Proteins are commonly purified by ion exchange chromatography (IEC) as a final step. Which of the
following statements is NOT true?
1. above the isoelectric point, the proteins bind to cation exchangers
2. even proteins of similar isoelectric point can be conveniently separated by IEC, Because interaction with
the support is determined by the surface charge distribution of the protein rather than the net charge.
3. in general, proteins can be eluted by increasing ionic strength
4. above the isoelectric point, the proteins bind to anion exchangers.

 What is Ion Exchange Chromatography (IEC)?

Ion exchange chromatography (IEC) is a powerful technique used in protein purification, especially as a final polishing step. It separates proteins based on their net surface charge by using charged resin beads:

• Cation exchangers carry a negative charge and bind positively charged proteins.

• Anion exchangers carry a positive charge and bind negatively charged proteins.

 Isoelectric Point and Protein Charge

The isoelectric point (pI) is the pH at which a protein has no net charge. Depending on the pH of the buffer used in IEC:

• Below the pI, proteins are positively charged → bind to cation exchangers.

• Above the pI, proteins are negatively charged → bind to anion exchangers.

Therefore, statement 1 is NOT true:

“Above the isoelectric point, the proteins bind to cation exchangers”
This is incorrect because proteins above their pI are negatively charged and bind to anion exchangers, not cation exchangers.

 Why the Other Statements Are True

• Statement 2:

Even proteins with similar pI values can behave differently on an ion exchanger due to differences in surface charge distribution — this affects how they interact with the column matrix.

• Statement 3:

Proteins are commonly eluted by increasing the ionic strength (e.g., salt concentration), which competes with the protein for binding sites on the resin.

• Statement 4:

As discussed, proteins above their pI are negatively charged and bind to anion exchangers.

 Conclusion

In ion exchange chromatography, understanding the relationship between pH and protein charge is crucial. The wrong assumption that negatively charged proteins bind to cation exchangers above their pI can lead to ineffective purification strategies.