this review, we summarize the main findings and discuss the implications with regard to molecular theories of solutions and specific ion effects. A preferential binding model is introduced in order to discuss protein-IL effects from a statistical mechanics perspective. The value of the preferential binding coefficient determines the strength of the ion influence and indicates a shift of the chemical equilibrium either to the native or the denatured state of the protein. We highlight the role of water in order to explain the self-association behavior of the IL species and discuss recent experimental and simulation results in the light of the observed binding effects.

Formation of Dendrimer Nanoassemblies by Oligomerization-Induced Liquid-Liquid Phase Separation

Dendrimers are hyperbranched macromolecules with applications in host-guest chemistry, self-assembly, nanocatalysis, and nanomedicine. We show that dendrimer-based globular nanoparticles are formed by using dendrimer oligomerization to isothermally induce liquid-liquid phase separation (LLPS). We first determined that LLPS of aqueous mixtures of the fourth-generation amino-functionalized poly(amido amine) dendrimer is observed by lowering temperature in the presence of sodium sulfate. In relation to LLPS, we experimentally characterized the effect of salt and dendrimer concentrations on the LLPS temperature and salt-dendrimer isothermal partitioning. Our results were theoretically examined using a two-parameter thermodynamic model. We then showed that the addition of a small amount of glutaraldehyde, which leads to the formation of soluble dendrimer oligomers by chemical cross-linking, increases the LLPS temperature. This implies that a dendrimer aqueous mixture, which is initially homogeneous at room temperature and exhibits LLPS only at relatively low temperatures, can exhibit LLPS at room temperature due to dendrimer oligomerization. The high dendrimer concentration inside the nanodroplets, produced from LLPS, accelerates dendrimer cross-linking, thereby yielding stable globular nanoparticles. These nanomaterials retain the host-guest properties of the initial dendrimers, indicating potential applications as nanocatalysts, extracting agents and drug carriers. Our work provides the basis for a new approach for obtaining dendrimer-based nanoassemblies by employing low-generation dendrimers as building blocks.

Extracellular chloride signals collagen IV network assembly during basement membrane formation

Basement membranes are defining features of the cellular microenvironment; however, little is known regarding their assembly outside cells. We report that extracellular Cl(-) ions signal the assembly of collagen IV networks outside cells by triggering a conformational switch within collagen IV noncollagenous 1 (NC1) domains. Depletion of Cl(-) in cell culture perturbed collagen IV networks, disrupted matrix architecture, and repositioned basement membrane proteins. Phylogenetic evidence indicates this conformational switch is a fundamental mechanism of collagen IV network assembly throughout Metazoa. Using recombinant triple helical protomers, we prove that NC1 domains direct both protomer and network assembly and show in Drosophila that NC1 architecture is critical for incorporation into basement membranes. These discoveries provide an atomic-level understanding of the dynamic interactions between extracellular Cl(-) and collagen IV assembly outside cells, a critical step in the assembly and organization of basement membranes that enable tissue architecture and function. Moreover, this provides a mechanistic framework for understanding the molecular pathobiology of NC1 domains.

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.

H-Bond Acceptor Parameters for Anions

UV/vis absorption titrations have been used to investigate the formation of H-bonded complexes between anionic H-bond acceptors (HBAs) and neutral H-bond donors (HBDs) in organic solvents. Complexes formed by three different HBDs with 15 different anions were studied in chloroform and in acetonitrile. The data were used to determine self-consistent HBA parameters (β) for chloride, bromide, iodide, phosphate diester, acetate, benzoate, perrhenate, nitrate, triflimide, perchlorate, hexafluorophosphate, hydrogen sulfate, methyl sulfonate, triflate, and perfluorobutyl sulfonate. The results demonstrate the transferability of H-bond parameters for anions between different solvents and different HBD partners, allowing reliable prediction of anion recognition properties in other scenarios. Carboxylates are the strongest HBAs studied, with β parameters (≈ 15) that are significantly higher than those of neutral organic HBAs, and the non-coordinating anion hexafluorophosphate is the weakest acceptor, with a β parameter comparable to that of pyridine. The effects of ion pairing with the counter-cation were found to be negligible, provided small polar cations were avoided in the less polar solvent (chloroform). There is no correlation between the H-bonding properties of the anions and the pK_a values of the conjugate acids.

