Ion-Exchange Chromatography and Protein Charge Separation

The chromatography technique that separates proteins based on the sign and magnitude of their net electric charges at a specific pH is Ion-exchange chromatography.

Principle of Separation

• Ion-exchange chromatography utilizes a stationary phase containing charged groups.
• Proteins in a sample, which possess different net charges depending on the pH of the buffer, interact with the charged stationary phase.
• Proteins with a net charge opposite to that of the stationary phase bind to it.
• Proteins with the same net charge or no net charge pass through the column.
• Separation is achieved by eluting the bound proteins using a gradient of increasing ionic strength or changing the pH, which disrupts the electrostatic interactions.

Why Other Options Are Incorrect

• Size exclusion chromatography separates molecules based on their size and shape (hydrodynamic volume), not charge.
• Affinity chromatography separates molecules based on specific reversible binding interactions between a ligand immobilized on the stationary phase and a specific protein.
• Isoelectric focusing separates molecules based on their isoelectric point (pI), the pH at which a molecule has no net electrical charge. While charge is involved, it specifically separates based on the pH at which net charge is zero, not the separation based on varying net charges at a given pH.

Therefore, ion-exchange chromatography is the method that directly separates based on the sign and magnitude of net electric charges at a given pH.