If SDS page is used, will the charge on all the proteins be same?

When using SDS-PAGE, the charge on all proteins is not exactly the same in absolute terms, but they are given a uniform negative charge relative to their size. SDS (sodium dodecyl sulfate) is an anionic detergent that binds to proteins in a ratio proportional to their length, imparting a large negative charge that overwhelms any intrinsic charge the proteins may have125. This means that while the absolute charge may vary slightly due to differences in protein size (since more SDS binds to larger proteins), the charge-to-mass ratio becomes uniform across all proteins. As a result, proteins are separated primarily based on their molecular weight rather than their native charge or shape125.

In practical terms, this uniform charge allows proteins to migrate towards the anode (positive electrode) based on their size, with smaller proteins moving faster than larger ones through the gel matrix13. However, the charge itself is not identical across all proteins; rather, it is proportional to their size due to the binding of SDS molecules5.