Ion-specific ice recrystallization provides a facile approach for the fabrication of porous materials

Ice recrystallization is of great importance to both fundamental research and practical applications, however understanding and controlling ice recrystallization processes remains challenging. Here, we report the discovery of an ion-specific effect on ice recrystallization. By simply changing the initial type and concentration of ions in an aqueous solution, the size of ice grains after recrystallization can be tuned from 27.4±4.1 to 277.5±30.9 μm. Molecular dynamics simulations show that the ability of the ion to be incorporated into the ice phase plays a key role in the ultimate size of the ice grains after recrystallization. Moreover, by using recrystallized ice crystals as templates, 2D and 3D porous networks with tuneable pore sizes could be prepared from various materials, for example, NaBr, collagen, quantum dots, silver and polystyrene colloids. These porous materials are suitable for a wide range of applications, for example, in organic electronics, catalysis and bioengineering.

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.

Anionic and cationic Hofmeister effects are non-additive for guanidinium salts

To understand specific ion effects on a molecular level we explore the effect of salts on the rotational mobility of a model amide using dielectric spectroscopy. Based on our previous studies on the effect of strong denaturing anions or cations, here we study the additivity of the anionic and cationic effect. Using salts consisting of denaturing spherical anions and spherical cations we find such salts to affect the amide according to what one expects based on the additive activity of the individual ions. The guanidinium (Gdm⁺) cation appears to be a notable exception, as our results suggest that GdmI (and accordingly GdmSCN) is less efficient in hindering the rotation of the amide than KI or GdmCl.

Exploring Fluorescent Dyes at Biomimetic Interfaces with Second Harmonic Generation and Molecular Dynamics

The adsorption of a DNA fluorescent probe belonging to the thiazole orange family at the dodecane/water and dodecane/phospholipid/water interfaces has been investigated using a combination of surface second harmonic generation (SSHG) and all-atomistic molecular dynamics (MD) simulations. Both approaches point to a high affinity of the cationic dye for the dodecane/water interface with a Gibbs free energy of adsorption on the order of -45 kJ/mol. Similar affinity was observed with a monolayer of negatively charged DPPG (1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol)) lipids. On the other hand, no significant adsorption could be found with the zwitterionic DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) lipids. This was rationalized in terms of Coulombic interactions between the monolayer surface and the cationic dye. The similar affinity for the interface with and without DPPG, despite the favorable Coulombic attraction in the latter case, could be explained after investigating the interfacial orientation of the dye. In the absence of a monolayer, the dye adsorbs with its molecular plane almost flat at the interface, whereas in the presence of DPPG it has to intercalate into the monolayer and adopt a significantly different orientation to benefit from the electrostatic stabilization.

Alkyl Chain Length Dependent Structural and Orientational Transformations of Water at Alcohol-Water Interfaces and Its Relevance to Atmospheric Aerosols

Although the hydrophobic size of an amphiphile plays a key role in various chemical, biological, and atmospheric processes, its effect at macroscopic aqueous interfaces (e.g., air-water, oil-water, cell membrane-water, etc.), which are ubiquitous in nature, is not well understood. Here we report the hydrophobic alkyl chain length dependent structural and orientational transformations of water at alcohol (C_nH_2n+1OH, n = 1-12)-water interfaces using interface-selective heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman multivariate curve resolution (Raman-MCR) spectroscopic techniques. The HD-VSFG results reveal that short-chain alcohols (C_nH_2n+1OH, n < 4, i.e., up to 1-propanol) do not affect the structure (H-bonding) and orientation of water at the air-water interface; the OH stretch band maximum appears at ∼3470 cm^-1, and the water H atoms are pointed toward the bulk water, that is, "H-down" oriented. In contrast, long-chain alcohols (C_nH_2n+1OH, n > 4, i.e., beyond 1-butanol) make the interfacial water more strongly H-bonded and reversely orientated; the OH stretch band maximum appears at ∼3200 cm^-1, and the H atoms are pointed away from the bulk water, that is, "H-up" oriented. Interestingly, for the alcohol of intermediate chain length (C_nH_2n+1OH, n = 4, i.e, 1-butanol), the interface is quite unstable even after hours of its formation and the time-averaged result is qualitatively similar to that of the long-chain alcohols, indicating a structural/orientational crossover of interfacial water at the 1-butanol-water interface. pH-dependent HD-VSFG measurements (with H₂O as well as isotopically diluted water, HOD) suggest that the structural/orientational transformation of water at the long-chain alcohol-water interface is associated with the adsorption of OH^- anion at the interface. Vibrational mapping of the water structure in the hydration shell of OH^- anion (obtained by Raman-MCR spectroscopy of NaOH in HOD) clearly shows that the water becomes strongly H-bonded (OH stretch max. ≈ 3200 cm^-1) while hydrating the OH^- anion. Altogether, it is conceivable that alcohols of different hydrophobic chain lengths that are present in the troposphere will differently affect the interfacial electrostatics and associated chemical processes of aerosol droplets, which are critical for cloud formation, global radiation budget, and climate change.

