What is the purpose of adding SDS in polyacrylamide gel electrophoresis?

The addition of sodium dodecyl sulfate (SDS) in polyacrylamide gel electrophoresis serves primarily to denature proteins and impart a uniform negative charge proportional to their mass. This ensures that proteins are separated based on size during electrophoresis. By binding to the proteins, SDS disrupts their secondary and tertiary structures, effectively neutralizing their intrinsic charges. As a result, all proteins migrate through the gel solely based on their molecular size, allowing for consistent and reliable separation.

Using SDS allows for improved resolution in separating proteins of different sizes, as each protein will have a similar charge-to-mass ratio due to the SDS binding. Without SDS, proteins would migrate based on both their size and charge, which could lead to inconsistent results. Additionally, SDS treatment does not increase sample concentration or stabilize the gel; rather, it primarily focuses on ensuring uniform movement through the gel matrix as proteins migrate during the electrophoretic process.