Solubility, Hydration and Swelling Power

Protein solubility is variable and is influenced by the number of polar and apolar groups and their arrangement along the molecule. Generally, proteins are soluble only in strongly 

At low ionic strengths, the solubility rises with increase in ionic strength and the solubility minimum (isoelectric point) is shifted from pH 5.4 to pH 5.2. This shift is due to preferential binding of anions to the protein. 

If a protein has enough exposed hydrophobic groups at the isoelectric point, it aggregates due to the lack of electrostatic repulsion via intermolecular hydrophobic bonds, and (isoelectric) precipitation will occur. If on the other hand, intermolecular hydrophobic interactions are only poorly developed, a protein will remain in solution even at the isoelectric point, due to hydration and steric repulsion. 

As a rule, neutral salts have a two-fold effect on protein solubility. At low concentrations they increase the solubility ( salting in effect) by suppressing the electrostatic protein-protein interaction (binding forces). 

Protein solubility is decreased ( salting out effect) at higher salt concentrations due to the ion hydration tendency of the salts. The following relationship applies (Sq solubility at p = 0 K salting out constant)  

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