NMR solution structure of the RED subdomain of the Sleeping Beauty transposase

DNA transposons can be employed for stable gene transfer in vertebrates. The Sleeping Beauty (SB) DNA transposon has been recently adapted for human application and is being evaluated in clinical trials, however its molecular mechanism is not clear. SB transposition is catalyzed by the transposase enzyme, which is a multi-domain protein containing the catalytic and the DNA-binding domains. The DNA-binding domain of the SB transposase contains two structurally independent subdomains, PAI and RED. Recently, the structures of the catalytic domain and the PAI subdomain have been determined, however no structural information on the RED subdomain and its interactions with DNA has been available. Here, we used NMR spectroscopy to determine the solution structure of the RED subdomain and characterize its interactions with the transposon DNA.

Complexing Cations by Poly(ethylene oxide): Binding Site and Binding Mode

The binding of K⁺ and Ba²⁺ cations to short poly(ethylene oxide) (PEO) chains with ca. 4-25 monomeric units in methanol was studied by determining the effective charge of the polymer through a combination of electrophoretic NMR and diffusion NMR experiments. These cations were previously found to bind to long PEO chains in a similar strong manner. In addition, ¹H chemical shift and longitudinal spin relaxation rate changes upon binding were quantified. For both systems, binding was stronger for the short chains than that for the longer chains, which is attributed mainly to interactions between bound ions. For K⁺ ions, the equilibrium binding constant of a cation to a binding site was measured. For both cations, the binding site was estimated to consist of ca. six monomeric units that coordinated with the respective ions. For the systems with barium, a significant fraction of the bound ions are (BaAnion)⁺ ion pairs. This leads to a strong anion effect in the effective charge of the oligomers acquired upon barium ion binding. For K⁺, the coordinating oligomer segment remains rather mobile and individual oligomers exchange rapidly (≪s) between their free and ion-complexing states. In contrast, segmental dynamics slows significantly for the oligomer section that coordinates with the barium species, and for individual oligomers, binding and nonbinding sections do not exchange on the time scale of seconds. Hence, oligomers also exchange slowly (>s) between their free and barium complexing states.

Do guanidinium and tetrapropylammonium ions specifically interact with aromatic amino acid side chains?

Many ions are known to affect the activity, stability, and structural integrity of proteins. Although this effect can be generally attributed to ion-induced changes in forces that govern protein folding, delineating the underlying mechanism of action still remains challenging because it requires assessment of all relevant interactions, such as ion-protein, ion-water, and ion-ion interactions. Herein, we use two unnatural aromatic amino acids and several spectroscopic techniques to examine whether guanidinium chloride, one of the most commonly used protein denaturants, and tetrapropylammonium chloride can specifically interact with aromatic side chains. Our results show that tetrapropylammonium, but not guanidinium, can preferentially accumulate around aromatic residues and that tetrapropylammonium undergoes a transition at ∼1.3 M to form aggregates. We find that similar to ionic micelles, on one hand, such aggregates can disrupt native hydrophobic interactions, and on the other hand, they can promote α-helix formation in certain peptides.

Decyltrimethylammonium Bromide Micelles in Acidic Solutions: Counterion Binding, Water Structuring, and Micelle Shape

Wide-angle neutron scattering experiments combined with empirical potential structural refinement modeling have been used to study the detailed structure of decyltrimethylammonium bromide micelles in the presence of acid solutions of HCl or HBr. These experiments demonstrate considerable variation in micelle structure and water structuring between micelles in the two acid solutions and in comparison with the same micelles in pure water. In the presence of the acids, the micelles are smaller; however, in the presence of HCl the micelles are more loosely structured and disordered while in the presence of HBr the micelles are more compact and closer to spherical. Bromide ions bind strongly to the micelle surface in the HBr solution, while in HCl solutions, ion binding to the micelle is similar to that found in pure water. The hydration numbers of the anions and extent of counterion binding follow the predictions of the Hofmeister series for these species.

A first study on copolymers of a methacrylate containing the 2-(hydroxyimino)aldehyde group and OEGMA. RAFT polymerization and assessment of thermal and photoresponsive polymer behavior

RAFT copolymerization of 4-[(hydroxyimino)aldehyde]butyl methacrylate with OEGMA₄₇₅ and first assessment of photoisomerization and thermal behavior of the copolymers in solution.

Molecular origin of urea driven hydrophobic polymer collapse and unfolding depending on side chain chemistry

Urea ambivalence: molecular simulations show that collapse and unfolding of aqueous polymers occur in response to urea screening of nonpolar molecular surfaces.

