Ion hydration: Implications for cellular function, polyelectrolytes, and protein crystallization

Analysis of microtubules in isolated axoplasm from the squid giant axon

Biochemical specialization of cellular microtubules has emerged as a primary mechanism in specifying microtubule dynamics and function. However, study of specific subcellular populations of cytoplasmic microtubules has been limited, particularly in the nervous system. The complexity of nervous tissue makes it difficult to distinguish neuronal microtubules from glial microtubules, and axonal microtubules from dendritic and cell body microtubules. The problem is further compounded by the finding that a large fraction of neuronal tubulin is lost during standard preparations of brain tubulin, and this population of stable microtubules is enriched in axons. Here, we consider a unique biological model that provides a unique opportunity to study axonal microtubules both in situ and in vitro: isolated axoplasm from the squid giant axon. The axoplasm model represents a powerful system for addressing fundamental questions of microtubule structure and function in the axon.

Salt-induced control of supramolecular order in biocatalytic hydrogelation

Biocatalytic action and specific ion effects are both known to have dramatic effects on molecular self-assembly and hydrogelation. In this paper, we demonstrate that these effects are highly cooperative. Biocatalytic hydrogelation of Fmoc peptides in the presence of salts combines kinetic (through enzymatic catalysis) and thermodynamic (specific ion and protein templating) contributions when applied in combination. Spectroscopic data (obtained by fluorescence spectroscopy and circular dichroism) revealed that hydrophobic interactions are greatly affected, giving rise to differential chiral organization and supramolecular structure formation. The kinetic effects of catalytic action could be removed from the system by applying a heat/cool cycle, giving insight into the thermodynamic influence of both protein and salt on these systems and showing that the effects of catalysis, templating, and salts are cooperative. The variable molecular interactions are expressed as variable material properties, such as thermal stability and mechanical strength of the final gel-phase material. To gain more insight into the role of the enzyme, beyond catalysis, in the underlying mechanism, static light scattering is performed, which indicates the different mode of aggregation of the enzyme molecules in the presence of different salts in aqueous solution that may play a role to direct the assembly via templating. Overall, the results show that the combination of specific salts and enzymatic hydrogelation can give rise to complex self-assembly behaviors that may be exploited to tune hydrogel properties.

Choline alkylsulfates--new promising green surfactants

In this work we show how a new promising green and highly water-soluble surfactant can be designed based on recent progress in the knowledge of counterion-headgroup binding and crystallization behavior. The result is the combination of a most classical surfactant anion, dodecylsulfate (DS), with choline (Ch), a natural green cation. The advantage of the physiological metabolite choline is its bulky structure that prevents ChDS from easy crystallization and thus leads to a considerable lowering of the Krafft point down to 0°C. The counterion-headgroup binding is reflected by the aqueous phase behavior of ChDS. Conductivity, surface tension, and cryo-TEM measurements allow the characterization of the dilute micellar region, while the penetration scan technique enables the establishment of a preliminary aqueous phase diagram. In addition, the influence of different mono- and divalent salts on the solubility of ChDS is investigated. The results are compared to the alkali sulfate and alkylcarboxylate homologs, and reveal that ChDS is less sensitive towards addition of salts than, for instance, choline carboxylates due to an increased counterion-headgroup association. Further, cytotoxicity tests on HeLa and SK-Mel 28 cells are presented and compared to other surfactants, showing that ChDS is no more harmful than its sodium counterpart SDS. Taken together, our findings highlight that the harmless green cation choline is of great potential for the design of new surfactants.

