Electrophoretic separation of proteins via complexation with a   polyelectrolyte
  We suggest to augment standard isoelectronic focusing for separation of proteins in a gradient of pH by a similar focusing in the presence of a strongly charged polyelectrolyte (PE). Proteins which have the same isoelectric point but different "hidden" charge of both signs in pI point make complexes with PE, which focus in different pH. This is a result of charge inversion of such proteins by adsorbed PE molecules, which is sensitive to the hidden charge. Hence the hidden charge is a new separation parameter. 
E. M. Baskin, B. I. Shklovskii and G. V. Zilberstein