Which amino acid disrupts an alpha helix when placed within it because its rigid ring locks the phi bond and its imino nitrogen cannot participate in the regular intrachain hydrogen bonding pattern?
- A Alanine
- B Proline ✓
- C Glutamate
- D Leucine
Correct answer: B. Proline
Explanation
Proline is a helix breaker: its cyclic side chain fixes the backbone phi angle near -65 degrees, incompatible with the alpha-helical geometry, and lacking an amide hydrogen it cannot donate the i to i+4 hydrogen bond. Alanine and leucine are among the strongest helix formers, and glutamate is also helix-favoring. Glycine destabilizes helices due to conformational freedom but lacks proline's structural rigidity.
Reference: Lippincott Illustrated Reviews: Biochemistry, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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