An amino acid that possesses a secondary amino group (imino group) within a rigid ring structure is:
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Correct answer: Proline (pyrrolidine ring)
Proline is unique because its side chain loops back and bonds covalently to the nitrogen of its own amino group, forming a rigid five-membered pyrrolidine ring. This cyclic structure makes it a secondary (imino) amine rather than a primary amine and introduces conformational kinks in alpha-helices, which are critical for collagen structure.
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