Which statement correctly describes alpha-helix geometry?

Which statement correctly describes alpha-helix geometry?

• A. An alpha-helix is typically left-handed with about 3.6 residues per turn.
• B. An alpha-helix is typically right-handed with about 3.6 residues per turn.
• C. An alpha-helix is typically right-handed with about 12 residues per turn.
• D. An alpha-helix is typically left-handed with about 2 residues per turn.

Answer: (B)

Proteins’ alpha-helix is a right-handed spiral with about 3.6 amino acid residues per turn. This geometry arises because the backbone forms stable hydrogen bonds between the carbonyl of residue i and the amide proton of residue i+4, which sets up a repeating pattern every four residues. The rise per residue is about 1.5 Å, so one full turn spans roughly 5.4 Å along the helix axis. The typical dihedral angles (phi around -57° and psi around -47°) favor a right-handed twist that matches this 3.6-residue per turn geometry. Left-handed helices do occur only rarely and are not the standard form, and 12 residues per turn or 2 residues per turn would imply a fundamentally different hydrogen-bonding pattern and a much different pitch, which is not characteristic of the canonical alpha-helix.

Proteins’ alpha-helix is a right-handed spiral with about 3.6 amino acid residues per turn. This geometry arises because the backbone forms stable hydrogen bonds between the carbonyl of residue i and the amide proton of residue i+4, which sets up a repeating pattern every four residues. The rise per residue is about 1.5 Å, so one full turn spans roughly 5.4 Å along the helix axis. The typical dihedral angles (phi around -57° and psi around -47°) favor a right-handed twist that matches this 3.6-residue per turn geometry. Left-handed helices do occur only rarely and are not the standard form, and 12 residues per turn or 2 residues per turn would imply a fundamentally different hydrogen-bonding pattern and a much different pitch, which is not characteristic of the canonical alpha-helix.