Normal hemoglobin is a tetramer, consisting of two molecules of β-globin and two molecules of α-globin; normal hemoglobin molecules do not associate with each other. in sickle-cell disease, the change in a single amino acid results in mutant hemoglobin tetramers, which associate with each other and assemble into large fibers. based on this information alone, what can we conclude about the changes in the structure of sickle cell hemoglobin as compared to normal hemoglobin?