Which structural feature is primarily associated with left-handed α-helices?

The correct choice reflects that left-handed α-helices are associated with the specific region of the Ramachandran plot where these conformations are represented. The Ramachandran plot is a graphical representation of the phi (φ) and psi (ψ) dihedral angles of amino acid residues in protein structures. In this plot, the regions indicate the sterically allowed conformations of peptide bonds.

Left-handed α-helices occupy a very small area in the Ramachandran plot, typically showing φ angles around -60° and ψ angles around +30°. This configuration is substantially less common than the right-handed α-helices, which dominate the structure of most proteins. Right-handed α-helices are aligned with the more favorable regions of the plot, demonstrating higher stability due to their compatible dihedral angles and the ability to form more favorable hydrogen bonds between the residues.

In contrast, left-handed helices, while theoretically possible and represented in the plot, do not favorably interact in the same manner, leading to their rare occurrence in natural proteins.