Sickle-cell anemia is a genetic defect whereby a person's red blood cell assumes a sickle-cell shape. This genetic defect involves the substitution of valine for glutamic acid at position six of the β-globin chain of hemoglobin. To understand how this can produce a defective red blood cell, you must be able to recognize the structural differences between valine and glutamic acid. How does the structural difference between valine and glutamic lead to the improper folding of the hemoglobin protein?