The planar character of the peptide group limitsthe conformational flexibility of the polypeptide chain • The αhelix and the βsheet allow the polypeptide chain to adopt favorable φand ψangles and to form hydrogen bonds • Fibrous proteins contain long stretches of regular secondary structure, such as the coiled coils in α keratinand the triple helix in collagen • Not all polypeptide segments form regular secondary structure such as αhelices or βsheets