Which one of the following statement is NOT TRUE for an a-helix structure of a protein?

The hydrogen bonds in secondary structure of proteins are hydrogen bonds and disulfide bonds. The hydrogen bonds either form alpha helix or beta pleated sheet structure. Alpha helix can be either right handed or left handed. Left handed alpha helix is less stable than right handed alpha helix. Only right handed alpha helix has been found in protein structure. Beta pleated sheet structure is formed when hydrogen bonds are formed between the carbonyl oxygen and amide hydrogen of two or more adjacent extended polypeptide chains. Thus hydrogen bonding in beta pleated sheet structure is interchain. The structure is not absolutely planar due to the angles of bonds. Ref: Textbook of Medical Biochemistry By Chatterjea page 90.