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

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.

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.