Which of the following side chains of amino acids can be specifically targeted for covalent bond formation during chemical cross-linking?

The side chains of amino acids that can be specifically targeted for covalent bond formation during chemical cross-linking include sulfhydryls. The sulfhydryl group (-SH) is characteristic of cysteine. When two cysteine residues come into proximity, their sulfhydryl groups can undergo oxidation to form a covalent bond known as a disulfide bond (S-S bond). This bond plays a crucial role in stabilizing the three-dimensional structures of proteins and can significantly affect their function.

In contrast, while hydroxyl groups (found in serine and threonine) can participate in various reactions, they are less likely to form stable cross-links compared to disulfide bonds. Aldehydes and esters are reactive groups that can form covalent bonds, but they are not usually associated with amino acid side chains in the context of cross-linking proteins directly. Instead, they might be involved in different types of biochemical reactions or modifications, but they do not specifically denote amino acid side chains typically targeted for covalent cross-linking in proteins.