Proteins become insoluble at high salt concentrations primarily because the salt ions compete for water molecules needed to keep the proteins dissolved.
Understanding Salting Out
The phenomenon where proteins precipitate out of solution at high salt concentrations is known as salting out. This process is the opposite of "salting in," where low salt concentrations can actually increase protein solubility. At high salt levels, the interactions between the protein molecules and the solvent (water) are disrupted, leading to aggregation and precipitation of the protein.
The Role of Salt Ions
According to explanations, including those by John Gamble Kirkwood, the behavior of proteins in high salt solutions is related to the interaction between the salt ions and the solvent.
- Competition for Water: Salt ions, such as Na⁺ and Cl⁻, are highly charged and attract a significant number of water molecules to form hydration shells around them.
- Decreased Solvation Power: At high salt concentrations, there is an abundance of these salt ions. This high concentration means many water molecules are tied up hydrating the salt ions.
- Impact on Proteins: As the reference notes, this abundance of salt ions decreases the solvating power of the solution for the proteins. Fewer free water molecules are available to hydrate the protein surface.
This reduced hydration weakens the favorable protein-solvent interactions.
From Soluble to Insoluble
When protein-solvent interactions become weaker than protein-protein interactions (such as hydrophobic interactions between protein molecules), the proteins begin to associate with each other.
- Proteins lose their surrounding water shell.
- Hydrophobic patches on protein surfaces, normally shielded by water, are exposed.
- Proteins aggregate together via hydrophobic and other interactions.
- These large aggregates become too heavy to remain suspended and precipitate out of the solution, making the protein insoluble.
The reference clearly states that "the abundance of the salt ions decreases the solvating power of salt ions, resulting in the decrease in the solubility of the proteins and precipitation results." This highlights the direct link between the high salt concentration, the change in the solvent's properties, and the loss of protein solubility.
Practical Applications of Salting Out
Salting out is a widely used technique in biochemistry and protein purification.
- Protein Fractionation: By gradually increasing the salt concentration, different proteins can be selectively precipitated from a mixture based on their varying solubilities. This allows for the separation and purification of specific proteins. Ammonium sulfate is a commonly used salt for this purpose due to its high solubility and minimal denaturation effects on proteins.
- Concentration: Salting out can be used to concentrate a dilute protein solution before further purification steps.
- Storage: High salt conditions can sometimes be used to store protein precipitates, helping to stabilize them.
| Salt Concentration | Effect on Protein Solubility | Process Name |
|---|---|---|
| Low | Can increase solubility | Salting In |
| High | Decreases solubility, causes precipitation | Salting Out |
Understanding why proteins become insoluble in high salt, as explained by the mechanism involving salt ions reducing the solvating power of water, is fundamental to many laboratory techniques involving proteins.
