Some structural proteins are found to be in a coiled form. The alpha helix is a spiral formed by coiling of the polypeptide chain around the fibre axis. Each turn of the helix is 5.4 Å long and contains 3.6 amino acid residues, length of each residue being 1.5 Å. The helical structure is stabilized by noncovalent hydrogen bonds linking the amide nitrogen with carbonyl oxygen.

                                             ________ H   –––– – – – – – – –  O    

The side chains of the resides project outwards. The alpha helix may be either right handed or left handed . The α-keratin of hair and wool exists in right handed helical form.

Pauling and Corey proposed yet another arrangement of protein-β-pleated sheet form which occurs in an extended form. Some fibrous proteins, like β-keratin, and collagens exist in sheet like structure where maximum number of hydrogen bonds are formed perpendicular to the long axis of the polypeptide chains.