At the isoelectric point how does ionic strength affect protein solubility?

[Brisket]

At the isoelectric point, protein have no net charges and therefore will be less soluble. Why does increasing ionic strength result in an increase in solubility? Do the ions form a solvation shell?

 

Also when salting out the ionic strength of the solution is increase (salt = more ions) and this results in a precipitate. The ions disrupt the protein/solvent interaction and cause the protein to aggregate and therefore precipitate out.

 

Can someone please help explain this concept of protein solubility with ionic strength at isoelectric point. Thanks

[CharonY]

This is actually a bit complicated as it is not only a single principle applied here. As you mentioned, at high ionic strengths you will salt out the proteins.

If you just look at the zeta potential as a function of pH you will see a) the pI shifts and the potential curve tends to flatten out (i.e. aggregation becomes more likely).

 

What you describe at the pI is usually in the context of IEF which utilizes much lower concentrations. The major aspect at low salt concentration is best explained by Debye-Hückel , though precise models are much more complex and suffers from the lack of data on protein surface properties and have troubles integrating ion specificity.

 

It is easiest to think of proteins of having patches with different surface charges, even if the net charge is 0 at pI. It is the assumed that electrostatic attractive interactions are the dominant effects (at low salt concentrations) and depending on the types and properties of patches various small amount of anions or cations assist in salting in.

One can imagine that the ions act in concert to disrupt protein-protein interactions at low concentration, resulting in increased solubility. At higher concentration this effect reverts.