Hofmeister Series Of Ions Research Articles

Ions effect on tunable coacervate and its relevance to the Hofmeister series

Ion induced self-assembly allows a better understanding of the natural self-assemblies such as proteins, colloids and polymers, etc. However, little is known about ions mediated tunable coacervates. Herein, we show that the Hofmeister series of ions can be used to explain the tunable coacervates induced by different types of ions. The assembly of benzenesulfonate and polyetheramine undergoes the transformation from bilayer vesicles to the network structure of bottom coacervate with the increased concentration of inorganic salt, and the tunable coacervates of surfactants was found to be highly dependent on the ion species in solution. The affinity and compatibility of cations and surfactant follows Li+ > Na+ > K+, which determines the disturbance range of water molecules in the hydration shell of surfactant, resulting in different states of the final aggregates varying from precipitate, upper coacervate to bottom coacervate. The chaotropic anions (e.g., I−) have more pronounced dehydration effect on surfactants compared with kosmotropic anions (e.g., F−), and the ability of anions induced the transition from the bottom to the upper coacervate follows I−> Br− > Cl− >F−. Since the structural transformation of bottom coacervate requires conformational entropy, the release of water molecules is thermodynamically favorable for the transition of coacervates, which is also verified by the fact that the occurrence of dehydration drives the bottom coacervate upward to the upper coacervate during the heating process. These results provide a close link between the Hofmeister series and the controllable coacervates, which is of importance for further progress in ion specificity effects and creates a basic physicochemistry skeleton to understand this important phenomenon.

Counteranions in the Stimulation Solution Alter the Dynamics of Exocytosis Consistent with the Hofmeister Series.

We show that the Hofmeister series of ions can be used to explain the cellular changes in exocytosis observed by single-cell amperometry for different counteranions. The formation, expansion, and closing of the membrane fusion pore during exocytosis was found to be strongly dependent on the counteranion species in solution. With stimulation of chaotropic anions (e.g., ClO4–), the expansion and closing time of the fusion pore are longer, suggesting chaotropes can extend the duration of exocytosis compared with kosmotropic anions (e.g., Cl–). At a concentration of 30 mM, the two parameters (e.g., t1/2 and tfall) that define the duration of exocytosis vary with the Hofmeister series (Cl– < Br– < NO3– ≤ ClO4– < SCN–). More interestingly, fewer (e.g., Nfoot/Nevents) and smaller (e.g., Ifoot) prespike events are observed when chaotropes are counterions in the stimulation solution, and the values can be sorted by the reverse Hofmeister series (Cl– ≥ Br– > NO3– > ClO4– > SCN–). Based on ion specificity, an adsorption-repulsion mechanism, we suggest that the exocytotic Hofmeister series effect originates from a looser swelling lipid bilayer structure due to the adsorption and electrostatic repulsion of chaotropes on the hydrophobic portion of the membrane. Our results provide a chemical link between the Hofmeister series and the cellular process of neurotransmitter release via exocytosis and provide a better physical framework to understand this important phenomenon.

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.

Salt and pH effect on thermoresponsive behavior of multiwalled carbon nanotube (MWCNT)/poly(N-vinylcaprolactam) dispersion

The influence of different salts and pH on the stability of the aqueous dispersion of multiwalled carbon nanotubes (MWCNTs) non-covalently modified by thermosensitive poly(N-vinylcaprolactam) (PVCL) was studied. The PVCL/MWCNT dispersion was prepared by adding MWCNTs into aqueous PVCL solution with the subsequent ultrasonication and centrifugation. The morphology of MWCNTs and MWCNT/PVCL composites was characterized by TEM. The aggregation temperatures of the MWCNT/PVCL dispersion and the lower critical solution temperature (LCST) of PVCL aqueous solution were determined for different salt types and concentrations as well as pH in the temperature range between 20 and 60 °C. Specific ion effect on LCST of PVCL was analyzed in terms of the Hofmeister series. In the presence of kosmotropes, both the MWCNT/PVCL dispersion aggregation temperature and the LCST of PVCL are well consistent with the Hofmeister series of ions. However, opposite trends were observed for LCST of PVCL and the MWCNT/PVCL dispersion aggregation temperature when adding chaotropic anions. The dynamic light scattering (DLS) technique was used to study the PVCL aggregation kinetics at a temperature above the LCST at different pH values. These results demonstrate that specific thermoresponsive properties of the MWCNT/PVCL dispersion at pH < 3 correlate with the behavior of the individual PVCL macromolecules in acid media above the LCST.

