Test of proteins
(i) Biuret test : On adding a dilute solution of copper sulphate to alkaline solution of protein, a violet colour is prepared. This test is due to the presence of peptide (-CO-NH-) linkage.
(ii) Xanthoproteic test : Some proteins give yellow colour with concentrated nitric acid (formation of yellow stains on fingers while working with nitric acid in laboratory). The formation of yellow colour is due to reaction of nitric acid with benzenoid structures. Thus, when a protein solution is warmed with nitric acid a yellow colour may be developed which turns orange on addition of NH4OH solution.
(iii) Millon's test : When millon's reagent (mercurous and mercuric nitrate in nitric acid) is added to a protein solution, a white precipitate which turns brick red on heating, may be formed. This test is given by proteins which yield tyrosine on hydrolysis. This is due to presence of phenolic group.
(iv) Ninhydrin test : This test is given by all proteins. When a protein is boiled with a dilute solution of ninhydrin, a violet colour is provided.
(v) Nitroprusside test : Proteins containing -SH group give this test. When sodium nitroprusside solution is added to proteins having -SH group, a violet colour is developed.
(vi) Molisch's test : This test is given by those proteins which contain carbohydrate residue. On adding a few drops of alcoholic solution of a-naphthol and concentrated sulphuric acid to the protein solution, a violet ring is created.
(vii) Hopkins-Cole test : On adding concentrated sulphuric acid down the side containing a solution of protein and glyoxalic acid, a violet colour is developed.
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