Which amino acid yields a non-blue-purple color with ninhydrin under standard conditions?

• A. Glycine
• B. Arginine
• C. Proline
• D. Lysine

Answer: (C)

In the ninhydrin test, the color mainly depends on the type of amine present in the amino acid. Most amino acids have a primary amino group that reacts with ninhydrin to form Ruhemann's purple, a deep blue–purple color after heating.

Proline is different because its amino group is a secondary amine incorporated into a five-membered ring. This structural difference changes the reaction pathway with ninhydrin, so it does not form Ruhemann's purple. Instead, the product is a yellow (or pale) color under standard conditions. That’s why proline yields a non-blue-purple result, making it the correct choice. The other amino acids listed typically produce the blue–purple Ruhemann’s purple color.