. The pI is called . The rule of calculating pI of an amino acid is that first, write the dissociation equation from fully protonated form to fully deprotonated form, label the charge of each form; second, identify the zwitterionic form (zero charge) and find the closest pKs (left and right side in the dissociation equation); third, average these two pKs. Write the dissociation equations for amino acids, glutamate, histidine, and calculate their pIs.