Opening image: Bacteriorhodopsin. Only the seven transmembrane α-helices are shown. The model also shows six proline residues in CPK colors.
For an unfolded polypeptide in solution, the hydration of the peptide groups and polar side chains allows a large number of floppy, extended conformations to exist. Water, however, is a poor solvent for the nonpolar side chains. Thus, in water there is a strong tendency for the polypeptide chain to fold into a single unique three-dimensional structure, referred to as its tertiary structure.