Xanthoproteic Test

Xanthoproteic test is usually performed to detect the presence of aromatic amino acids present in the given sample. Xanthoproteic test is generally used to detect the presence of phenolic or indolic groups i.e. All types of aromatic amino acids. As a result Yellow precipitate remains due to presence of xanthoproteic acid. The term ‘Xanthoproteic’ refers as (‘Xantho’ = Yellow and ‘Proteic’ = Protein) thus this test is also known as Yellow Protein Test. This test is only qualitative test that provides the information about presence of aromatic amino acids i.e. presence of benzene ring.

Why Xanthoproteic Test?

• This test is used to detect the presence of only aromatic amino acids like Tryptophan, Tyrosine etc.
• If xanthoproteic test gives positive result then and then only you need to perform further tests like Sulfur test, Hopkins-Kole test, Millon’s test, Sakaguchi test, etc.

Principle of Xanthoproteic Test

Nitration of the phenyl rings in the aromatic amino acids due to the HNO3 gives a yellow color due to the nitro substitution products. Addition of alkali produces orange color due to the formation of Na-salt (nitro derivatives). Amino acids containing a benzene ring such as phenylalanine do not give this test a positive test because the phenyl group in phenylalanine is very stable, which does not react with nitric acid under the conditions of xanthoproteic test. However, phenylalanine can give positive results after extended periods of heating.

Requirements of Xanthoproteic Test

Instruments and Materials

• Test tubes
• Test tube stand
• Pipettes
• Dropper
• Bunsen burner

Chemicals and Reagents

• 3 ml of test solution
• 40% of NaOH solution
• 1ml of conc. HNO3

Procedure of Xanthoproteic Test

1. Take a dry and sterile test tube.
2. Add 3 ml of the given sample solution into the test tube with the help of dropper.
3. Add a 1 ml of concentrated sulfuric acid (HNO3).
4. Boil the test solution gently on a Bunsen burner.
5. The solution turns yellow; then cool the test solution.
6. Add 40% of NaOH solution till solution turns alkaline.
7. Observe the change in colur.
8. Formation of a deep yellow or orange color confirms that test is positive.

Observation and Inferences of Xanthoproteic Test

Formation of a deep yellow or orange color gives this test positive.

Results and Interpretation of Xanthoproteic Test

Formation of a deep yellow or orange color gives this test positive and confirms the presence of amino acids that containing aromatic group. All other results gives this test negative and confirms the presence of non-aromatic amino acids.

References and Sources

• https://microbenotes.com/xanthoproteic-test/
• https://www.onlinebiologynotes.com/xanthoproteic-test-objective-principle-reagents-procedure-and-result/
• https://en.wikipedia.org/wiki/Xanthoproteic_reaction
• http://biocheminfo.com/2020/04/04/xanthoproteic-test-principle-reaction-reagents-procedure-and-result-interpretation/
• https://byjus.com/jee-questions/do-all-amino-acids-give-a-positive-xanthoproteic-test/
• https://www.oxfordreference.com/view/10.1093/oi/authority.20110803125154569
• http://ecoursesonline.iasri.res.in/mod/page/view.php?id=53513

Further Readings

1. 8 Qualitative Tests for Protein
2. Ninhydrin Test
3. Acid Fast Staining
4. Algae
5. Aseptic Transfer Technique
6. Bacterial Flagella, Fimbriae and Pili
7. Bacterial Growth and Nutrition
8. Growth Curve of Bacteria
9. Instruments Used in Microbiology Laboratory
10. MacConkey agar
11. McFarland Standards
12. Monochrome Staining
13. Negative Staining
14. Nutritional Requirements of Micro-Organisms
15. Serial Dilution in Microbiology
16. Spread Plate Technique
17. Streak Plate Technique