Principles of Biochemistry

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.