Secondary structure of protein refers to

To understand the secondary structure of a protein, we focus on the hydrogen bonding.We imagine that the peptide chains as linear structures in nature, and they form orderly hydrogen-bonded arrangements. In particular, the carbonyl oxygen atoms form hydrogen bonds with the amide hydrogens. This tendency leads to orderly patterns of hydrogen bonding: the helix and the pleated sheet. These hydrogen-bonded arrangements, if present, are called the secondary structure of the protein.