Why do positively charged proteins move toward the cathode during electrophoresis?

In electrophoresis, molecules migrate based on their electric charge when placed in an electric field. The movement of proteins toward the cathode—the negative terminal—is governed by the fundamental principle of electrostatics: opposite charges attract.

Because the cathode carries a negative charge, it exerts an attractive force on any protein molecule that possesses a net positive charge. Proteins exhibit this behavior primarily at low pH values, where their side chains become protonated, resulting in a net positive charge that drives them to migrate toward the negative terminal. Conversely, at high pH values, proteins typically carry a net negative charge and migrate toward the anode, or positive terminal.1