All proteins with isoelectric point above pH 7.0 will have positive charge and move towards negative electrode. As they move towards negative electrode due to pH increase (due to pH gradient) net positive charge on protein would decrease. When protein reaches pH which equals isolectric point of protein, net charge on the protein becomes zero and electrophoretic mobility becomes nil (protein remains confined near isoelectric point in the pH gradient.

All proteins with isoelectric point below 7.0 will have negative charge and move towards positive electrode. As they move towards positive electrode there is a decrease in the net negative charge on the protein due to pH decrease ( due to pH gradient). When protein reaches pH which equals isolectric point of protein, net charge on the protein becomes zero and electrophoretic mobility becomes nil (protein remains confined near isoelectric point in the pH gradient). Please refer to Fig. 1 for detail. This process of separating proteins based on isoelectric point is called isoelectric focusing.

Figure 1: Process of separating proteins based on isoelectric point