The isoelectric point pI of a peptide is the pH at which the peptide doesn't migrate in an electric field. Since the peptide is zwitterionic, there are the same numbers of positive charges as negative charges on the peptide population. The pI can be estimated fairly accurately within 0.1 or 0.2 pH units from the pK values of all the proton dissociable groups in the peptide. Utilizing pK values from the table at the right approximate the pI value of the following hexapeptide
Trp-His-Glu-Tyr-Gly-Asp