each ionizable group of an amino acid can exist in one of two states, charged or neutral. the electric charge on the functional group is determined by the relationship between its pka and the ph of the solution. this relationship is described by the henderson–hasselbalch equation. histidine has three ionizable functional groups. complete the equilibrium equations by assigning the proper pka for each ionization and the net charge on the histidine molecule in each ionization state.