Macromolecular Crowding Modifies the Impact of Specific Hofmeister Ions on the Coil–Globule Transition of PNIPAM

Macromolecular crowding alters many biological processes ranging from protein folding and enzyme reactions in vivo to the precipitation and crystallization of proteins in vitro. Herein, we have investigated the effect of specific monovalent Hofmeister salts (NaH2PO4, NaF, NaCl, NaClO4, and NaSCN) on the coil–globule transition of poly­(N-isopropylacrylamide) (PNIPAM) in a crowded macromolecular environment as a model for understanding the specific-ion effect on the solubility and stability of proteins in a crowded macromolecular environment. It was found that although the salts (NaH2PO4, NaF, and NaCl) and the macromolecular crowder (polyethylene glycol) lowered the transition temperature almost independently, the macromolecular crowder had a great impact on the transition temperature in the case of the chaotropes (NaClO4 and NaSCN). The electrostatic repulsion between the chaotropic anions (ClO4 – or SCN–) adsorbed on PNIPAM may reduce the entropic gain of water associated with the excluded volume effect, leading to an increase in the transition temperature, especially in the crowded environment. Furthermore, the affinity of the chaotropic anions for PNIPAM becomes small in the crowded environment, leading to further modification of the transition temperature. Thus, we have revealed that macromolecular crowding alters the effect of specific Hofmeister ions on the coil–globule transition of PNIPAM.