Energetic parameters of β-casein/quercetin activated and thermodynamically stable complex formation accessed by Surface Plasmon Resonance

Characterizing the energetics and molecular dynamics of binding between proteins and bioactive compounds is strategic. Using surface plasmon resonance, we demonstrated that β-casein (β-cas) and quercetin (Qct) form supramolecular complexes driven by an increase in entropy (ΔH° = 25.86 and TΔS° =53.49 kJ∙mol^-1 at 25 °C). It was possible to infer that the β-cas/Qct complex was formed via an activated complex synthesized by an entropic reduction (TΔS^‡_(a)= -15.31 kJ mol^-1 and TΔS^‡_(d)= -68.80 kJ mol^-1 at 25 °C) and an enthalpic increase (ΔH^‡_(a) = 30.87 and ΔH^‡_(d) =5.0 kJ∙mol^-1 at 25 °C). Independent of the nature of the Hofmeister ions, the salts KCl or KSCN increased complex stability by decreasing both the kinetic and thermodynamic enthalpy values, through shielding of the electrostatic interactions at the electric double layer of the interacting molecules. An increase in temperature favored both the association of the free interacting molecules and the dissociation of the thermodynamically stable β-cas/Qct complexes. These results provide insights into the β-cas/Qct interaction process and contribute to the understanding of how Hofmeister ions can modulate intermolecular interactions between proteins and small molecules.