Polypeptides and proteins must fold properly to be functional. Hydrogen bonds between the oxygen and nitrogen atoms of the peptide bonds are crucial to which level of protein structure?

Secondary structure

Explanation:

Stricture of proteins can be categories into 4

Primary structure, secondary structure, tertiary structure and quaternary structure.

Primary structure are polypeptide chains. The polypeptide chains are formed by amino acids held together by peptide bonds.

Secondary structure refers to the folding of polypeptides chains. Two most common secondary structures are alpha helix and beta pleated structure. These folding occurs because of hydrogen bonding between the oxygen of the carbonyl of one amino acid and the hydrogen of another amino acid.

Therefore, hydrogen bonding is crucial for secondary structure of the protein.