1. What are the principles and the purpose of using differential salt precipitation in protein purification?
2. 6-phosphogluconate dehydrogenase has a pI of 6. Explain why the buffer used for a chromatography on DEAE-cellulose must have a pH greater than 6 but less than 9 in order to ensure the enzyme is efficiently bound to the column?
3. We load a DEAE-cellulose column adjusted to a pH of 6.5 with the following mixture of proteins: ovalbumin (pI = 4.6), urease (pI = 5.0), and myoglobin (pI = 7.0). The proteins are eluted first with a buffer of weak ionic strength at pH of 6.5, and then the same buffer containing increasing amounts of sodium chloride is used to elute the proteins. What order are the proteins eluted?