Protein secondary structures can be divided into two categories:

(a) α-Helix structure

The NH group of an amino acid residue forms an H-bond with the -NH group of another amino acid residue in this structure. The adjacent turn of the right-handed screw (α –helix ) is grouped together.

(b) β-pleated sheet structures

The name comes from the fact that it resembles the pleated folds of draperies. The peptide chains are arranged side by side in this structure after extending out to near maximum extension. The peptide chain is held together by an intermolecular hydrogen bond.