Reaction of amino acids with ninhydrin

Figure 10.11 The ninhydrin reaction. The overall reaction of amino acids with ninhydrin is ...

The reaction of amino acids with ninhydrin can detect amino acids on a wide variety of substrates. For example, if a kidnapper touches a ransom note with his fingers, the dermal ridges on his fingers leave traces of amino acids from skin secretions. Treatment of the paper with ninhydrin and pyridine causes these secretions to turn purple, forming a visible fingerprint. 

Figure 6-9 Reaction of amino acids with ninhydrin.

The reactions of amino acids with ninhydrin, carbon dioxide, metal ions, and glucose are described below. The last three are of physiological importance. 

Much more important than these reactions is the color test of amino acids with ninhydrin (78 — 81). The reactions with isatin or alloxan (82) belong to the same group. The course of the reaction of amino acids with ninhydrin is not yet completely understood. At present it is supposed that in the first step Strecker s degradation of amino acids takes place accompanied by the formation of carbon dioxide, ammonia, and an aldehyde possessing one carbon atom less than the original amino acid. Ninhydrin (I) is supposed to be reduced simultaneously to diketohydrindol (II), which reacts with the liberated ammonia and a second molecule of ninhydrin to form the intensely blue anion of diketohydrindenediketohydrindamine (III). 

Tible 1 Color reactions of amino acids with fluorescein isothiocyanate — ninhydrin (extract from original table). 

Since the early detection of amino acids with ninhydrin, many derivatization procedures have investigated these solutes. 4-Dimethylaminoazobenzene-4 -sulfonyl chloride (DABS-C1) is performing well, but the best seems to be FMOC (9-fluorenyl chloroformate) (63). More generally, each class of reactions replaces the active hydrogens of the OH, NH, and SH groups. 

Amino acids are chemically reactive at their a-amino groups and their side chains. The carboxyl groups are relatively unreactive unless activated. A very useful reaction is the oxidative deamination of amino acids with ninhydrin. The reaction produces a blue pigment, which can be used for the detection of amino acids both qualitatively and quantitatively. The series of reactions producing the blue complex is given in Equation (4.2). 

Saifer, A., and Oreskes, I., Color reaction of amino acids with alloxan, isatin, and ninhydrin in circular paper chromatography. Anal. Chem. 28, 501-504 (1956). 

Deamination of amino acids by carbonyl compounds has long been known in organic chemistry. Classical instances are the Strecker reaction of amino acids with alloxan and rimilar reactions with o-quinones, isatin, ninhydrin, methylglyoxal, and the like. In such cases deamination is usually associated with decarboxylation of the amino acid. Condensation of the amino and carbonyl groups to yield SchiiTs bases, and tautomeric transformation of these, are generally assumed as intermediary steps. 

Unlike ninhydrin, the reaction of amino acids with OPA does not destroy the amino acids, but adds the fluorescent tag to the primary amine. This allows OPA to also be used as a precolumn derivative, since the derivatization product is unique for each amino acid. 

Suggest a reasonable mechanism for the reaction of an a-amino acid with ninhydrin. J... 

Write the equation for the reaction of a given amino acid with ninhydrin reagent. 

One product of the reaction of an amino acid with ninhydrin is the extensively conjugated purple ninhydrin product. The other major product is the aldehyde derived from the side chain of the amino acid. When valine reacts, the resulting aldehyde is 2-methylpropanal. The other products are carbon dioxide and water. 

The use of a detecting agent can be exemplified by that of ninhydrin. This reagent is used for the location of amino acids with which it gives purple spots on warming, through the following reaction sequence. 

The reaction of amino acids (and peptides) with ninhydrin has been known for nearly a century. Ninhydrin reacts rapidly with all primary amines to produce characteristic blue-colored spots. 

Predict products of the following reactions of amino acids esterification, acylation, reaction with ninhydrin. 

Reaction of an a-Amino Acid with Ninhydrin (Section 27.2D) Treatment of an a-amino acid with ninhydrin gives a purple-colored solution. Treatment of proline with ninhydrin gives an orange-colored solution. 

STEPS IN THE REACTION OF AN AMINO ACID WITH NINHYDRIN TO FORM A COLORED PRODUCT... 

If ninhydrin is added to each of the fractions, the concentration of amino acid in each fraction can be determined by the amount of absorbance at 570 nm because the colored compound formed by the reaction of an amino acid with ninhydrin has a A ax of 570 nm (Section 14.19). This information combined with each fraction s rate of passage through... 

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