What is the isoelectric point of histidine?

The isoelectric point (pI) of an amino acid is the pH at which the molecule carries no net electric charge. For histidine, which has an imidazole side chain, the pI is particularly significant due to the unique properties of its side group that can be protonated or deprotonated depending on the pH of the environment.

Histidine has three ionizable groups: the alpha-carboxyl group, the alpha-amino group, and the side chain's imidazole group. The pKa values for these groups are approximately 2.1 for the carboxyl group, 9.1 for the amino group, and around 6.0 for the imidazole side chain. The isoelectric point is calculated by averaging the pKa values of the two ionizable groups that are protonated at the isoelectric state.

Given the pKa of the imidazole side chain, the pI can be calculated by taking the average of the pKa of the side chain (approximately 6.0) and the pKa of the amino group (around 9.1). This evaluation leads to a pI around 7.59. Thus, the isoelectric point