What amino acid is commonly found in α-helices?

Alanine is commonly found in α-helices due to its small side chain, which allows it to fit well within the helical structure. The α-helix is a common secondary structural motif in proteins, characterized by its right-handed coil and the presence of hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues down the chain. Alanine's side chain is simple, consisting of a single methyl group, which introduces minimal steric hindrance, enabling the polypeptide chain to adopt stable helical conformations.

In contrast, proline's unique structure introduces a kink in the polypeptide chain due to its cyclic nature, leading to a lack of the amide hydrogen necessary for hydrogen bonding in the α-helix. Glycine, while small, has a flexible backbone that can disrupt the stability of the helix because it does not impose the same structural constraints as other amino acids. Cysteine, known for its thiol group and ability to form disulfide bonds, does not have the ideal properties for fitting into an α-helix. Thus, alanine's characteristics make it particularly favorable for incorporation into this type of secondary structure in proteins.