In paper electrophoresis, amino acids and peptides can be separated by their differential migration in an electric field. To the center of a strip of paper is applied a mixture of the following three peptides in a single small spot: Gly-Lys, Gly-Asp, and Gly-Ala. The paper is soaked in a pH = 6 buffer, a positively charged electrode (anode) is attached to the left side of the paper, and a negatively charged electrode (cathode) is attached to the right side. A voltage is applied across the ends of the paper for a time, after which the peptides have separated into three spots: one near the cathode, one near the anode, and one in the center, at the location of the original spot. Which peptide is in each spot? Explain.