Protein Chemistry: Structure, Properties, and Research Methods - Shendryk, A.N. 2022

Protein Structure
Behavior of Proteins in Solutions
Protein solubility and its determining factors - Dependence of protein solubility on medium pH

Protein solubility is significantly influenced by several factors, the primary ones being:

> solution pH;

> Ionic strength of the medium;

> dielectric Properties of the solvent;

> Temperature.

Let us examine specific experimental data on the solubility of native ß-lactoglobulin as a function of pH at various NaCl concentrations (see Fig. 2.7).

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Fig. 2.7 Solubility of ß-lactoglobulin as a function of pH at various NaCl concentrations, T = 25∘C. NaCl concentrations are indicated on the curves

As can be seen from the figure, regardless of the salt content in the solution, the minimum protein solubility occurs in the pH range around 5.2 - 5.3. On both sides of this point, solubility increases rapidly. The pH value at which the solubility of ß-lactoglobulin is minimal coincides with its isoelectric point. This feature is practically a rule for Globular Proteins. At the isoelectric point, protein molecules carry no net charge and can readily aggregate into a solid phase. At pH values differing from pI, charges appear on the molecules, which prevent their "sticking together".

In the absence of salts, certain proteins are virtually insoluble at pH = pI. This property is frequently utilized to separate protein mixtures with different pI values. This Separation technique is known as isoelectric precipitation. By varying the pH, one can sequentially precipitate individual proteins from a mixture at their respective pH = pI points.