# What is the difference in the effect of DTT (Dithiotreitol) and SDS (sodium dodecyl sulfate) for PAGE?

In class, we have studied that when running a polyacrylamide gel with our protein sample to separate the proteins by molecular size, we can use several techniques, among which we have denaturing PAGE and SDS-PAGE. Basically, we focused on the latter, in which SDS is a detergent added to negatively charge all proteins in the sample (at a constant mass:charge ratio), as well as denature them. Then, the professor mentioned that we also add reagents such as DTT or beta-mercaptoethanol to reduce the disulphide bonds in between proteins.

My question is: if SDS already denatures proteins (so we have primary structure only), why would we be interested in adding DTT?

$$\ce{\underset{\text{cystein}}{2 R-S-H} ->[][- 2e- \ -2 H+] \underset {\text{disulphide bridge}}{\ce{R-S-S-R}}}$$