Proline is frequently found at the ends of alpha helices and in beta turns but is rarely found within an alpha helix. The structural reason is:

• A Its bulky indole side chain causes steric clash with neighboring residues
• B Its sulfhydryl group forms intrachain disulfide bonds that kink the helix
• C Its positively charged guanidinium group forms disruptive salt bridges within the helix
• D Its cyclic pyrrolidine ring restricts rotation about the N-Calpha bond, fixing the phi angle near -65 degrees ✓

Explanation

Proline is technically an imino acid: its side chain cyclizes back onto the backbone nitrogen, forming a rigid pyrrolidine ring. This locks the phi torsion angle and removes the amide hydrogen, so proline can neither adopt the required helix geometry nor donate the i-to-i+4 hydrogen bond. It therefore acts as a helix breaker while fitting well into tight beta turns.

Reference: Harper's Illustrated Biochemistry, 31st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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