Thermodynamic complexation mechanism of zinc ion with 8-hydroxyquinoline-5-sulfonic acid in molecular crowding environment

Abstract

A crowded molecular environment, which is universally observed in biological cells, changes the thermodynamics and kinetics of biochemical reactions compared to bulk solution. Although molecular crowding has received significant attention, a fundamental explanation of the molecular crowding effect based on thermodynamic parameters, including enthalpy (ΔH) and entropy changes (ΔS), is insufficient. Herein, the complexation mechanism of zinc ions with quinolinol derivative in a molecular crowding environment was investigated using polyethylene glycol (PEG) based on the changes in ΔH and ΔS. For 1:1 complexation, which is only promoted by the osmotic pressure effect, ΔH and ΔS decreased with increasing PEG concentration (CPEG) because the number of dehydration molecules decreased. In the 1:2 complexation system, which is affected by the osmotic pressure and volume exclusion effects, no obvious dependences of ΔH and ΔS on CPEG were observed. This is because the osmotic pressure and volume exclusion effects contributed oppositely to ΔH and ΔS. The results presented herein provide fundamental knowledge for protein-metal interactions, and it is expected that these data will attract the attention of many physical chemists and biochemists.