Why is the presence of proline destablizes a-helix?

A. Proline residue introduces a destablizing kink in the a-helix

B. In proline, the nitrogen atom is a part of a rigid ring, and rotation about N-C bond is not possible

C. The nitrogen atom of a proline residue in a peptide linkage has no subsituent hydrogen to participate in hydrogen bonds with other residues

D. Proline residue works as a helix linker

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