Electrostatic interaction may cause the molecules of a particular protein to self-associate at the isoelectric pH, called the pI (Figure 2.21). For example, the common milk protein -lactoglobulin is a polyampholyte with an isoelectric point of about 5.3.

Above or below this pH, the molecules all have either negative or positive charges and repel one another. This protein is therefore very soluble at either acidic or basic pH. At the isoelectric point the net charge is zero, but each molecule still carries surface patches of both positive and negative charge. The charge-charge interactions, together with other kinds of intermolecular interactions such as van der Waals forces, make the molecules tend to clump together and precipitate. Therefore, -lactoglobulin, like many other proteins, has minimum solubility at its isoelectric point (Figure 2.21d).