α-Helix structure:
In this structure, formation of hydrogen bonding between amide groups within the same chain causes the peptide chains to coil upto a spiral structure. This is called α-helix. The α-helix structure is found in many proteins such as myosin (found in muscles) and keratin (found in hair, wool and nails)as shown bellow.
The α-helix structure is also known as 3.613 helix because each turn of the helix contains approximately 3.6 amino acids and a 13 membered ring formed by hydrogen bonding. The helix is held in its shape primarily by hydrogen bonds between one amide group and carbonyl group which is 3.6 amino acids units away.