In an alpha helical polypeptide, the backbone hydrogen bonds are between
(1) NH of n and CO of n + 4 amino acids
(2) CO of n and NH of n + 3 amino acids
(3) CO of n and NH of n + 4 amino acids
(4) NH of n and CO of n + 3 amino acids

Hydrogen Bonding Pattern in α-Helical Polypeptides

Protein secondary structures, such as α-helices, rely on hydrogen bonding for stability. These bonds occur between the backbone NH and CO groups of different residues within the polypeptide chain.

Correct Answer:

The correct option is:

(3) CO of n and NH of n + 4 amino acids

Explanation of the Answer Choices

1. How Hydrogen Bonds Form in α-Helices

• In an α-helix, the carbonyl oxygen (C=O) of residue n forms a hydrogen bond with the amide hydrogen (N-H) of residue n+4.
• This stabilizes the helical structure, making it compact and thermodynamically favorable.

2. Why Are Other Options Incorrect?

• (1) NH of n and CO of n + 4 (Incorrect order of donor and acceptor groups).
• (2) CO of n and NH of n + 3 (This is the bonding pattern seen in 3₁₀ helices, not α-helices).
• (4) NH of n and CO of n + 3 (Incorrect order and corresponds to different helical structures).

Significance of Hydrogen Bonding in α-Helices

1. Contribution to Protein Stability

• Hydrogen bonds prevent the helix from collapsing or unwinding.
• The optimal length and spacing of these bonds maintain a right-handed helical twist.

2. Role in Protein Function and Folding

• Many enzymes and structural proteins contain α-helices due to their stability.
• Proper hydrogen bonding ensures correct folding, preventing diseases like Alzheimer’s and Parkinson’s.

3. Applications in Bioengineering

• Understanding α-helical hydrogen bonding aids in designing synthetic peptides.
• It helps in the development of drug delivery systems and biomaterials.

Conclusion

The hydrogen bonding pattern in α-helices follows a CO (n) to NH (n+4) interaction, ensuring stability, structural integrity, and proper protein folding. This fundamental principle is crucial for biochemistry, structural biology, and protein engineering.