Which bonds link amino acids in a protein backbone?

• A. Hydrogen bonds
• B. Ionic bonds
• C. Disulfide bonds
• D. Peptide (amide) bonds

Answer: (D)

Peptide bonds form the backbone of a protein by covalently linking consecutive amino acids, created when the carboxyl group of one amino acid reacts with the amino group of the next, releasing water. This connection, called a peptide (amide) bond, stacks the amino acids in a chain with a repeating -C(O)-NH- linkage. The bond is covalent and has partial double-bond character, which makes it planar and restricts rotation about the bond itself, though the chain can still rotate around neighboring bonds to allow folding.

Other interactions don’t link the backbone in the same way: hydrogen bonds help stabilize the folded structure without linking amino acids in the chain, ionic bonds form between charged side chains, and disulfide bonds connect cysteine side chains. The backbone linkage specifically is the peptide (amide) bond.