What is called a `3.6_(13)` helix of protein ? 

Explanation

Secondary structure of proteins are two types : `alpha` helix structure and `beta` - pleated sheet structure, A `3.6_(13)` helix of protein is `alpha` helical secondary structure. Hydrogen bonds presents within the same chain between -NH- and CO - groups.
These intramolecular hydrogen bonds stabilise the helix. In each turn ofhelix, 3.6 amino acid residues are : present on an average. A 13 membered chelate ring is , formed due to intermolecular hydrogen bonding.