Interaction of Lysozyme with Sulfated β-Cyclodextrin: Dissecting Salt and Hydration Contributions

This article investigates the thermodynamic driving force of the interaction between lysozyme (Lys) and sulfated β-cyclodextrin (β-CDS), with a particular emphasis on the elusive role of hydration during polyelectrolyte-protein binding. Using isothermal titration calorimetry (ITC), the binding affinity was quantified across varying temperatures and salt concentrations, employing a recently developed thermodynamic framework that explicitly separates the contributions from counterion release and hydration effects. The study reveals that while counterion release is minimal in the Lys/β-CDS system, hydration effects become a dominant factor influencing the binding free energy ΔG_b, especially as experimental temperature deviates from the characteristic temperature T₀. It demonstrates that hydration contributions can substantially weaken binding at increased salt concentration c_s. The high characteristic temperature T₀ and the salt-dependent heat capacity change indicate a complex interplay of water structure and ion association-significantly departing from commonly linear interpretations of ΔG_b vs. log c_s based solely on counterion release effects. This work advances the understanding of polyelectrolyte-protein interactions by providing the first direct quantification of the hydration effect in such complexes and may have an impact on the rational design of biomolecular assemblies and therapeutic carriers.