Towards understanding specific ion effects in aqueous media using thermodiffusion

Electric field-induced control of protein crystal morphology

In a previous study (D. Ray, et al., J. Phys. Chem. Lett., 2024, 15, 8108-8113), we found that an alternating electric field considerably affects the location of the crystallization boundary and the liquid-liquid phase separation line as well as crystallization kinetics in lysozyme solutions containing sodium thiocyanate (NaSCN). The present study extends this work by investigating the influence of the same electric field on the microscopic appearance of lysozyme crystals as they form from a supersaturated solution. We observe a variety of distinct crystal morphologies, which we classify as single- and multi-arm crystals, flower-like crystal structures, whiskers, and sea-urchin crystals. Crystal morphologies exhibit significant variations with changes in protein and salt concentrations, and the electric field strongly alters the morphology-state diagram in the protein-versus-salt concentration plane. This alteration is likely due to the field effect on protein-protein interactions. We believe the effect is mediated by the field-enhanced adsorption of SCN- ions to the surface of lysozyme, ultimately driving the observed changes in crystallization behavior. These findings offer insights into how electric fields can be used to control crystal formation and morphology in protein systems.

Analyzing the concentration-dependent Soret coefficient minimum in salt solutions: an overview

Temperature gradients often cause the separation of the components in liquid mixtures by a process called thermodiffusion and quantified by the Soret coefficient. In recent years, the existence of minima in the Soret coefficient as a function of concentration has been investigated by experiments and simulations for various aqueous salt solutions. In this paper, we analyze the data of ten 1 : 1 electrolytes (lithium, sodium and potassium chloride, lithium, sodium and potassium iodide, potassium acetate, sodium and potassium thiocyanate and guanidinium chloride) in water, together with those of newly measured Soret coefficients for aqueous cesium iodide solutions. The latter were measured in the temperature range between 15 °C and 45 °C and concentrations between 0.5 and 3 moles per kg of the solvent using thermal diffusion forced Rayleigh scattering. We analyze the data by expressing the Soret coefficients as products of two factors, one purely thermodynamic factor and one being the ratio of two Onsager coefficients. It turns out that the ratio of Onsager coefficients is the main factor responsible for the non-monotonic behavior of the Soret coefficients, contrary to recent findings using computer simulations of binary Lennard-Jones mixtures. Moreover, for salts with the same anion, we find that the thermodynamic factors increase with increasing Pauling radii of the cations, while the Onsager ratios increase monotonically with the radii of the hydrated cations.

Deciphering the guanidinium cation: Insights into thermal diffusion

Thermophoresis, or thermodiffusion, is becoming a more popular method for investigating the interactions between proteins and ligands due to its high sensitivity to the interactions between solutes and water. Despite its growing use, the intricate mechanisms behind thermodiffusion remain unclear. This gap in knowledge stems from the complexities of thermodiffusion in solvents that have specific interactions as well as the intricate nature of systems that include many components with both non-ionic and ionic groups. To deepen our understanding, we reduce complexity by conducting systematic studies on aqueous salt solutions. In this work, we focused on how guanidinium salt solutions behave in a temperature gradient, using thermal diffusion forced Rayleigh scattering experiments at temperatures ranging from 15 to 35 °C. We looked at the thermodiffusive behavior of four guanidinium salts (thiocyanate, iodide, chloride, and carbonate) in solutions with concentrations ranging from 1 to 3 mol/kg. The guanidinium cation is disk-shaped and is characterized by flat hydrophobic surfaces and three amine groups, which enable directional hydrogen bonding along the edges. We compare our results to the behavior of salts with spherical cations, such as sodium, potassium, and lithium. Our discussions are framed around how different salts are solvated, specifically in the context of the Hofmeister series, which ranks ions based on their effects on the solvation of proteins.

Non-monotonic Soret coefficients of aqueous LiCl solutions with varying concentrations

We investigate the thermodiffusive properties of aqueous solutions of lithium chloride, using thermal diffusion forced Rayleigh scattering in a concentration range of 0.5-2 mole per kg of solvent and a temperature range of 5 to 45 °C. All solutions exhibit non-monotonic variations of the Soret coefficient ST with a concentration exhibiting a minimum at about one mole per kg of solvent. The depth of the minimum decreases with increasing temperature and shifts slightly towards higher concentrations. We compare the experimental data with published data and apply a recent model based on overlapping hydration shells. Additionally, we calculate the ratio of the phenomenological Onsager coefficients using our experimental results and published data to calculate the thermodynamic factor. Simple linear, quadratic and exponential functions can be used to describe this ratio accurately, and together with the thermodynamic factors, the experimental Soret coefficients can be reproduced. The main conclusion from this analysis is that the minimum of the Soret coefficients results from a maximum in the thermodynamic factor, which appears itself at concentrations far below the experimental concentrations. Only after multiplication by the (negative) monotonous Onsager ratio does the minimum move into the experimental concentration window.

On the microscopic origin of Soret coefficient minima in liquid mixtures

Temperature gradients induce mass separation in mixtures in a process called thermodiffusion and quantified by the Soret coefficient. The existence of minima in the Soret coefficient of aqueous solutions at specific salt concentrations was controversial until fairly recently, where a combination of experiments and simulations provided evidence for the existence of this physical phenomenon. However, the physical origin of the minima and more importantly its generality, e.g. in non-aqueous liquid mixtures, is still an outstanding question. Here, we report the existence of a minimum in liquid mixtures of non-polar liquids modelled as Lennard-Jones mixtures, demonstrating the generality of minima in the Soret coefficient. The minimum originates from a coincident minimum in the thermodynamic factor, and hence denotes a maximization of non-ideality mixing conditions. We rationalize the microscopic origin of this effect in terms of the atomic coordination structure of the mixtures.

Overlapping hydration shells in salt solutions causing non-monotonic Soret coefficients with varying concentration

We develop an intuitive picture that overlapping hydration shells in salt solutions cause non-monotonic Soret coefficients with varying concentration.