Question

You are attempting to purify a protein that has a pI of 9.0 from a mixture of similarly sized proteins, and have access to different purification columns. For this protein, you would select as your first column for purification an:

(a) Anion exchange

(b) Cation exchange

(c) Gel filtration

(d) Heparin sulfate

(e) Ammonium sulfate

Answer 1

as the pH of a solution increases, deprotonation of the acidic and basic groups on proteins occur, so that carboxyl groups are converted to carboxylate anions (R-COOH to R-COO-) and ammonium groups are converted to amino groups (R-NH3+ to R-NH2). In proteins the isoelectric point (pI) is defined as the pH at which a protein has no net charge. When the pH > pI, a protein has a net negative charge and when the pH < pI, a protein has a net positive charge. The pI varies from protein to protein. This is the reason ion exchange is useful for separating proteins. If a buffer containing more than one protein is used with an anion exchange resin, the the most negatively-charged protein will be most attracted to the stationary phase and will therefore elute last and the protein with the highest positive charge will elute first.

Thus Answer is a) Anion Exchange