What chemical side bonds are formed when two sulfur-type chains are joined together?

• A. Peptide bonds
• B. Hydrogen bonds
• C. Salt bonds
• D. Disulfide bonds

Answer: (D)

Disulfide bonds form when two sulfur-containing side chains come together to create a covalent link. In proteins, this usually means the thiol groups of cysteine residues are oxidized so that their sulfurs connect, forming an S-S bond. This covalent bridge can link two parts of a protein or even two separate protein chains, providing a strong, stabilizing cross-link that helps maintain structure, especially under stress.

This is different from peptide bonds, which join amino acids along the protein backbone; hydrogen bonds, which are weaker, noncovalent interactions between polar groups; and salt bonds, which are ionic attractions between positively and negatively charged side chains. Disulfide bonds stand out because they involve sulfur and create a durable cross-link that can be reversed only by reduction.