Ion Specificity and Nonmonotonic Protein Solubility from Salt Entropy

Area of Science:

• Biophysical Chemistry
• Protein Science
• Solution Chemistry

Background:

• Protein solubility is modulated by salt concentration, a phenomenon known as salting out or salting in.
• The specific salt used significantly influences the magnitude and direction of this effect, as described by the Hofmeister series.

Purpose of the Study:

• To develop a theoretical framework that captures the dual effects of salt on protein solubility.
• To elucidate the role of electrostatic interactions, protein-ion interactions, and ion-solvent interactions in determining salt specificity.
• To explain observed phenomena like the reversal of the Hofmeister series and to predict protein solubility.

Main Methods:

• A theoretical model incorporating electrostatic interactions, nonelectrostatic protein-ion interactions, and ion-solvent interactions.
• Inclusion of an effective solvated ion radius to account for ion-solvent interactions and their impact on translational entropy.
• Application of the theory to explain lysozyme cloud point measurements and solubility of protein crystals.

Main Results:

• The effective solvated ion radius significantly impacts salt's translational entropy, leading to salt-specific effects on protein solubility.
• At low salt concentrations, the entropic cost of ion confinement dominates; at high concentrations, depletion attraction favors aggregation.
• The theory successfully explains the reversal of the Hofmeister series for lysozyme and semi-quantitatively describes the solubility of lysozyme and chymosin crystals.

Conclusions:

• A unified theory explains both salting in and salting out phenomena by considering electrostatic and ion-specific interactions.
• Ion size and its entropic contribution are critical for understanding salt specificity in protein solubility.
• The developed model provides guidelines for predicting salting in or salting out effects based on salt properties and concentration.