Why does sickle-cell hemoglobin (hbs) migrate slower than normal hemoglobin (hba) during gel electrophoresis?

The cause a translocation of the amino acid in position of a normal beta globin, transforming glutamic acid into valine. Then the HBs has less charge due to loss of the glutamic acid.Sickle-cell anemia is a hereditary disease make  by hemoglobin S in its homozygous form. 

an adaptation is a change or process by which an organism takes to be better suited for its environment. for example; migration is a behavioral adaptation, this is needed because birds are not built to last extremely cold climates. there are different adaptations because there are different species needing to adjust to different things, bears don't migrate; they hibernate.

i am positive it is c. condensing but i may be wrong