Which amino acid is not commonly found in α-helices?

Proline is not commonly found in α-helices due to its unique structural properties. The cyclic structure of proline introduces a rigid and restrictive angle in the peptide backbone, which disrupts the hydrogen bonding pattern required to stabilize the α-helix. In an α-helix, the backbone carbonyl oxygen of one amino acid forms a hydrogen bond with the amide hydrogen of another amino acid located four residues away. The presence of proline interferes with this bonding because it lacks a free amide hydrogen and contributes to structural constraints that are unfavorable for helix formation.

In contrast, the other amino acids listed—serine, arginine, and lysine—can easily accommodate the helical structure. Serine contains a hydroxyl group that can participate in hydrogen bonding without disrupting the helix. Arginine has a long side chain with a positive charge that can also stabilize helical structures, often through interactions with anionic residues. Lysine, being positively charged, can similarly stabilize helical regions through interactions with other negatively charged side chains, while also participating in hydrogen bonding. Thus, proline's unique properties lead to its rarity in α-helices, making it the correct answer.