Question

How can one extract a negatively charged protein from a cation exchange column other than using NaCl?

Answer 1

In cation exchange chromatography, the stationary phase consists of negatively charged resin. The positively charged proteins get attracted to the negatively charged resin and are retained on the stationary phase. The retained proteins are eluted from the column by using either the salts or buffer system. The salt in water produces cations which replace the proteins attached to the resin and allow its elution.

The buffer system acts differently. Proteins have a net surface charge e.g. positively charged protein that can change with pH depending on the protein’s isoelectric point (pI). Thus, the elution using the buffer system takes advantage of this property. The buffer system pH is gradually changed. As the pH becomes equal to the isoelectric point of the protein, the net charge on the protein becomes zero. Thus, the protein separates from the stationary phase and elutes from the column. The gradual change in pH results in gradual elution of proteins. The commonly used buffers for the cation exchange chromatography are phosphate buffers, HEPES buffer, citric acid buffer, malonic acid buffer, MOPS buffer etc.