Influence of counterions on micellization of tetramethylammonium perfluorononanoic carboxylate in 1-butyl-3-methylimidazolium ionic liquid

The influence of counterions on micellization of perfluorononanoic carboxylate ammonium salts in water and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF(4)) solutions was investigated by surface tension and (19)F NMR measurements and freeze-fracture transmission electron microscopy (FF-TEM) observations. Changes in the counterions of the fluorocarbon surfactants have different effects on the two solvents. With the increase of counterion volume, the critical micelle concentration (cmc) value of relevant fluorinated surfactant decreases in aqueous solutions. This is because the counterions with larger size, such as (+)N(CH(3))(4), can be little hydrated, which can screen the electrostatic repulsion of the headgroups of the fluorocarbon surfactant and thus facilitate micelle formation. However, the fluorocarbon surfactants can dissolve and form micelles in [bmim]BF(4) only when they provide with largest counterion such as (+)N(CH(3))(4). This is because the counterion, (+)N(CH(3))(4), disperses the charge of the cations, which could weaken the electrostatic interaction between the ion pair of the surfactant, leading to a higher degree of counterion binding. The thermodynamic parameters estimated from the temperature dependence of the cmc values tell us that the micelle formation for tetramethylammonium perfluorononanoic carboxylate (C(8)F(17)COON(CH(3))(4), PFNT) in ionic liquids (ILs) is an entropy-driven process at low temperature but an enthalpy-driven process at high temperature. The driving force of the micellization for fluorocarbon surfactants in [bmim]BF(4) is the solvophobic effect, due to the hydrophobic and oleophobic properties of fluorocarbon chains.

Specific Cation Effects on the Bimodal Acid-Base Behavior of the Silica/Water Interface

Using nonresonant second harmonic generation spectroscopy, we have monitored the change in surface charge density of the silica/water interface over a broad pH range in the presence of different alkali chlorides. Planar silica is known to possess two types of surface sites with pKa values of ∼4 and ∼9, which are attributed to different solvation environments of the silanols. We report that varying the alkali chloride electrolyte significantly changes the effective acid dissociation constant (pKa(eff)) for the less acidic silanol groups, with the silica/NaClaq and silica/CsClaq interfaces exhibiting the lowest and highest pKa(eff) values of 8.3(1) and 10.8(1), respectively. Additionally, the relative populations of the two silanol groups are also very sensitive to the electrolyte identity. The greatest percentage of acidic silanol groups was 60(2)% for the silica/LiClaq interface in contrast to the lowest value of 20(2)% for the silica/NaClaq interface. We attribute these changes in the bimodal behavior to the influence of alkali ions on the interfacial water structure and its corresponding effect on surface acidity.

Hyperbranched polymers with thermoresponsive property highly sensitive to ions

The salt effects on the water solubility of thermoresponsive hyperbranched polyethylenimine and polyamidoamine possessing large amounts of isobutyramide terminal groups (HPEI-IBAm and HPAMAM-IBAm) were studied systematically. Eight anions with sodium as the counterion and ten cations with chloride as the counterion were used to measure the anion and cation effects on the cloud point temperature (T(cp)) of these dendritic polymers in water. It was found that the T(cp) of these dendritic polymers was much more sensitive to the addition of salts than that of the traditional thermoresponsive linear polymers. At low anion concentration, the electrostatic interaction between anions and the positively charged groups of these polymers was dominant, resulting in the unusual anion effect on the T(cp) of these polymers in water, including (1) T(cp) of these dendritic polymers decreasing nonlinearly with the increase of kosmotropic anion concentration; (2) the chaotropic anions showing abnormal salting-out property at low salt concentration and the stronger chaotropes having much pronounced salting-out ability; (3) anti-Hofmeister ordering at low salt concentration. At moderate to high salt concentration, the specific ranking of these anions in reducing the T(cp) of HPEI-IBAm and HPAMAM-IBAm polymers was PO(4)(3-) > CO(3)(2-) > SO(4)(2-) > S(2)O(3)(2-) > F(-) > Cl(-) > Br(-) > I(-), in accordance with the well-known Hofmeister series. At moderate to high salt concentration, the specific ranking order of inorganic cations in reducing the T(cp) of HPEI-IBAm polymer was Sr(2+) ≈ Ba(2+) > Na(+) ≈ K(+) ≈ Rb(+) > Cs(+) > NH(4)(+) ≈ Ca(2+) > Li(+) ≈ Mg(2+). This sequence was only partially similar to the typical Hofmeister cation series, whereas at low salt concentration the cation effect on T(cp) of the dendritic polymer was insignificant and no obvious specific ranking order could be found.

