Why does phenylalanine gives negative result in Xanthoproteic test?

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.

Which of the following will give a positive reaction to the Xanthoproteic test *?

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.

What is responsible for the color reaction of proteins with Xanthoproteic test?

Nitric acid reacts with proteins to form yellow nitrated products. This reaction is known as the xanthoproteic reaction. These color changes are caused by nitrated aromatic rings in the protein. The xanthoproteic test is specific for aromatic compounds such as tyrosine, tryptophan and phenylalanine.

What happens when The xanthoproteic reaction is positive?

The test gives a positive result in amino acids carrying aromatic groups, especially in the presence of tyrosine. If the test is positive the proof is neutralized with an alkali, turning dark yellow.

What do you need to know about The xanthoproteic test?

Xanthoproteic Test Definition. Xanthoproteic test is a biochemical test for the detection of amino acids containing phenolic or indolic groups like phenylalanine, tyrosine, and tryptophan (aromatic amino acids). The test is named Xanthoproteic test due to the formation of a yellow precipitate of xanthoproteic acid.

How does The xanthoproteic test differentiate tyrosine and tryptophan?

To differentiate tyrosine and tryptophan from other amino acids. The Xanthoproteic test is based on the fact that aromatic groups in the amino acids are nitrated by heating with concentrated HNO 3 to yield intensely yellow-colored nitro derivative.

How to do The xanthoproteic test for NaOH?

Take 1ml test solution in dry test tube. Similarly, take 1ml distilled water in another test tube as control. Add 1ml of conc. HNO3 in all test tubes and mix well. Cool the solution under tap water. Now add 2ml of 40 % NaOH to all test tubes.