SDS is an amphiphilic molecule that has both hydrophilic group and hydrophobic group. Once SDS is added to a protein solution, it can interact with both the hydrophilic and hydrophobic side-chain groups. Since the 3D structure of a protein is mainly held together by hydrophobic interactions and hydrogen bonds between the side chains, the addition of SDS can interfere with both, resulting in the unfolding and denaturation.