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Biochemistry

This test detects aromatic amino acids like tyrosine and tryptophan in proteins. It works by causing the aromatic rings in these amino acids to undergo nitration when treated with concentrated nitric acid, producing yellow nitro derivatives. Phenylalanine gives a negative or weak result despite having an aromatic ring, because it is difficult to nitrate under normal conditions. Adding an alkali like sodium hydroxide to the yellow solution causes the color to change to orange. The test involves treating a protein solution like egg white with concentrated nitric acid and sodium hydroxide to detect the presence of aromatic amino acids.

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109 views3 pages

Biochemistry

This test detects aromatic amino acids like tyrosine and tryptophan in proteins. It works by causing the aromatic rings in these amino acids to undergo nitration when treated with concentrated nitric acid, producing yellow nitro derivatives. Phenylalanine gives a negative or weak result despite having an aromatic ring, because it is difficult to nitrate under normal conditions. Adding an alkali like sodium hydroxide to the yellow solution causes the color to change to orange. The test involves treating a protein solution like egg white with concentrated nitric acid and sodium hydroxide to detect the presence of aromatic amino acids.

Uploaded by

Womcidi
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Lect.6. Bio Chemistry Laboratory Reem.S.Najm.

C2

Xanthoproteic test
Objective:

This test is used for aromatic amino acids which give positive result
from other amino acids. Such tyrosine , and tryptophan react with
Xanthoproteic test, phenyl alanine does not respond with this test.

Principle:

Xanthoproteic test is used to detect amino acids containing an


aromatic nucleus (tyrosine, tryptophan and phenylalanine) in a
protein solution which gives yellow color nitro derivatives on heating
with conc. HNO3. The aromatic benzene ring undergoes nitration to
give yellow colored product. Phenylalanine gives negative or weakly
positive reaction though this amino acid contains aromatic nucleus
because it is difficult to nitrate under normal condition. On adding
alkali to these nitro derivative salts, the color change for yellow to
orange.

1
Lect.6. Bio Chemistry Laboratory Reem.S.Najm.

Reactions

Reagents:
1-test solution: 1 % tyrosine, 1 % tryptophan, 1 % phenylalanine, 5
% egg white (albumin).

2-Nitric acid Con .


3-NaOH % 44 .

2
.Lect.6. Bio Chemistry Laboratory Reem.S.Najm

Procedures:
Take 1ml of albumin solution in dry test tube.
Add 1ml of conc. HNO3 in all test tubes and mix well
Cool the solution under tap water, becomes yellow.
Now add 2ml of 40 % NaOH to all test tubes, becomes
orange.

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