Question

Ninhydrin with evolution of $CO_2$ forms a blue complex with

• A. Peptide bond
• B. Serotonin
• C. $\alpha$ - Amino acid
• D. Histamine

Hint:

Ninhydrin + $\alpha$-amino acid = Blue/Purple + $CO_2$.
Ninhydrin + Proline = Yellow.
Biuret test = Detects Peptide bonds (Blue to Violet).
Don't confuse these two famous protein tests!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The Ninhydrin test is a classical biochemical test for detecting certain nitrogenous compounds. The question asks to identify which compound specifically produces a blue complex along with the release of carbon dioxide gas.
Detailed Explanation:

• The Ninhydrin Reaction:
Ninhydrin (triketohydrindene hydrate) is a powerful oxidizing agent.
When it reacts with an $\alpha$-amino acid, it undergoes an oxidative deamination and decarboxylation reaction.

• Products of the Reaction:
The reaction releases Carbon Dioxide ($CO_2$), an aldehyde, and Ammonia ($NH_3$).
The reduced ninhydrin (hydrindantin) then reacts with the released ammonia and another molecule of ninhydrin to form a deep blue-purple complex.
This colored complex is known as Ruhemann's Purple.

• Why $\alpha$-amino acids?
Only $\alpha$-amino acids (where the amino group is on the carbon adjacent to the carboxyl group) have the correct structure to undergo the decarboxylation required to release $CO_2$ in this specific test.
Proline and hydroxyproline (imino acids) react with ninhydrin but produce a yellow color instead of blue, and they do not release $CO_2$ in the same way.

• Applications:
This test is used in laboratories to detect amino acids and proteins. It is also used in forensics to develop latent fingerprints, as the ninhydrin reacts with the amino acids in the sweat left on a surface.

Step 3: Final Answer:
Ninhydrin produces a blue complex and releases $CO_2$ specifically when reacting with $\alpha$-amino acids.