Weak Anion Binding to Poly(N-isopropylacrylamide) Detected by Electrophoretic NMR

Effect of Hofmeister Anions Series on the Langmuir Film of Tetronic 90R4 and Tetronic 701 Block Copolymers

The air-water interfacial behavior of Tetronic 90R4 and Tetronic 701 was studied in the presence of sodium salts with different anions namely PO 4 3 - , SO 4 2 - , MoO 4 2 - , WO 4 2 - , Cl-, Br-, NO 3 - , I-, and SCN-. Their presence in the subphase altered the arrangement of both tetronic molecules at the air-water interface. The limiting mean molecular area of the Langmuir film for both tetronics was found to be ion specific; it increased following the series PO 4 3 - < SO 4 2 - < MoO 4 2 - < WO 4 2 - < Cl-< Br-< NO 3 - < I-< SCN-, which was found to be aligned with the Hofmeister series of anions. Furthermore, the study explored the effects of the hydration enthalpy, free energy, viscosity BJD coefficient, and polarizability of these anions on the interfacial behavior of tetronics. The Langmuir-Blodgett film morphology was also examined in the presence of these salt species using SEM. Morphologies were explained considering kosmotropic and chaotropic nature of these anions.

Destabilization of the D2 domain of Thermotoga maritima arginine binding protein induced by guanidinium thiocyanate and its counteraction by stabilizing agents

D2 is a structural and cooperative domain of Thermotoga maritima Arginine Binding Protein, that possesses a remarkable conformational stability, with a denaturation temperature of 102.6°C, at pH 7.4. The addition of potassium thiocyanate causes a significant decrease in the D2 denaturation temperature. The interactions of thiocyanate ions with D2 have been studied by means of isothermal titration calorimetry measurements and molecular dynamics simulations. It emerged that: (a) 20-30 thiocyanate ions interact with the D2 surface and are present in its first solvation shell; (b) each of them makes several contacts with protein groups, both polar and nonpolar ones. The addition of guanidinium thiocyanate causes a marked destabilization of the D2 native state, because both the ions are denaturing agents. However, on adding to the solution containing D2 and guanidinium thiocyanate a stabilizing agent, such as TMAO, sucrose or sodium sulfate, a significant increase in denaturation temperature occurs. The present results confirm that counteraction is a general phenomenon for globular proteins.

From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush

HYPOTHESIS

Specific ion effects govern myriad biological phenomena, including protein-ligand interactions and enzyme activity. Despite recent advances, detailed understanding of the role of ion hydrophobicity in specific ion effects, and the intersection with hydrotropic effects, remains elusive. Short chain fatty acid sodium salts are simple amphiphiles which play an integral role in our gastrointestinal health. We hypothesise that increasing a fatty acid's hydrophobicity will manifest stronger salting-out behaviour.

EXPERIMENTS

Here we study the effect of these amphiphiles on an exemplar thermoresponsive polymer brush system, conserving the carboxylate anion identity while varying anion hydrophobicity via the carbon chain length. Ellipsometry and quartz crystal microbalance with dissipation monitoring were used to characterise the thermoresponse and viscoelasticity of the brush, respectively, whilst neutron reflectometry was used to reveal the internal structure of the brush. Diffusion-ordered nuclear magnetic resonance spectroscopy and computational investigations provide insight into polymer-ion interactions.

FINDINGS

Surface sensitive techniques unveiled a non-monotonic trend in salting-out ability with increasing anion hydrophobicity, revealing the bundle-like morphology of the ion-collapsed system. An intersection between ion-specific and hydrotropic effects was observed both experimentally and computationally; trending from good anti-hydrotrope towards hydrotropic behaviour with increasing anion hydrophobicity, accompanying a change in hydrophobic hydration.

Relevance of the Cation in Anion Binding of a Triazole Host: An Analysis by Electrophoretic Nuclear Magnetic Resonance

Triazole hosts allow cooperative binding of anions via hydrogen bonds, which makes them versatile systems for application in anion binding catalysis to be performed in organic solvents. The anion binding behavior of a tetratriazole host is systematically studied by employing a variety of salts, including chloride, acetate, and benzoate, as well as different cations. Classical nuclear magnetic resonance (1H NMR) titrations demonstrate a large influence of cation structures on the anion binding constant, which is attributed to poor dissociation of most salts in organic solvents and corrupts the results of classical titration techniques. We propose an approach employing electrophoretic NMR (eNMR), yielding drift velocities of each species in an electric field and thus allowing a distinction between charged and uncharged species. After the determination of the dissociation constants KD for the salts, electrophoretic mobilities are measured for all species in the host-salt system and are analyzed in a model which treats anion binding as a consecutive reaction to salt dissociation, yielding a corrected anion binding constant KA. Interestingly, dependence of KA on salt concentration occurs, which is attributed to cation aggregation with the anion-host complex. Finally, by the extrapolation to zero salt concentration, the true anion-host binding constant is obtained. Thus, the approach by eNMR allows a fully quantitative analysis of two factors that might impair classical anion binding studies, namely, an incomplete salt dissociation as well as the occurrence of larger aggregate species.

Denaturation of proteins: electrostatic effects vs. hydration

The unfolding transition of proteins in aqueous solution containing various salts or uncharged solutes is a classical subject of biophysics. In many cases, this transition is a well-defined two-stage equilibrium process which can be described by a free energy of transition ΔG u and a transition temperature T m. For a long time, it has been known that solutes can change T m profoundly. Here we present a phenomenological model that describes the change of T m with the solute concentration c s in terms of two effects: (i) the change of the number of correlated counterions Δn ci and (ii) the change of hydration expressed through the parameter Δw and its dependence on temperature expressed through the parameter dΔc p/dc s. Proteins always carry charges and Δn ci describes the uptake or release of counterions during the transition. Likewise, the parameter Δw measures the uptake or release of water during the transition. The transition takes place in a reservoir with a given salt concentration c s that defines also the activity of water. The parameter Δn ci is a measure for the gain or loss of free energy because of the release or uptake of ions and is related to purely entropic effects that scale with ln c s. Δw describes the effect on ΔG u through the loss or uptake of water molecules and contains enthalpic as well as entropic effects that scale with c s. It is related to the enthalpy of transition ΔH u through a Maxwell relation: the dependence of ΔH u on c s is proportional to the dependence of Δw on temperature. While ionic effects embodied in Δn ci are independent of the kind of salt, the hydration effects described through Δw are directly related to Hofmeister effects of the various salt ions. A comparison with literature data underscores the general validity of the model.