What forces are responsible for the stability of `alpha`-helix. Why is it also known as `3.6_(13)` helix?

Explanation

The stability of `alpha`-helix structure is due to intramolecular H-bonding between `-NH- and -CO-` groups of the same polypeptide chain. The `alpha`-helix is termed as `3.6_13` helix since each turn of the helix has approximately `3.6` amino acids and the hydrogen bonding leads to the formation of a 13 -membered ring.