... system. When a solution of proteins is subjected to an electrical potential their net charge will cause the molecules to migrate towards the positive electrode (anode) or the negative electrode (cathode.) The direction in which they move depends upon the proteins net charge. If the protein molecule has a positive charge (cations), it will move towards the negative electrode (cathode), if however the protein molecule is negatively charged (anion) it will migrate towards the positive electrode (anode.) Each amino acid has an isoelectric point (pI) this is the point at which the molecule has an average net charge of zero, this is the pH level at which there is no migration in an electric field. A zwitterion is an amino acid which contains both negatively charged and positively charged groups (see fig. 2.) As they are electrically neutral there is no migration in an electric field. Fig 2. A Zwitterion The pI ...

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