Charge on a Protein

The charge on the protein affects its behavior in ion exchange chromatography. Proteins contain many ionizable groups on the side chains of their amino acids as well as their amino - and carboxyl - termini. These include basic groups on the side chains of lysine, arginine and histidine and acidic groups on the side chains or glutamate, aspartate, cysteine and tyrosine. The pH of the solution, the pK of the side chain and the side chainís environment influence the charge on each side chain. The relationship between pH, pK and charge for individual amino acids can be described by the Henderson-Hasselbalch equation, which is described in detail elsewhere:

In general terms, as the pH of a solution increases, deprotonation of the acidic and basic groups on proteins occur, so that carboxyl groups are converted to carboxylate anions (R-COOH to R-COO-) and ammonium groups are converted to amino groups (R-NH3+ to R-NH2). In proteins the isoelectric point (pI) is defined as the pH at which a protein has no net charge. When the pH > pI, a protein has a net negative charge and when the pH < pI, a protein has a net positive charge. The pI varies for different proteins.