What is the primary function of SDS in SDS-PAGE?

The primary function of SDS (sodium dodecyl sulfate) in SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) is to denature proteins and coat them with a negative charge. This process is critical for the separation of proteins based on their molecular weight.

When proteins are exposed to SDS, the detergent disrupts non-covalent interactions that maintain their three-dimensional structures. As a result, proteins unfold into a linear form. Simultaneously, SDS molecules bind to the polypeptide chains, imparting a uniform negative charge proportional to the length of the protein. This uniformity in charge allows proteins to be separated effectively when an electric field is applied during electrophoresis. Since the proteins are now all negatively charged and of different lengths, they migrate through the gel matrix toward the positive electrode, with smaller proteins moving faster than larger ones. Thus, this combination of denaturation and charge coating is essential for accurate analysis of protein sizes in gel electrophoresis.