At isoelectric pH, proteins:

Zwitterions  are  one  example  of  an  isoelectric  species—the form of a molecule that has an equal number of positive and negative charges and thus is electrically neutral. The isoelectric pH, also called the pI, is the pH midway between pKa  values for the ionizations on either side of the isoelectric species. The pKa  values of all functional groups of an amino acid or of a peptide dictate its net charge at a given pH. pI, the isoelectric pH, is the pH at which an amino acid bears no net charge, and thus does not move in a direct current electrical field. Harper's Illustrated Biochemistry 30 th edition page no 20, 24