Serine, threonine, glutamine, and asparagine are polar but neutral (uncharged) amino acids. These side chains can form multiple hydrogen bonds, so they prefer to project into the aqueous phase. If they are on the inside of the protein they are hydrogen-bonded to other buried polar groups.
and can be phosphorylated within a protein chain. This post-translational modification is often used by the cell to turn on or off a critical biological process. The phosphate ester concentrates two negative charges in a small region at the surface of the protein molecule. This can cause a change in the conformation in the protein, or may lead to association of the phosphoprotein with another protein.
and are nearly as hydrophilic as their charged counterparts, glutamate and aspartate. The group has a large dipole moment and, when exposed at the protein surface, can form several hydrogen bonds to water molecules.