Effects of Hofmeister ions on the α-helical structure of proteins

The molecular conformation of proteins is sensitive to the nature of the aqueous environment. In particular, the presence of ions can stabilize or destabilize (denature) protein secondary structure. The underlying mechanisms of these actions are still not fully understood. Here, we combine circular dichroism (CD), single-molecule Förster resonance energy transfer, and atomistic computer simulations to elucidate salt-specific effects on the structure of three peptides with large α-helical propensity. CD indicates a complex ion-specific destabilization of the α-helix that can be rationalized by using a single salt-free computer simulation in combination with the recently introduced scheme of ion-partitioning between nonpolar and polar peptide surfaces. Simulations including salt provide a molecular underpinning of this partitioning concept. Furthermore, our single-molecule Förster resonance energy transfer measurements reveal highly compressed peptide conformations in molar concentrations of NaClO(4) in contrast to strong swelling in the presence of GdmCl. The compacted states observed in the presence of NaClO(4) originate from a tight ion-backbone network that leads to a highly heterogeneous secondary structure distribution and an overall lower α-helical content that would be estimated from CD. Thus, NaClO(4) denatures by inducing a molten globule-like structure that seems completely off-pathway between a fully folded helix and a coil state.

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.

Effective charge measurements reveal selective and preferential accumulation of anions, but not cations, at the protein surface in dilute salt solutions

Specific-ion effects are ubiquitous in nature; however, their underlying mechanisms remain elusive. Although Hofmeister-ion effects on proteins are observed at higher (>0.3 M) salt concentrations, in dilute (<0.1 M) salt solutions nonspecific electrostatic screening is considered to be dominant. Here, using effective charge (Q*) measurements of hen-egg white lysozyme (HEWL) as a direct and differential measure of ion-association, we experimentally show that anions selectively and preferentially accumulate at the protein surface even at low (<100 mM) salt concentrations. At a given ion normality (50 mN), the HEWL Q* was dependent on anion, but not cation (Li(+), Na(+), K(+), Rb(+), Cs(+), GdnH(+), and Ca(2+)), identity. The Q* decreased in the order F(-) > Cl(-) > Br(-) > NO(3)(-) ∼ I(-) > SCN(-) > ClO(4)(-) ≫ SO(4)(2-), demonstrating progressively greater binding of the monovalent anions to HEWL and also show that the SO(4)(2-) anion, despite being strongly hydrated, interacts directly with the HEWL surface. Under our experimental conditions, we observe a remarkable asymmetry between anions and cations in their interactions with the HEWL surface.

Ion-specific weak adsorption of salts and water/octanol transfer free energy of a model amphiphilic hexapeptide

An amphiphilic hexapeptide has been used as a model to quantify how specific ion effects induced by addition of four salts tune the hydrophilic/hydrophobic balance and induce temperature-dependant coacervate formation from aqueous solution. The hexapeptide chosen is present as a dimer with low transfer energy from water to octanol. Taking sodium chloride as the reference state in the Hofmeister scale, we identify water activity effects and therefore measure the free energy of transfer from water to octanol and separately the free energy associated to the adsorption of chaotropic ions or the desorption of kosmotropic ions for the same amphiphilic peptide. These effects have the same order of magnitude: therefore, both energies of solvation as well as transfer into octanol strongly depend on the nature of the electrolytes used to formulate any buffer. Model peptides could be used on separation processes based on criteria linked to "Hofmeister" but different from volume and valency.

