Fill blanks with:N−Cα−C N − C α − CCα−C C α − CN−Cα N − C αbackboneside chaina stable secondary structurea stable tertiary structurea stable functionadoptchangeα−NH group α − N H groupα−COOH group α − C O O H groupH-bond donor H -bond donorelectron residue in a polypeptide chain has two backbone bonds: Each residue in a polypeptide chain has two backbone bonds: blank, about which rotation is permitted. The angle of rotation about the blank bond is referred to as ϕ (phi) and that about the blank bond is called ψ (psi).These angles describe the blank conformation of any particular residue in any protein, and allow one to predict whether a combination of amino acids in a polypeptide will be able to form and maintain blank. conformation of any particular residue in any protein, and allow one to predict whether a combination of amino acids in a polypeptide will be able to form and maintain blank. Proline is not able to blank the ideal ϕ and ψ angles for an α helix, owing to the presence of the rigid imidic group as its side chain. Furthermore, proline does not have an blank that acts as a stabilizing blank in the middle of the helix. As a consequence, the secondary structure is predicted to be blank..