Which structure is unable to embed itself within a phospholipid bilayer?

Peripheral membrane proteins are unable to embed themselves within the phospholipid bilayer. 

Peripheral membrane proteins are a type of membrane protein that are present on the surface of the cell membrane where they temporarily and loosely adhere to the phospholipid bilayer. Peripheral proteins have specific amino acid sequences that allow them to interact with integral proteins or the hydrophilic phosphate heads of the lipid molecules that make up the phospholipid bilayer. However, they are unable to enter and embed themselves within the inner hydrophobic space of the cell membrane. 

The weak interactions between the peripheral proteins and the cell membrane are often reversible, allowing them to associate and dissociate based on cellular needs.