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4.2.3.1 Reaction with Ninhydrin
Ninhydrin oxidatively decarboxylates amino acids to CO2, NH3, and an aldehyde with one less carbon atom than the parent amino acid. The reduced compound hydrindantin reacts with another molecule of ninhydrin and the liberated ammonia, to form a purple complex (Ruhemann’s purple) that maximally absorbs light of wavelength 570 nm. This blue color forms the basis of a amino acids that can detect as little as 1 g of aminoacids.
Amines, other than amino acids also react with ninhydrin, forming a blue color, but without releasing CO2. The evolution of CO2 thus indicates an ∝ amino acid. NH3 and peptides also react, but more slowly than ∝ amino acids.
Proline and 4-hydroxy proline produce a yellow color with ninhydrin. |