Negligible cation effect on the vibrational relaxation dynamics of water molecules in NaClO4 and LiClO4 aqueous electrolyte solutions

Negligible cation effects on the vibrational relaxation dynamics of water molecules in NaClO₄ and LiClO₄ aqueous solutions.

Hydration structure of osmolyte TMAO: concentration/pressure-induced response

The effect of solute concentration/pressure on the TMAO hydration structure was studied to understand its protective action under abiotic stressors.

Orientational ordering of water in extended hydration shells of cations is ion-specific and is correlated directly with viscosity and hydration free energy

Specific ion effects in aqueous solutions are investigated at the molecular, nanoscopic and macroscopic levels.

Fluorescent and colorimetric molecular recognition probe for hydrogen bond acceptors

We report on the development of a dual molecular recognition probe for hydrogen bond acceptors.

Ion pairing controls rheological properties of "processionary" polyelectrolyte hydrogels

We demonstrated recently that polyelectrolytes with cationic moieties along the chain and a single anionic head are able to form physical hydrogels due to the reversible nature of the head-to-body ionic bond. Here we generate a variety of such polyelectrolytes with various cationic moieties and counterion combinations starting from a common polymeric platform. We show that the rheological properties (shear modulus, critical strain) of the final hydrogels can be modulated over three orders of magnitude depending on the cation/anion pair. Our data fit remarkably well within a scaling model involving a supramolecular head-to-tail single file between cross-links, akin to the behaviour of pine-processionary caterpillar. This model allows the quantitative measure of the amount of counterion condensation from standard rheology procedure.

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.

Development of a simple assay system for protein-stabilizing efficiency based on hemoglobin protection against denaturation and measurement of the cooperative effect of mixing protein stabilizers

We have elucidated the cooperative stabilization of proteins by sugars, amino acids, and other protein-stabilizing agents using a new and simple assay system. Our system determines the protein-stabilizing ability of various compounds by measuring their ability to protect hemoglobin from denaturation. Hemoglobin denaturation was readily measured by quantitative changes in its ultraviolet–visible absorption spectrum. The efficiency of our assay was confirmed using various sugars such as trehalose and sucrose that are known to be good protein stabilizers. We have also found that mixtures of two different types of protein stabilizers resulted in a cooperative stabilizing effect on protein.

Salt Effect Accelerates Site-Selective Cysteine Bioconjugation

Highly efficient and selective chemical reactions are desired. For small molecule chemistry, the reaction rate can be varied by changing the concentration, temperature, and solvent used. In contrast for large biomolecules, the reaction rate is difficult to modify by adjusting these variables because stringent biocompatible reaction conditions are required. Here we show that adding salts can change the rate constant over 4 orders of magnitude for an arylation bioconjugation reaction between a cysteine residue within a four-residue sequence (π-clamp) and a perfluoroaryl electrophile. Biocompatible ammonium sulfate significantly enhances the reaction rate without influencing the site-specificity of π-clamp mediated arylation, enabling the fast synthesis of two site-specific antibody-drug conjugates that selectively kill HER2-positive breast cancer cells. Computational and structure-reactivity studies indicate that salts may tune the reaction rate through modulating the interactions between the π-clamp hydrophobic side chains and the electrophile. On the basis of this understanding, the salt effect is extended to other bioconjugation chemistry, and a new regioselective alkylation reaction at π-clamp cysteine is developed.

Binding of Monovalent and Multivalent Metal Cations to Polyethylene Oxide in Methanol Probed by Electrophoretic and Diffusion NMR

Complex formation in methanol between monodisperse polyethylene oxide (PEO) and a large set of cations was studied by measuring the effective charge acquired by PEO upon complexation. Quantitative data were obtained at a low ionic strength of 2 mM (for some salts, also between 0.5 and 6 mM) by a combination of diffusion nuclear magnetic resonance (NMR) and electrophoretic NMR experiments. For strongly complexing cations, the magnitude of the acquired effective charge was on the order of 1 cation per 100 monomer units. For monovalent cations, the relative strength of binding varies as Na⁺ < K⁺ ≈ Rb⁺ ≈ Cs⁺, whereas Li⁺ exhibited no significant binding. All polyvalent cations bind very weakly, except for Ba²⁺ that exhibited strong binding. Anions do not bind, as is shown by the lack of response to the chemical nature of anionic species (perchlorate, iodide, or acetate). Diffusion experiments directly show that the acetate anion with monovalent cations does not associate with PEO. Considering all cations, we find that the observed binding does not follow any Hofmeister order. Instead, binding occurs below a critical surface charge density, which indicates that the degree of complexation is defined by the solvation shell. A large solvation shell prevents the binding of most multivalent ions.