Like the derivation for glycine, the process can be applied to the amphoteric zwitterionic species H₂A, and it can be shown that the pI of aspartic acid is the average of the pKa values for the two ionizable groups involved in the protonation and deprotonation of H₂A. These groups are the -COOH (pK a 1 ) and the -COOH (pK a 2 ).