Which statement correctly describes the geometry of a typical alpha-helix in terms of handedness and residues per turn?

Which statement correctly describes the geometry of a typical alpha-helix in terms of handedness and residues per turn?

• A. An alpha-helix is typically right-handed with about 3.6 residues per turn.
• B. An alpha-helix is typically left-handed with about 3.6 residues per turn.
• C. An alpha-helix has about 6 residues per turn.
• D. An alpha-helix has two residues per turn.

Answer: (A)

The main idea here is how an alpha-helix actually packs in space, focusing on its twist direction and how many residues fit into each turn. A typical alpha-helix is right-handed, meaning it winds clockwise when you trace it from one end to the other. Each residue adds about 100 degrees of rotation around the helix axis, so roughly 3.6 residues complete a full 360-degree turn. That exact geometry gives a rise per turn of about 5.4 Å and a rise per residue of ~1.5 Å, with stabilizing hydrogen bonds between the carbonyl of residue i and the amide hydrogen of residue i+4. The combination of right-handed twist and ~3.6 residues per turn is what makes the classic alpha-helix so stable and characteristic. The other options don’t fit because they describe a different handedness or a number of residues per turn that would alter the hydrogen-bonding pattern and overall geometry in ways not observed for typical protein alpha-helices.

The main idea here is how an alpha-helix actually packs in space, focusing on its twist direction and how many residues fit into each turn. A typical alpha-helix is right-handed, meaning it winds clockwise when you trace it from one end to the other. Each residue adds about 100 degrees of rotation around the helix axis, so roughly 3.6 residues complete a full 360-degree turn. That exact geometry gives a rise per turn of about 5.4 Å and a rise per residue of ~1.5 Å, with stabilizing hydrogen bonds between the carbonyl of residue i and the amide hydrogen of residue i+4. The combination of right-handed twist and ~3.6 residues per turn is what makes the classic alpha-helix so stable and characteristic. The other options don’t fit because they describe a different handedness or a number of residues per turn that would alter the hydrogen-bonding pattern and overall geometry in ways not observed for typical protein alpha-helices.