a) Xanthoproteic reaction

The addition of concentrated nitric acid to protein solutions generally causes the formation of a white precipitate which turns yellow upon heating, the colour becoming orange when the solution is made alkaline. Insoluble proteins are turned yellow and orange on the surface. The xanthoproteic reaction is due to nitration of the phenyl rings present in tyrosine, phenylalanine, and tryptophan to give yellow nitrosubstitution products, which become orange-coloured upon the addition of alkali. Most proteins give the xanthoproteic reaction.

b) Biuret Reaction

When protein solutions are made strongly alkaline with sodium or potassium hydroxide and when very dilute copper sulphate is added, a purplish to pinkish violet is obtained, the colour depending upon the complexity of protein. Proteins give a purplish violet colour while proteoses and peptones give a pink colour. Peptides give a very light pink colour and gelatin gives a blue colour. This biuret reaction is used as a test for the presence of proteins in biological materials. It is also used as an excellent method for the quantitative estimation of proteins.