Buffer selection is an important part of the formula-tion process, because of the pH dependence of protein solubility and physical and chemical stability. Buffer systems regularly encountered in biotech formula-tions are phosphate, citrate, and acetate. A good example of the importance of the isoelectric point (its negative logarithm is equal to pI) is the solubility profile of human growth hormone (hGH, pI around 5) as presented in Figure 4.
Even short, temporary pH changes can cause aggregation. These conditions can occur, for example during the freezing step in a freeze-drying process, when one of the buffer components is crystallizing and the other is not. In a phosphate buffer, Na2HPO4 crystallizes faster than NaH2PO4. This causes a pronounced drop in pH during the freezing step. Other buffer components do not crystallize, but form amorphous systems and then pH changes are minimized.