Describe how to tell if a substance is a protein by using the xanthoproteic test?

Answer 1

Nitration of benzoid radicals present in the amino acid side chain occurs due to reaction with nitric acid giving the solutions a yellow colouration.

This test is answered by aromatic amino acids like tyrosine and tryptophan. Phenyl alanine gives a weak positive reaction. When a preotein solution is heated with concentrated nitric acid, the benzene ring under goes nitration to form yellow nitro derivatives. When treated with NaOH the sodium salt formed is tense orange in color.

Answer 2

To determine if a substance is a protein using the xanthoproteic test, you first need to add nitric acid to the substance. If a yellow color develops upon addition of nitric acid followed by a warming step, this indicates the presence of proteins. The yellow color is due to the formation of nitro derivatives of phenylalanine residues in proteins.