Effect of Salts on the Water Sorption Kinetics of Dried Pasta

The water sorption kinetics of dried pasta were measured in the 20-90 °C range in 1.83 mol/L of NaCl and at 80 °C in 1.83 mol/L of LiCl, KCl, NaBr and NaI solutions in order to elucidate the role of salt in the kinetics. At the temperatures higher than 70.8 °C, the change in the enthalpy of sorption, ΔH, in the 1.83 mol/L NaCl solution was 33.1 kJ/mol, which was greater than the ΔH value in water, and the activation energy for the sorption, E, in the salt solution was 25.6 kJ/mol, which was slightly lower than the E value in water. The Hofmeister series of ions was an index for their effect on the equilibrium amount of the sorbed solution of pasta. The apparent diffusion coefficient of water into pasta was not correlated with the crystal radius of the salts, but was with the Stokes radius of the hydrated ions. Equations were formulated to predict the amount of sorbed solution under any condition of temperature and NaCl concentration.

Water Mobility: A Bridge between the Hofmeister Series of Ions and the Friction of Zwitterionic Surfaces in Aqueous Environments

In this work, we systematically studied the effects of monovalent ions in the Hofmeister series on the friction of zwitterionic surfaces with carboxybetaine self-assembled monolayers (CB-SAMs) as model surfaces using molecular dynamics simulations. The friction coefficients between two CB-SAM surfaces under shear were calculated in the presence of LiCl, NaCl, KCl, CsCl, CsF, CsBr, or CsI solutions at 1 M. Results show that there is a strong correlation between the order of ions in the Hofmeister series and the friction of the CB-SAM surfaces. For salt solutions with the same anion, the friction of the surfaces in a solution with kosmotropic cations has larger friction than that in a solution with chaotropic ones. The same relationship between the order of ions in the Hofmeister series and the friction of the surfaces can be found for the salt solutions with the same cation but different anions. The analysis of water near the surfaces further suggests that these surfaces are hydrated with a similar amount ...

Prediction and interpretation of the hydration entropies of monovalent cations and anions

The hydration entropy of the alkali metal cations and halide anions are calculated directly from molecular dynamics simulations of the hydrated ion and bulk water using theory previously applied to the hydration of noble gases [Irudayam and Henchman, J. Phys.: Condens. Matter 22, 284108 (2010)]. Extensions are included to account for differential hydrogen-bonding of first-shell waters with themselves, the ion, and bulk water. The entropies, enthalpies and Gibbs energies agree reasonably with simulation and experiment when the effect of force field is taken into account. The anions' entropy losses are mostly vibrational and librational, consistent with their stronger hydration. The cations' entropy losses are mostly orientational which imply fewer hydrogen-bond arrangements because the cations substantially inhibit the ability of surrounding water molecules to accept hydrogen bonds. Owing to the many entropy terms and different decompositions, it is shown that the terms, kosmotropes and chaotropes must be appropriately applied so as not to lead to contradictions. It is also proposed that the number of hydrogen-bond arrangements helps explain the ordering in Hofmeister series of ions whereby anions increase this number but cations decrease it.

ION SERIES AND THE PHYSICAL PROPERTIES OF PROTEINS

1. Ions with the opposite sign of charge as that of a protein ion diminish the swelling, osmotic pressure, and viscosity of the protein. Ions with the same sign of charge as the protein ion (with the exception of H and OH ions) seem to have no effect on these properties as long as the concentrations of electrolytes used are not too high. 2. The relative depressing effect of different ions on the physical properties of proteins is a function only of the valency and sign of charge of the ion, ions of the same sign of charge and the same valency having practically the same depressing effect on gelatin solutions of the same pH while the depressing effect increases rapidly with an increase in the valency of the ion. 3. The Hofmeister series of ions are the result of an error due to the failure to notice the influence of the addition of a salt upon the hydrogen ion concentration of the protein solution. As a consequence of this failure, effects caused by a variation in the hydrogen ion concentration of the solution were erroneously attributed to differences in the nature of the ions of the salts used. 4. It is not safe to draw conclusions concerning specific effects of ions on the swelling, osmotic pressure, or viscosity of gelatin when the concentration of electrolytes in the solution exceeds M/16, since at that concentration the values of these properties are near the minimum characteristic of the isoelectric point.