Which statement about protein secondary structure is true?

• A. Secondary structure is due to interactions of the backbone and is stabilized by hydrogen bonds
• B. Secondary structure is formed by interactions of side chains only
• C. Alpha helices are stabilized by ionic bonds between side chains
• D. Beta sheets are stabilized by disulfide bonds

Answer: (A)

The key idea here is that protein secondary structure is defined by interactions along the peptide backbone, not by side chains. The regular hydrogen-bonding pattern between backbone atoms stabilizes these motifs. In alpha helices, a hydrogen bond forms between the carbonyl oxygen of one residue and the amide hydrogen four residues away, along the same chain, producing a tight helical wheel of backbone interactions. In beta sheets, hydrogen bonds form between backbone amide and carbonyl groups on adjacent strands, which can be parallel or antiparallel, again driven by backbone chemistry. The side chains project outward and influence overall folding, but they aren’t the main stabilizing force for the secondary structure itself.

The other statements don’t fit because they imply side-chain interactions or other bonds govern secondary structure. Side chains only would miss the backbone hydrogen-bonding pattern; alpha helices aren’t stabilized primarily by ionic bonds between side chains; beta sheets aren’t stabilized by disulfide bonds—the latter are covalent links between cysteines that can stabilize overall tertiary or quaternary structure, not the characteristic backbone-based hydrogen bonding of beta sheets.