Xanthoproteic test: Principle, Reaction, Reagents, Procedure and Result Interpretation

Xanthoproteic test

Detection of aromatic amino acids

Yellowish substance produced by heating protein with nitric acid is xanthoprotein, hence the name xanthoproteic for the test. This test is performed to identify the amino acid having benzene ring in them like tyrosine, tryptophan, and phenylalanine.

Principle:

Treating aromatic amino acids with the concentrated nitric acid leads to the nitration of the aromatic ring and formation of yellow nitro-products (nitro derivatives). When the strong basic solution is added the colour of obtained products turns darker (from yellow to orange). The positive result of xanthoproteic reaction gives: tyrosine, tryptophan and phenylalanine (only after extended heating time). It is difficult to nitrate phenylalanine under normal conditions so it does not respond to this test without extended heating.

Reaction:

Reagents:

  • Test solution: 1 % tyrosine, 1 % tryptophan, 1 % phenylalanine,1% any other amino acid, protein solution (albumin)
  • Nitric acid
  • 40 % NaOH

Procedure:

  • To 1 ml of test solution, add 1ml of concentrated HNO3
  • Mix and heat (In case of protein solution, initially white precipitate appears due to denaturation of protein, which turns yellow on heating).
  • Cool under tap water
  • Add 1 ml of 40% NaOH to make the solution alkaline
  • Observe whether the mixture turns orange red in colour.

Result Interpretation:

Positive test: Colour changes to orange indicating the presence of aromatic amino acids

Negative test:  Colour doesnot changes indicating absence of aromatic amino acids

References:

  • http://www.biologydiscussion.com/proteins/qualitative-and-quantitative-tests-for-amino-acids-and-proteins/13065
  • http://ecoursesonline.iasri.res.in/mod/page/view.php?id=53513
  • https://www.onlinebiologynotes.com/xanthoproteic-test-objective-principle-reagents-procedure-and-result/
  • https://www.merriam-webster.com/medical/xanthoproteic%20test
About Anup Basnet 30 Articles
Lecturer of Biochemistry in St. Xavier's College, Maitighar, Kathmandu, Nepal. Also Visiting Faculty of: Central Department of Microbiology (Tribhuvan University(TU), Nepal), Central Department of Biotechnology (Tribhuvan University (TU), Nepal), Amrit Science Campus (ASCOL) (Kathmandu, Nepal).

Be the first to comment

Leave a Reply

Your email address will not be published.


*