Disulfide bonds are covalent bonds between which atoms?

• A. Ionic bonds between side chains
• B. Covalent bonds between sulfur atoms of two cysteines
• C. Hydrogen bonds between backbone
• D. Peptide bonds between adjacent amino acids

Answer: (B)

Disulfide bonds are covalent linkages formed by oxidation of the thiol groups on cysteine residues. Each cysteine has a -SH side chain, and when two such thiols are oxidized, their sulfur atoms form a covalent S–S bond. This disulfide bridge can connect different parts of the same protein or different protein chains, helping to stabilize the overall three-dimensional structure. It’s different from ionic interactions between charged side chains, hydrogen bonds between backbone or side chains, or peptide bonds that covalently join adjacent amino acids in the backbone.