Hydrotrope-induced inversion of salt effects on the cloud point of an extended surfactant

We report on the effects of electrolytes spanning a range of anions (NaOc, NaSCN, NaNO(3), NaBr, NaCl, NaBu, NaOAc, Na(2)SO(4), Na(2)HPO(4), and Na(2)CO(3)) and cations (LiCl, NaCl, KCl, CsCl, and choline chloride) on the aqueous solubility of an extended surfactant. The surfactant is anionic with a long hydrophobic tail as well as a significant fraction of propylene oxide groups and ethylene oxide groups (C(12-14)-PO(16)-EO(2)-SO(4)Na, X-AES). In the absence of electrolytes, X-AES exhibits a cloud-point temperature that decreases with increasing surfactant concentration. After the addition of salts to the surfactant solutions, various shifts in the solubility curves are observed. These shifts follow precisely the same Hofmeister series that is found for salting-in and salting-out effects in protein solutions. In the presence of different concentrations of sodium xylene sulfonate (SXS), the solubility of the surfactant increases. In this context, SXS can be considered to be a salting-in salt. However, when the electrolytes are added to an aqueous solution of X-AES and SXS the Hofmeister series reverses for divalent anions such as Na(2)SO(4), Na(2)HPO(4), and Na(2)CO(3). Studies on the phase behavior and micelle structures using polarization microscopy, freeze-etch TEM, and NMR measurements indicate a dramatic change in the coexisting phases on the addition of SXS.

Phase behavior and rheological properties of salt-free catanionic TTAOH/DA/H2O system in the presence of hydrophilic and hydrophobic salts

In the cationic and anionic (catanionic) surfactant mixed system, tetradecyltrimethylammonium hydroxide (TTAOH)/decanoic acid (DA)/H(2)O, abundant phase behaviors were obtained in the presence of hydrophilic and hydrophobic salts. The microstructures of typical L(α) phases with the different compositions were characterized by the transmission electron microscope (TEM) images. Aqueous double-phase transition induced by addition of hydrophilic salts was observed when the cationic surfactant was in excess. Salt-induced reversible vesicle phases could be obtained when the anionic surfactant was excess, whereas the vesicle phase at lower salinity behaves highly viscoelastic but is much less viscoelastic with high salinity which was demonstrated by measuring their rheological properties. The L(α) phase with the positive membrane charges can be finally transferred into an L(1) phase with added salts. The ion specificity of hydrophilic and hydrophobic salts is discussed, and the order of cations is summarized, which is significant for the further study of the Hofmeister effects on catanionic surfactant mixed systems.

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.

Interfacial and emulsifying properties of designed β-strand peptides

The structural and surfactant properties of a series of amphipathic β-strand peptides have been studied as a function of pH. Each nine-residue peptide has a framework of hydrophobic proline and phenylalanine amino acid residues, alternating with acidic or basic amino acids to give a sequence closely related to known β-sheet formers. Surface activity, interfacial mechanical properties, electronic circular dichroism (ECD), droplet sizing and zeta potential measurements were used to gain an overview of the peptide behavior as the molecular charge varied from ±4 to 0 with pH. ECD data suggest that the peptides form polyproline-type helices in bulk aqueous solution when highly charged, but may fold to β-hairpins rather than β-sheets when uncharged. In the uncharged state, the peptides adsorb readily at a macroscopic fluid interface to form mechanically strong interfacial films, but tend to give large droplet sizes on emulsification, apparently due to flocculation at a low droplet zeta potential. In contrast, highly charged peptide states gave a low interfacial coverage, but retained good emulsifying activity as judged by droplet size. Best emulsification was generally seen for intermediate charged states of the peptides, possibly representing a compromise between droplet zeta potential and interfacial binding affinity. The emulsifying properties of β-strand peptides have not been previously reported. Understanding the interfacial properties of such peptides is important to their potential development as biosurfactants.

