The graph shows the titration curve for a protein containing 124 amino acids with 16 basic and 20 acidic substituents. The curve is smooth without clear breaks because 29 groups are titrated in the pH interval shown. The 29 end points are so close together that a nearly uniform rise results. The isoionic point is the pH of the pure protein with no ions present except H⁺ and OH^-. The isoelectric point is the pH at which the average charge on the protein is zero. Is the average charge of the protein positive, negative, or neutral at its isoionic point? How do you know?

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12 10 - Isoelectric point Isoionic point 6 2 15 10 5 10 H* added OH added per molecule | per molecule 4. Hd