Hofmeister ions control protein dynamics

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.

Insights into graphene oxide interaction with human serum albumin in isolated state and in blood plasma

The interactions of graphene oxide (GO), a 2-dimensional nanomaterial with hydrophilic edges, hydrophobic basal plane and large flat surfaces, with biological macromolecules, are of key importance for the development of novel nanomaterials for biomedical applications. To gain more insight into the interaction of GO flakes with human serum albumin (HSA), we examined GO binding to HSA in its isolated state and in blood plasma. Calorimetric data reveal that GO strongly stabilizes free isolated HSA against a thermal challenge at low ionic strength, indicating strong binding interactions, confirmed by the drop in ζ-potential of the HSA/GO assemblies compared to bare GO flakes. However, calorimetry also revealed that the HSA-GO molecular interaction is hampered in blood plasma, the ionic strength being particularly important for the interactions. Molecular modelling calculations are in full concert with these experimental findings, indicating a considerably higher binding affinity for HSA to GO in its partially unfolded state, characteristic to low-ionic-strength environment, than for the native protein conformation, observed under physiological conditions. Therefore, for the first time we demonstrate an impeded interaction between HSA and GO nanoflakes in blood plasma, and suggest that the protein is protected from the plausible toxic effects of GO under native conditions.

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.

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.

The Ca2+-activated Cl- channel ANO1/TMEM16A regulates primary ciliogenesis

The Ca²⁺-activated Cl⁻ channel ANO1/TMEM16A is located in the primary cilium, and blocking it pharmacologically or knocking it down with shRNA interferes with ciliogenesis. Before ciliogenesis, the channel is organized into a torus-shaped structure (the “nimbus”) enriched in proteins required for ciliogenesis.

Many cells possess a single, nonmotile, primary cilium highly enriched in receptors and sensory transduction machinery that plays crucial roles in cellular morphogenesis. Although sensory transduction requires ion channels, relatively little is known about ion channels in the primary cilium (with the exception of TRPP2). Here we show that the Ca²⁺-activated Cl⁻ channel anoctamin-1 (ANO1/TMEM16A) is located in the primary cilium and that blocking its channel function pharmacologically or knocking it down with short hairpin RNA interferes with ciliogenesis. Before ciliogenesis, the channel becomes organized into a torus-shaped structure (“the nimbus”) enriched in proteins required for ciliogenesis, including the small GTPases Cdc42 and Arl13b and the exocyst complex component Sec6. The nimbus excludes F-actin and coincides with a ring of acetylated microtubules. The nimbus appears to form before, or independent of, apical docking of the mother centriole. Our data support a model in which the nimbus provides a scaffold for staging of ciliary components for assembly very early in ciliogenesis and chloride transport by ANO1/TMEM16A is required for the genesis or maintenance of primary cilia.