Proteins that contain tyrosine develop yellow colour when heated with `HNO_(3)`. This test is known as

Options

A. Ninhydrin test
B. Biuret as
C. Xanthoproteic test
D. Millon's test

Answer

C

Explanation

When a protein is treated with conc. `HNO_(3)`, a yellow colour is produced. This test, called xanthoproteic test, is given by a protein which consists of `alpha`-amino-acids containing a benzene ring such as tyrosine, phenylalanine, etc, and the yellow colour is due to the nitration of the benzene ring. An important example of this test is that when cone. `HNO_(3)` is spilled on our hands the skin turns yellow due to nitration of benzene ring of the amino acids of the proteins present in our skin.
When proteins are boiled a dilute aqueous solution of ninhydrin (2,2-dihydroxyindane - 1,3- dione), a blue - viloet (intense purple) colour is produced. This test, called ninhydrin test, is actually given by almost all `alpha`-amino acids. Since proteins and peptides on hydrolysis give `alpha`-amino acids, ninhydrin test is also given by them.
An alkaline solution of a protein produces a violet colouration on treatment with a few drops of `1% CuSO_(4)` solution. This test is called biuret test and the colour is due to the formation of a complex of `Cu^(2+)` ions with and`
and - NH-` groups of the peptide linkage when Million's reagent (a solution of mercurous nitrate and mercuric nitrate in nitric acid containing little nitrous acid) is added to the aqueous solution of a protein, a white ppt. is formed. This test, called million's test is given by all proteins containing phenolic `alpha`-amino acids, i.e., tyrosine. Gelatine, which does not contain phenolic `alpha`-amino acids does not give this test.