Proteins are amphoteric in nature; that is, they have both acidic and basic properties. In the electric field, their migration depends upon the net charge carried by the molecule.

The net charge is influenced by the pH value. Each protein has a specific pI value at which it will not move, because at this point the net charge of the protein is always zero. However, the total charge of the protein at this point is always maximum. Thus, proteins exist as dipolar ions, internal salts, or zwitterions at pI and are present in solution as

(+ H₃N)_m—R—(COO^—)_n