Regarding the geometry of a right-handed alpha helix, which set of parameters is correct?
- A 4.0 residues per turn, 1.2 angstrom rise per residue, hydrogen bonds parallel to the helix axis
- B 3.0 residues per turn, 3.6 angstrom rise per residue, hydrogen bonds between residue i and i+3
- C 3.6 residues per turn, 1.5 angstrom rise per residue, hydrogen bonds between C=O of residue i and N-H of residue i+4 ✓
- D 3.6 residues per turn, 5.4 angstrom rise per residue, hydrogen bonds between adjacent strands
Correct answer: C. 3.6 residues per turn, 1.5 angstrom rise per residue, hydrogen bonds between C=O of residue i and N-H of residue i+4
Explanation
The alpha helix has 3.6 residues per turn, a rise of 1.5 angstrom per residue giving a pitch of 5.4 angstrom per turn, and hydrogen bonds from the A=O of residue i to the N-H of residue i+4, running nearly parallel to the helix axis. Option D quotes the correct pitch figure but mislabels it as rise per residue and describes beta-sheet hydrogen bonding. Option B describes features closer to a 310 helix.
Reference: Lehninger Principles of Biochemistry, 7th ed.
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