Disulfide bonds are _____ bonds.

• A. Ionic
• B. Hydrogen
• C. Covalent
• D. Metallic

Answer: (C)

Disulfide bonds are covalent bonds. They form when two thiol groups from cysteine side chains are oxidized to create a single S–S linkage, meaning the two sulfur atoms share electrons to hold the partners together. This sharing of electrons is the hallmark of a covalent bond, making disulfide bridges strong links that help pin together parts of a protein or even separate protein molecules. This is different from ionic bonds, which rely on electrostatic attraction between charged atoms, and from hydrogen bonds, which are weaker attractions involving partial charges rather than full electron sharing. In proteins, disulfide bridges contribute to stability of the folded structure and can be reduced back to free thiols under reducing conditions.