Question

You have the following mixture of proteins: Protein A pI=4.3 Protein B pI=8.1 Protein C pI=6.8 The mixture of proteins is in a buffer at pH=7.8. You apply the mixture to a cation exchange column. Which protein(s) elute (do not bind to the column)?

I believe A and C will elute because they are negatively charged. Not sure if i am correct or not. Explanation Please!

Answer 1

The corret answer is proteins A and C will elute that is they don't bind to the column.

The buffer pH (7.8) is above the pI value of protein A (4.3) and protein C (6.8), so protein A and C will get negative charge and protein B will get positive charge. In cation exchange column, the column resin will have negatively charged ions and bind positively charged proteins. So, protein A and C can't bind to column and elute out whereas protein B will bind to the column.