Ninhydrin

• 2,2- Dihydroxyhydrinden -1 ,3- dione
• 2,2- Dihydroxyindan -1 ,3- dione
• Indantrionhydrat
• 2,2- dihydroxy-1 ,3- dione indene
• 2,2- dihydroxy- 1H-indene -1, 3 (2H) -dione

Colorless crystals

Fixed

Decomposition above 250 ° C.

Poorly in water (20 g · l-1 at 20 ° C)

• Soluble in boiling water and ethanol, badly in diethyl ether

Attention

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Ninhydrin is the hydrate of the indan -1 ,2,3- trione, and serves as a reagent for the detection of ammonia and primary amino groups, in particular amino acids.

Properties

Ninhydrin reacts with the amino group of amino acids with elimination of water to a Schiff base ( imine ). After decarboxylation of the carboxy group of the amino acid and subsequent cleavage of the amino acid residue is produced amino ninhydrin. This dimerizes with ninhydrin to form a violet dye ( peace 's purple ). The intense violet color can be explained by resonance with the participation of hydrogen bonding within the system of conjugated double bonds.

The intensity of the color is the concentration of the dye (see Lambert-Beer 's law), and thus the concentration of amino acid to be determined proportionally. For quantitative analysis of multiple samples can be compared with a known concentration as standards with the sample to be examined spectrophotometrically at 570 nm. In the reaction with the secondary amino acids, such as proline, or hydroxyproline, produces a yellow colored complex, which can be measured photometrically at 400 nm.

Ninhydrin also has a role in the volume control on the solid-phase peptide synthesis (SPPS ) and in the solid-phase organic synthesis ( SPOS) and is part of the Kaiser test.

The reaction is relatively sensitive, the detection limit is 0.001 to 0.1 mg.

Use

Ninhydrin is used primarily for the detection of amino acids and proteins. For proteins, however, the test is successful only if relatively short oligopeptides are present, as ninhydrin reacts with free amino groups, and these are hardly present in long-chain polypeptides. The proof is carried out in solution ( by heating in a water bath ). Ninhydrin is often used as a spray reagent, for example, in paper chromatography or thin layer chromatography.

Another application based on this reaction is the preparation of fingerprints. Since the welding occur amino acids, they can respond with ninhydrin and make the finger or hand prints visible.

This mechanism is also used in medicine as a so-called Moberg test application for the detection of peripheral nerve lesions. Since the regulating for the sweat sympathetic fibers run from the exit from the spinal cord together with the peripheral nerves, the sweat is disturbed, which can be detected in a detached skin print by ninhydrin.

An exception is the amino acid proline or hydroxyproline, which have as a secondary amino acids instead of a primary ( NH 2) a secondary (NH) amino group. Proline and hydroxyproline react with a different reaction mechanism to a yellow-red product having an absorption maximum at 440 nm

Reaction mechanism

The reaction mechanism is more complex than usually presented; so that Friedmann and Williams deal. In detail, such as running in the presentation to see - following reactions: First, the ninhydrin is dehydrated to Trion. On the central carbonyl oxygen, the amino group of the amino acid attacks and condensed to form the imine. This has an open seven-membered ring structure, which enables a cyclic shift of electrons under decarboxylation. By hydrolysis of the imine of the amino acid residue is cleaved off as the aldehyde and the primary amine remaining. In a condensation reaction arises when a second Ninhydrinmolekül elimination of water, the rest 's purple.

Special

Ninhydrin is one of the few compounds that contradict the Erlenmeyer rule.