Final answer:
The correct answer is a. left-handedness. The feature not commonly observed in α-helices is left-handedness, as α-helices are characterized by a right-handed coiled structure, a helical turn every 3.6 amino acids, a cylindrical shape, and outward-pointing amino acid side chains.
Step-by-step explanation:
The α-helix (alpha helix) is a common motif in the secondary structure of proteins and is characterized by a right-handed coiled structure. Each helical turn of the α-helix consists of 3.6 amino acid residues, with the R groups (side chains) extending outward, allowing for proper spacing and minimal steric hindrance. This structure is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen four residues ahead. The cylindrical shape of the α-helix contributes to the stability of this secondary protein structure.
The α-helix represented in proteins is almost always right-handed, making left-handedness an incorrect feature for α-helices. Right-handed α-helices can be found in various proteins, including keratin in hair and nails, and play an important role in their functional structure.