What type of chemical bonding is responsible for stabilizing the regular α-helix secondary structure of a polypeptide chain?

Answer and explanation

Correct answer: C. Intramolecular H-bonds between C=O of residue n and N-H of residue n+4 along the chain

  1. Step 1: The -helix is one of the most common secondary protein structures, discovered by Linus Pauling and Robert Corey.
  2. Step 2: In an -helix, a single polypeptide chain coils tightly into a right-handed screw.
  3. Step 3: The helical conformation is stabilized exclusively by intramolecular hydrogen bonds formed between the carbonyl oxygen () of each amino acid residue () and the amide hydrogen () of the fourth amino acid residue along the chain ().
  4. Step 4: All hydrogen bonds lie parallel to the helical axis.
Answer
Intramolecular hydrogen bonds between the C=O group of one amino acid residue and the N-H group of the fourth residue ahead (n and n+4)
Why not the others: 'Intermolecular disulfide covalent bonds...' describes tertiary cross-links between cysteine residues, not the -helix; 'Coordinate covalent bonds...' describes metalloprotein coordination, not secondary structure; 'Ionic salt bridges between alanine...' is impossible as alanine is non-polar and uncharged.

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