Effects of salt on the lower critical solution temperature of poly (N-isopropylacrylamide)

Classical molecular dynamics simulations were performed to investigate the effects of salt on the lower critical solution temperature (LCST) of Poly (N-isopropylacrylamide) (PNIPAM). PNIPAM is often studied as a protein proxy due to the presence of a peptide bond in its monomer unit. PNIPAM is a temperature sensitive polymer which exhibits hydrophobic-hydrophilic phase transition at its LCST. The presence of salt in the solution will shift its LCST, typically to a lower temperature. This LCST shift follows the so-called Hofmeister series. Molecular dynamics (MD) simulations of PNIPAM in 1 M of NaCl, NaBr, NaI, and KCl were carried out to elucidate the effects of different salt on LCST and protein stability. Our results suggest that direct interactions between the salt cations and the polymer play a critical role in the shift of LCST and subsequently on protein stability. Further, cations have a much stronger affinity with the polymer, whereas anions bind weakly with the polymer. Moreover, the cation-polymer binding affinity is inversely correlated with the cation-anion contact pair association constant in solution.

A single mutation promotes amyloidogenicity through a highly promiscuous dimer interface

Light chain amyloidosis is a devastating protein misfolding disease characterized by the accumulation of amyloid fibrils that causes tissue damage and organ failure. These fibrils are composed of monoclonal light chain protein secreted from an abnormal proliferation of bone marrow plasma cells. We previously reported that amyloidogenic light chain protein AL-09 adopts an altered dimer while its germline protein (kappaI O18/O8) forms a canonical dimer observed in other light chain crystal structures. In solution, conformational heterogeneity obscures all NMR signals at the AL-09 and kappaI O18/O8 dimer interfaces, so we solved the nuclear magnetic resonance structure of two related mutants. AL-09 H87Y adopts the normal dimer interface, but the kappaI Y87H solution structure presents an altered interface rotated 180 degrees relative to the canonical dimer interface and 90 degrees from the AL-09 arrangement. Our results suggest that promiscuity in the light chain dimer interface may promote new intermolecular contacts that may contribute to amyloid fibril structure.

Hosting anions. The energetic perspective

Hosting anions addresses the widely spread molecular recognition event of negatively charged species by dedicated organic compounds in condensed phases at equilibrium. The experimentally accessible energetic features comprise the entire system including the solvent, any buffers, background electrolytes or other components introduced for e.g. analysis. The deconvolution of all these interaction types and their dependence on subtle structural variation is required to arrive at a structure-energy correlation that may serve as a guide in receptor construction. The focus on direct host-guest interactions (lock-and-key complementarity) that have dominated the binding concepts of artificial receptors in the past must be widened in order to account for entropic contributions which constitute very significant fractions of the total free energy of interaction. Including entropy necessarily addresses the ambiguity and fuzziness of the host-guest structural ensemble and requires the appreciation of the fact that most liquid phases possess distinct structures of their own. Apparently, it is the perturbation of the intrinsic solvent structure occurring upon association that rules ion binding in polar media where ions are soluble and abundant. Rather than specifying peculiar structural elements useful in anion binding this critical review attempts an illumination of the concepts and individual energetic contributions resulting in the final observation of specific anion recognition (95 references).

Investigation of protein conformation and interactions with salts via X-ray absorption spectroscopy

Nitrogen K-edge spectra of aqueous triglycine were measured using liquid microjets, and the effects of Hofmeister-active salts on the spectra were observed. Spectra simulated using density functional theory, sampled from room temperature classical molecular dynamics trajectories, capture all major features in the measured spectra. The spectrum of triglycine in water is quite similar to that in the presence of chaotropic sodium bromide (and other halides), which raises the solubility of proteins. However, a new feature is found when kosmotropic Na(2)SO(3), which lowers solubility, is present; this feature results from excitations of the nitrogen atom in the terminal amino group of triglycine. Both direct interactions between this salt and the protonated amino terminus, as well as corresponding changes in the conformational dynamics of the system, contribute to this new feature. These molecular measurements support a different mechanism for the Hofmeister effect than has previously been suggested based on thermodynamic measurements. It is also shown that near edge X-ray absorption fine structure (NEXAFS) is sensitive to strong direct interaction between certain salts and charged peptides. However, by investigating the sensitivity of NEXAFS to the extreme structural differences between model beta-sheets and alpha-helices, we conclude that this technique is relatively insensitive to secondary structure of peptides and proteins.