Ninhydrin
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Ninhydrin is a chemical used to detect ammonia, or primary amines. When reacting with these free amines, a deep blue or purple color is evolved. Ninhydrin is most commonly used to detect fingerprints, as amines left over from peptides and proteins (terminal amines or lysine residues) sloughed off in fingerprints react with ninhydrin.
Ninhydrin can also be used to monitor deprotection in solid phase peptide synthesis, when the growing peptide chain is deprotected, a ninhydrin test yields blue, if the next peptide residue is coupled then the test is colorless or yellow.
Ninhydrin is also used in amino acid analysis of proteins; all of the amino acids are hyrdolyzed and reacted with ninhydrin, they are then quantified colormetrically after separation by chromatography.
A solution suspected of containing ammonium ion can be tested by ninhydrin; dotting it onto a solid support (such as silica gel) and treatement with ninhydrin should dramatically turn purple if the solution contains this species.
Mechanism to generate the ninhydrin chromophore
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Ninhydrin.png
Image:Ninhydrin.png
Note that in order to generate the ninhydrin chromophore, the amine is oxidized to a schiff base by redox transfer from the ninhydrin moiety. Thus only ammonia and primary amines can proceed past this step. At this step, there must also be proton (H* in the diagram) for Schiff base transfer, so an amine adjacent to a tertiary carbon cannot be detected by the ninhydrin test.