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Thermophoresis or thermodiffusion is the mass transport driven by a temperature gradient. This thesis focuses on the thermophoretic motion of ionic compounds in a biological context and is motivated by a practical application, in which thermodiffusion is used to monitor protein-ligand reactions. Proteins are complex molecules containing non-ionic and ionic groups. While recent studies of non-ionic compounds found a strong correlation between thermodiffusion and hydration, it is unclear how this correlation changes when molecules are charged. To separate ionic from non-ionic contributions, it is reasonable to look first into the thermophoretic motion of simple salts without large organic side groups and to study in the next step complex protein-ligand systems, which typically contain hydrophobic and hydrophilic groups. The systematic studies of aqueous solutions of simple salts should reveal differences between ionic and non-ionic systems and should give further information about ion and ion specific effects. Due to the high complexity of protein-ligand systems, complementary methods should be used to gain a better understanding of the interactions between different components that are present in the system. This will help to understand how the thermophoretic behavior of the free protein differs from that of the protein-ligand complex formed. Study of the thermophoretic behavior of ionic systems indicates that several correlations, which were found for aqueous solutions of non-ionic solutes are no longer valid for ionic solutes. For non-ionic solutes hydrogen bonds primarily influence the thermophoretic behavior. In case of ionic solutes, although both electrostatic interactions and hydrogen bonds are present, it is found that thermophoretic behavior is influenced by electrostatic interactions. Focusing on the specific ion effects for ionic systems in the context of the Hofmeister series, a change of the anion is found to influence the thermophoretic behavior more than a change of the cation. Further, a correlation between thermophoretic behavior and hydrophilicity of the ionic solutes is found, which underlines the sensitivity of thermodiffusion to changes in hydration. Based on this sensitivity, a preliminary model is developed for describing the non-monotonous variation of Soret coefficient ST with concentration for aqueous solutions of alkali iodide salts. To study the thermodiffusion of binding reactions, we also use complementary methods such as Isothermal Titration Calorimetry (ITC) and a thermophoretic microfluidic cell. As systems, we have chosen EDTA-CaCl2 and protein-ligand systems (binding of Bovine Carbonic Anhydrase I (BCA I) with two aryl sulfonamide ligands). To gain deeper insight into the complex formation reactions thermophoretic data (non-equilibrium process) are compared with thermodynamic data (equilibrium process) to establish a mathematical relation between ST and Gibb’s free energy ΔG. For EDTA-CaCl2 and protein-ligand systems, the derived relation holds valid, which enables calculation of ΔG at a particular temperature from ST.

Thermal gradients induce a motion of solute molecules towards hot or cold regions. The motion of solutes in a solvent induced by a thermal gradient is termed thermodiffusion. Soret coefficient is used to quantify the ratio of the established concentration gradient to the thermal gradient. This has become an important tool to monitor protein-ligand binding , as it is very sensitive to the nature of solute-water interactions. Buffers, which are used to stabilize the proteins, are multicomponent mixtures with different kinds of salts. Hence to obtain a better understanding of protein-ligand systems, the first step is the investigation of salt solutions. We investigated aqueous solutions of five potassium salts: potassium chloride, potassium bromide, potassium thiocyanate, potassium acetate and potassium carbonate where the corresponding anions cover the full Hofmeister range. We study the thermophoresis of all salt solutions between 15 to 45oC in a molality range between 1 – 5 mol/kg using thermal diffusion forced Rayleigh scattering. We examine, in particular, whether the correlations found for non-ionic solutes are carried over to ionic solutes. Our results are discussed in the context with the ionic and hydrogen bonding contributions to thermophoresis. This study shows how sensitive the thermophoretic phenomenon is to the ion species and its position in the Hofmeister series and how a change of the ion influences thermophoretic properties.

Thermal gradients induce a motion of solute molecules towards hot or cold regions. The motion of solutes in a solvent inducedby thermal gradient is termed as thermodiffusion. Soret coefficient ST is used to quantify the ratio of the established concentrationgradient to the thermal gradient. This phenomenon being very sensitive to the nature of solute-solvent interactions , hasgained a lot of interest in the recent years, especially in the context of biological interactions. Many solutions of interest inthermodiffusion or thermophoresis involve aqueous solutions with ions . To identify the underlying principles, we systematicallyinvestigate the thermodiffusion of aqueous alkali halide solutions. There have been previous reports where the behavior of certainaqueous halide salts under thermal gradient have been studied and for some salts a minimum being observed in the ST with saltconcentration . In this work, we measured the thermodiffusion behavior of salts (KCl, KBr, KI, NaI and LiI) in water in atemperature range of 15-45o C, with concentration being varied from 0.5-4 mol/kg by infrared thermal diffusion forced Rayleighscattering (IR-TDFRS). We find for the iodide salts a minimum of ST with concentration, while KCl and KBr show monotonousbehavior. We will discuss the temperature and concentration dependence of ST using the knowledge how different salts distortthe hydration layer. Furthermore, our systematic studies are discussed in context of recent simulations . These results willhelp to obtain a better understanding of the thermophoretic response of protein ligand systems typically studied in buffer solutionscontaining alkali halides to maintain the stability.