Proteomics/Protein Separations - Chromatography/Anion exchangers

Anion Exchangers
While the resin involved with an anion exchanger is positively charged, anion exchangers are named so because of their affinity to anions. To effectively bind proteins, the pH of the buffer in the system must be greater than the isoelectric point of the protein of interest, as proteins are negatively charged above their isoelectric point.

Anion exchangers can be classified as either weak or strong. The charged group on a weak exchanger is a weak base that easily loses its charge at a high pH due to deprotonation. On the other hand, strong anion exchangers are comprised of a charged group that is a strong base. Strong anion exchangers are able to maintain their positive charges across a variable pH range while weak anion exchangers tend to lose their charge as the pH increases. Examples of anion exchangers include the strong anion exchanger Q (quaternary resin), and the weak anion exchanger DEAE (diethylaminoethane).

Typically, the chromatography is performed using buffers at pH's between 7 and 10 and running a gradient from a solution containing just this buffer to a solution containing this buffer with 1M NaCl (Res1). The salt (in solution) competes for binding to the stationary phase, thus releasing the protein from its bound state. Proteins separate because the amount of salt needed to compete varies with the external charge of the protein. Anion exchange chromatography have been successfully used to clean up of a crude slurry, separation of proteins from each other, concentrating a protein, and the removal of negatively charged endotoxin from protein preparations (Res1).

Resources

 * 1) Proteinchemist.com Ion Exchange Chromatography (IEC)
 * 2) Whatman Advanced Ion Exchange Cellulose and Column Chromatography