The secondary structure sections of a polypeptide may further be organized into a tertiary structure by these interactions:

• Covalent disulfide bonds form between polar cysteine molecules.

• Most of the polar hydrophilic R groups are located on the surface, close to surrounding water molecules.

• Most nonpolar hydrophobic R groups are located on the interior, avoiding water.

This diagram shows tertiary structure of myoglobin, an oxygen carrier molecule in muscle cells.