2.2 The simplest arrangement of the polypeptide chain was proposed to be a helical structure called a-helix (Pauling and corey, 1951) 2.2.1 The polypeptide backbone is tightly wound around the long axis (rodlike). 2.2R groups protrude outward from the helical backbone 2.2.3 A single turn of the helix(corresponding to the repeating unit in a-keratin) extends about 5.6 Angstroms, including 3. 6 residues(each residue arises 1.5a and rotate 100 degrees about the helix axis)2.2 The simplest arrangement of the polypeptide chain was proposed to be a helical structure called a-helix (Pauling and Corey, 1951) 2.2.1 The polypeptide backbone is tightly wound around the long axis (rodlike). 2.2.2 R groups protrude outward from the helical backbone. 2.2.3 A single turn of the helix (corresponding to the repeating unit in a-keratin) extends about 5.6 Angstroms, including 3.6 residues (each residue arises 1.5 Å and rotate 100 degrees about the helix axis)