In SDS separations, migration is determined not by intrinsic electrical charge of polypeptides but by molecular weight
Sodium dodecylsulfate (SDS) is an anionic detergent which denatures secondary and non–disulfide–linked tertiary structures by wrapping around the polypeptide backbone. In so doing, SDS confers a net negative charge to the polypeptide in proportion to its length
When treated with SDS and a reducing agent, the polypeptides become rods of negative charges with equal “charge densities" or charge per unit length.