Acid Dissociation Behavior of 8-Hydroxyquinoline-5-Sulfonic Acid in Molecular Crowding Environment Modeled Using Polyethylene Glycol

Abstract

• Revealing the molecular crowding effect on the acid dissociation (AD) in PEG solution. • The p K a value depended on the concentration and the molecular weight of PEG. • Δ H and Δ S of AD in PEG solution were obtained. • The AD in PEG solution was caused by the dehydration of dissociated species. The thermodynamics and kinetics of biomolecular reactions in a molecular crowding environment are different from those in dilute solutions. Although biomolecular reactions in crowding solutions have received extensive attention from many researchers, fundamental thermodynamic reactions, such as acid dissociation, are not yet understood. Herein, we investigated the thermodynamic behavior of acid dissociation of 8-hydroxyquinolin-5-sulfonic acid in the molecular crowding. Two acid dissociation constants, K a1 and K a2 , in polyethylene glycol (PEG) of different molecular weights were determined by pH titration. The K a change was analyzed based on the change in osmotic pressure. The temperature dependences of K a1 and K a2 allowed us to obtain the enthalpy (Δ H ) and entropy changes (Δ S ) . The Δ H and Δ S values suggest that dehydration of the dissociated species was induced by the decrease in water activity upon the addition of PEG. The results show that the acid dissociation in the PEG solution can be explained by dehydration.