What happens to proteins in a cation-exchange column at a pH below their isoelectric point?

In a cation-exchange chromatography system, proteins are separated based on their net charge at a specific pH. The isoelectric point (pI) is the pH at which a protein carries no net charge. When the pH of the cation-exchange column is below the pI of the protein, the protein carries a net positive charge.

At this lower pH, the positively charged protein is attracted to the negatively charged groups in the cation-exchange resin. This interaction allows the protein to bind to the column. The nature of the cation-exchange mechanism relies on this attraction, where the cationic (positively charged) proteins will adhere to the negatively charged stationary phase, allowing for separation from other proteins based on their charge characteristics.

As a result, a protein in a cation-exchange column at a pH below its isoelectric point effectively binds to the column because its positive charges enable it to interact favorably with the negative charges present in the resin. This step is critical for isolating and analyzing proteins based on their charge properties.