Analytical color test: Which amino acid yields a characteristic yellow–brown color with the ninhydrin reaction due to its imino (secondary amine) nature?

Introduction / Context:
Ninhydrin is a classic reagent for detecting amino acids in chromatography and electrophoresis. Most α-amino acids yield a deep purple product (Ruhemann’s purple). However, imino acids behave differently and generate a distinct color, which is diagnostically useful.

Given Data / Assumptions:

• Proline is an imino acid: its α-amino group is secondary (incorporated in a pyrrolidine ring).
• Standard ninhydrin conditions are used (heating, mildly acidic aqueous/organic medium).
• Other listed amino acids have primary α-amino groups.

Concept / Approach:
The ninhydrin reaction mechanism involves oxidative deamination of primary amino groups to an aldehyde and formation of a chromophore (Ruhemann’s purple). Secondary amino groups (as in proline/hydroxyproline) cannot follow the same pathway and instead form a yellow to yellow-brown product.

Step-by-Step Solution:
Classify amino groups: primary vs secondary. Predict color outcome: primary → purple; secondary (imino) → yellow-brown. Identify imino acid → Proline (also hydroxyproline, not listed). Therefore the amino acid giving brownish/yellow product is proline.

Verification / Alternative check:
Textbook TLC plates show proline spots as yellow compared to purple for most amino acids, confirming the diagnostic difference.

Why Other Options Are Wrong:
Asparagine, alanine, valine, and glycine have primary amino groups and produce the purple chromophore.

Common Pitfalls:
Assuming all amino acids turn the same color with ninhydrin; forgetting that proline’s ring makes the amino group secondary.

Final Answer:
Proline.