Advances in Fingerprint Technology.pdf

Figure 5.5 The mechanism of formation of Ruhemann’s purple (X). (From Friedman,
M. and Williams, L.D., Bioorganic Chem., 3, 267, 1974.)
base with the amino acid. The ketimine formed (VII) undergoes decarboxylation
and cleavage, yielding the aldehyde and an intermediate amine (IX).
Condesation of the amine (IX) with another molecule of ninhydrin (I) follows
to form the chromophore, Ruhemann’s purple (X). A slight modification
of this mechanism was suggested in 1986 by Grigg and co-workers. 32
They indicated that the intermediate imine (VIII) exists as a 1,3-dipole (XI,
Figure 5.6), and that 1,3-dipole form is also the structure of the protonated
form of Ruhemann’s purple (XII, Figure 5.7).
The deprotonated form, which is the colored product, Ruhemann’s purple
(X), has a highly stabilized aza-allylic chromophore that is responsible
for the color (XIII, Figure 5.7). A summary of the mechanistic approaches
was published by Bottom and co-workers. 33 Grigg’s modification, which
involves the formation of an azomethine ylide, is the currently accepted
mechanism. 34
Ninhydrin as a Latent Fingerprint Reagent
Shortly after the discovery of the ninhydrin reaction with amino acids, scientists
noticed that many common materials form blue-colored products
upon reaction with ninhydrin. In 1913, three years after the first report on
ninhydrin, Abderhalden and Schmidt in Germany wrote: 35