Cationic Hofmeister Series of Wettability Alteration in Mica-Water-Alkane Systems.
ABSTRACT: The specific interaction of ions with macromolecules and solid-liquid interfaces is of crucial importance to many processes in biochemistry, colloid science, and engineering, as first pointed out by Hofmeister in the context of (de)stabilization of protein solutions. Here, we use contact angle goniometry to demonstrate that the macroscopic contact angle of aqueous chloride salt solutions on mica immersed in ambient alkane increases from near-zero to values exceeding 10°, depending on the type and concentration of cations and pH. Our observations result in a series of increasing ability of cations to induce partial wetting in the order Na+, K+ < Li+ < Rb+ < Cs+ < Ca2+ < Mg2+ < Ba2+. Complementary atomic force microscopy measurements show that the transition to partial wetting is accompanied by cation adsorption to the mica-electrolyte interface, which leads to charge reversal in the case of divalent cations. In addition to electrostatics, hydration forces seem to play an important role, in particular for the monovalent cations.
Project description:This study examines the properties of a 4 × 2 matrix of aqueous cations and anions at concentrations up to 8.0 M. The apparent molar water volume, as calculated by subtracting the mass and volume of the ions from the corresponding solution density, was found to exceed the molar volume of ice in many concentrated electrolyte solutions, underscoring the nonideal behavior of these systems. The solvent properties of water were also analyzed by measuring the solubility of diketopiperazine (DKP) in 2.000 M salt solutions prepared from the same ion combinations. Solution rankings for DKP solubility were found to parallel the Hofmeister series for both cations and anions, whereas molar water volume concurred with the cation series only. The results are discussed within the framework of a desolvation energy model that attributes solute-specific changes in equilibria to solute-dependent changes in the free energy of bulk water.
Project description:Quantitative interpretation and prediction of Hofmeister ion effects on protein processes, including folding and crystallization, have been elusive goals of a century of research. Here, a quantitative thermodynamic analysis, developed to treat noncoulombic interactions of solutes with biopolymer surface and recently extended to analyze the effects of Hofmeister salts on the surface tension of water, is applied to literature solubility data for small hydrocarbons and model peptides. This analysis allows us to obtain a minimum estimate of the hydration b1 (H2O A(-2)), of hydrocarbon surface and partition coefficients Kp, characterizing the distribution of salts and salt ions between this hydration water and bulk water. Assuming that Na+ and SO4(2-) ions of Na2SO4 (the salt giving the largest reduction in hydrocarbon solubility as well as the largest increase in surface tension) are fully excluded from the hydration water at hydrocarbon surface, we obtain the same b1 as for air-water surface (approximately 0.18 H2O A(-2)). Rank orders of cation and anion partition coefficients for nonpolar surface follow the Hofmeister series for protein processes, but are strongly offset for cations in the direction of exclusion (preferential hydration). By applying a coarse-grained decomposition of water accessible surface area (ASA) into nonpolar, polar amide, and other polar surface and the same hydration b1 to interpret peptide solubility increments, we determine salt partition coefficients for amide surface. These partition coefficients are separated into single-ion contributions based on the observation that both Cl- and Na+ (also K+) occupy neutral positions in the middle of the anion and cation Hofmeister series for protein folding. Independent of this assignment, we find that all cations investigated are strongly accumulated at amide surface while most anions are excluded. Cation and anion effects are independent and additive, allowing successful prediction of Hofmeister salt effects on micelle formation and other processes from structural information (ASA).
Project description:Hofmeister effects have been recognized as important as Mendel's work was to genetics while remain largely controversial, especially for the mechanistic aspects. Here we demonstrated that complex colloids in electrolyte solutions show resembling aggregation kinetics as model colloid, and then quantitatively evaluated the resulting Hofmeister effects. Mechanism for the aggregation of complex colloids has been proposed that is closely associated with the charges of their constituents; despite that, electrostatic interactions play a minor role while polarization effect is evidenced to be the driving force for the aggregation processes. Polarization effect is further ascribed to arouse the resulting Hofmeister effects, which is supported by the fine correlation of activation energies vs. polarizability data of different alkali ions and the calculations of dipole moments for minerals with different charges and adsorbed alkali ions. Because of neglecting polarization effect, the prevailing DLVO theory is not sufficient to describe Hofmeister effects that are ubiquitous in nature. We speculate that polarization effect should also be responsible for Hofmeister effects of other charged systems such as proteins and membranes.
Project description:Anion effects on the cloud-point temperature for the liquid-liquid phase transition of lysozyme were investigated by temperature gradient microfluidics under a dark field microscope. It was found that protein aggregation in salt solutions followed 2 distinct Hofmeister series depending on salt concentration. Namely, under low salt conditions the association of anions with the positively charged lysozyme surface dominated the process and the phase transition temperature followed an inverse Hofmeister series. This inverse series could be directly correlated to the size and hydration thermodynamics of the anions. At higher salt concentrations, the liquid-liquid phase transition displayed a direct Hofmeister series that correlated with the polarizability of the anions. A simple model was derived to take both charge screening and surface tension effects into account at the protein/water interface. Fitting the thermodynamic data to this model equation demonstrated its validity in both the high and low salt regimes. These results suggest that in general positively charged macromolecular systems should show inverse Hofmeister behavior only at relatively low salt concentrations, but revert to a direct Hofmeister series as the salt concentration is increased.
Project description: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<sup>+</sup> and Li<sup>+</sup> ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li<sup>+</sup> adsorbs to the aqueous solution-vapor interface, while K<sup>+</sup> does not. Thus, we provide experimental validation of the "surfactant-like" behavior of Li<sup>+</sup> predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K<sup>+</sup> and Li<sup>+</sup> ions to a difference in the resilience of their hydration shells.
