Which of the following statements is incorrect about the secondary structure of protein?

• A

  It refers to the shape in which long polypeptide chain can exist.

• B

  It exists in two different type of structures, i.e. $\mathrm{\alpha }$-helix and $\mathrm{\beta }$-pleated sheet structure

• C

  Its structure arises due to regular folding of the backbone of the polypeptide chain

• D

  There is the covalent bonding between 

  

  and -NH groups of the peptide bond