Effects of Salt or Cosolvent Addition on Thermal Stability of a Protein: Relevance to those on Solubility of a Hydrophobic Solute in Water

Abstract

The thermal stability of a protein is changed upon addition of a salt or cosolvent. The solubility of a hydrophobic solute (e.g., argon or methane) in water is also influenced by the addition. Interestingly, the addition which decreases the solubility usually enhances the thermal stability. This suggests that the hydrophobic effect is a principal factor governing the stability change, because the decrease and increase in the solubility, respectively, are ascribed to enhancement and reduction of the effect. However, urea decreases the solubility but lowers the stability. Bromide and iodide ions decrease the solubility but lower the stability of a protein with a large, positive net charge. In these cases, the stability change is influenced by the changes in not only the hydrophobic effect but also other physical factors. We show for hydrophobic solutes that the integral equation theory where the solute and solvent particles are modeled as hard spheres with different diameters can reproduce the experimental data for the following items: salting out by an alkali halide and salting in by tetramethylammonium bromide, increase in solubility by a monohydric alcohol, and decrease in solubility by sucrose or urea. The orders of cation or anion species in terms of the power of decreasing the solubility can also be reproduced for alkali halides. With our model, the analyses are focused on the roles of entropy originating from the translational displacement of solvent particles. As the products, we clarify the pivotal physical origin of the hydrophobic effect and present a new view on the Hofmeister series. We show how the series is expressed when the hydrophobic effect dominates and how it is modified when other physical factors are also influential.