Project description:Aiming to improve understanding of the mechanisms behind specific anion effects in biological systems we have studied the effects of sodium salts of simple monovalent anions belonging to the Hofmeister series on the bilayers of the zwitterionic lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine using small-angle x-ray scattering and the osmotic stress technique. NaCl, NaBr, NaNO(3), NaI, and NaSCN were used in this investigation. The electrolytes were found to swell the bilayers and to increase the area per lipid headgroup at each value of the osmotic pressure, suggesting the association of anions with the bilayer-lipid interfaces. The effects follow the Hofmeister series with SCN(-) inducing the most pronounced changes. "Ion competition" experiments with mixed NaI/NaCl solutions at total salinity 0.1 and 0.5 M revealed that the effect of ions on the lipid equation-of-state is roughly linear at low concentrations, but strongly nonlinear at high concentrations. The experimental results are fitted in a companion article to provide "binding" or "partitioning" constants of anions in the lipid bilayers.
Project description:Gadolinium (Gd(3+)) vibronic sideband luminescence spectroscopy (GVSBLS) is used to probe, as a function of added Hofmeister series salts, changes in the OH stretching frequency derived from first-shell waters of aqueous Gd(3+) and of Gd(3+) coordinated to three different types of molecules: (i) a chelate (EDTA), (ii) structured peptides (mSE3/SE2) of the lanthanide-binding tags (LBTs) family with a single high-affinity binding site, and (iii) a calcium-binding protein (calmodulin) with four binding sites. The vibronic sideband (VSB) corresponding to the OH stretching mode of waters coordinated to Gd(3+), whose frequency is inversely correlated with the strength of the hydrogen bonding to neighboring waters, exhibits an increase in frequency when Gd(3+) becomes coordinated to either EDTA, calmodulin, or mSE3 peptide. In all of these cases, the addition of cation chloride or acetate salts to the solution increases the frequency of the vibronic band originating from the OH stretching mode of the coordinated waters in a cation- and concentration-dependent fashion. The cation dependence of the frequency increase scales with charge density of the cations, giving rise to an ordering consistent with the Hofmeister ordering. On the other hand, water Raman spectroscopy shows no significant change upon addition of these salts. Additionally, it is shown that the cation effect is modulated by the specific anion used. The results indicate a mechanism of action for Hofmeister series ions in which hydrogen bonding among hydration shell waters is modulated by several factors. High charge density cations sequester waters in a configuration that precludes strong hydrogen bonding to neighboring waters. Under such conditions, anion effects emerge as anions compete for hydrogen-bonding sites with the remaining free waters on the surface of the hydration shell. The magnitude of the anion effect is both cation and Gd(3+)-binding site specific.
Project description:The appearance of several disulfide bond isoforms in multiple cysteine containing venom peptides poses a significant challenge in their synthesis and purification under laboratory conditions. Recent experiments suggest that careful tuning of solvent and temperature conditions can propel the disulfide bond isoform equilibrium in favor of the most potent, native form. Certain aqueous ionic liquids (ILs) have proven significantly useful as solvents for this purpose, while exceptions have also been noted. To elucidate the molecular level origin behind such a preference, we report a detailed explicit solvent replica exchange molecular dynamics study of a conotoxin, AuIB, in pure water and four different aqueous IL solutions (~45-60% v/v). The ILs studied here are comprised of cations like 1-ethyl-3-methyl-imidazolium (Im21+) or 1-butyl-3-methyl-imidazolium (Im41+) coupled with either acetate (OAc-) or chloride (Cl-) as the counter anion. Our simulations unfold interesting features of the conformational spaces sampled by the peptide and its solvation in pure water and aqueous IL solutions. Detailed investigation into populations of the globular disulfide bond isoform of AuIB in aqueous IL solutions reveal distinct trends which might be related to the Hofmeister effect of the cation and anion of the IL and of specific interactions of the aqueous IL solutions with the peptide. In accordance with experimental observations, the aqueous [Im21][OAc] solution is found to promote the highest globular isoform population in AuIB.
Project description:The effect of salts on water behavior has been a topic of interest for many years; however, some recent reports have suggested that ions do not influence the hydrogen bonding behavior of water. Using an effective two-state hydrogen bonding model to interpret the temperature excursion infrared response of the O-H stretch of aqueous salt solutions, we show a strong correlation between salt effects on water hydrogen bonding and the Hofmeister order. These data clearly show that salts do have a measurable impact on the equilibrium hydrogen bonding behavior of water and support models which explain Hofmeister effects on the basis of solute charge density.
Project description:Polydopamine has been found to be a biocompatible polymer capable of supporting cell growth and attachment, and to have antibacterial and antifouling properties. Together with its ease of manufacture and application, it ought to make an ideal biomaterial and function well as a coating for implants. In this paper, atomic force microscope was used to measure the adhesive forces between polymer-, protein- or polydopamine-coated surfaces and a silicon nitride or polydopamine-functionalised probes. Surfaces were further characterised by contact angle goniometry, and solutions by circular dichroism. Polydopamine was further characterised with infrared spectroscopy and Raman spectroscopy. It was found that polydopamine functionalisation of the atomic force microscope probe significantly reduced adhesion to all tested surfaces. For example, adhesion to mica fell from 0.27 ± 0.7 to 0.05 ± 0.01 nN nm-1. The results suggest that polydopamine coatings are suitable to be used for a variety of biomedical applications.