A Zwitterion is formed when an amino acid is at its isoelectric point. Most Zwitterions have no net charge, therefore will not migrate in a charged field. This is because they accept a proton (H+) at their N-terminus, and lose a proton at their C-terminus. However, it is possible for the R-group of an amino acid to interact in the same way, which can give the Zwitterion a net charge eg. if the R-group contains a carboxyl group (COOH) it may also donate the H+ from this group, creating a net charge of +1.