Isoelectric pH.

• It is the pH at which biomolecules exist as dipolar ion possessing same amount of positive and negative charge on their surface with net zero.
• Biomolecules contain different ionizable groups on their surface.
• Depending on the pH of solution these ionizable groups act either as proton donor or as proton acceptor and thereby express charge on their surface.
• At one definite pH called isoelectric pH, every biomolecule expresses both the charges on its surface in equal amount and exists as zwitter ion, when the net charge on the surface of biomolecule is zero because positive and negative charge equalize each other. So zwitter ions are electrically neutral without any electrohoretic mobility during electrophoresis.
• At pH below the isoelectric pH biomolecules accept protons from the medium and exist as cations with positive charge on their surface and at pH above the isoelectric pH biomolecules release proton to the medium and exist as anions with negative charge on their surface.