Secondary structure of a protein is stabilized by what?

• A. Intermolecular forces between side chains due to ionic interactions
• B. Hydrogen bonds between R groups
• C. Hydrogen bonds between the polypeptide backbone
• D. Ionic bonds between amino acids

Answer: (C)

Hydrogen bonds between the polypeptide backbone stabilize the local folding patterns that define secondary structure, such as alpha helices and beta sheets. These bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another along the same chain, creating a regular, repeating network that holds these motifs together. The side chains (R groups) point outward and influence the overall folded shape more in tertiary structure, while they don’t establish the backbone hydrogen-bonding pattern that underpins secondary structure. Ionic interactions or hydrogen bonds involving side chains contribute to other levels of structure, but the main stabilizing force for secondary structure is backbone hydrogen bonding.