Xanthoproteic Test
Xanthoproteic Test
Xanthoproteic
test is for the
aromatic amino acids, tyrosine
and tryptophan. The third aromatic
amino acid, phenylalanine dose not
respond this test.
Principle
 On addition of concentrated nitric acid, the white
precipitate is formed due to denaturation of proteins.
The benzene ring (phenyl groups) present in tyrosine
and tryptophan undergoes nitration on heating with
nitric acid. The nitro phenyl group imparts yellow colour
to the solution.
 On addition of the alkali (NaOH), the nitro phenyl
group ionizes imparting orange colour to the solution.
 Nitration of phenylalanine does not take place under
these conditions, therefore it does not respond to this
test, although it also contains benzene ring.
Reagents
1. Concentrated nitric acid.
2. Sodium hydroxide, (40 % solution in
water).
Procedure
 Take 2 ml of the given solution in the test tube
and add 1 ml of concentrated nitric acid. Mix it,
white precipitate is formed.
 Heat the solution for 1 minute and then cool
under tap water. Some of the precipitate
redissolves, and the entire solution becomes
yellow.
 Add 2 ml of 40% sodium hydroxide solution and
mix.
Interpretation
 Appearance of deep yellow or orange colour
will indicate the presence of aromatic
amino acids, tyrosine and tryptophan in the
protein.
 Gelatin which contains no tryptophan and
very little tyrosine, gives only a faint colour.