Secrets of protiens

Proteins can be identified by three groups of tests:
1. Colour reaction,
2. Coagulation reaction, and
3. Precipitation reaction.
1. Colour Reaction:
It depends upon the characteristic radicles in the molecule.
ADVERTISEMENTS:
i. Biuret Tests (Piotrowski’s Rose’s):
To 2 to 3 ml of the protein solution in a test tube, when an equal volume of concentrated caustic soda solution and one drop or two drops of 1% copper sulphate solution are added, a distinct violet colour develops. This reaction is due to the presence of peptide linkage in the protein molecule.
ii. Xanthoproteic Reaction:
To 2 to 3 ml of the protein solution when few drops of concentrated nitric acid are added, a white precipitate forms. It is boiled for a minute the precipitate turns yellow and partly dissolves to furnish a yellow solution. It is then cooled and concentrated caustic soda is added till the reaction is alkaline. The yellow colour change to an orange one. This reaction is due to the presence of the phenyl group in the protein molecule.
iii. Millon’s Reaction:
To 5 ml of the protein solution when few drops of Millon’s reagent are added-a white precipitate forms which on heating changes to a brick-red coagulum. This is due to the presence of the tyrosine in the protein molecule.
iv. Molisch’s Reaction:
To 3 or 4 ml of protein solution, three or four drops of an alcoholic solution of thymol are added. The solution is well mixed and to it few drops of concentrated sulphuric acid are added when a distinct purple or violet ring develops at the junction of the two liquids. The reaction is due to the presence of carbohydrate radicles in the protein molecule.
v. Adamkiewicz’s Reaction:
To 3 or 4 ml of protein solution excess of glacial acetic acid is added and heated. It is cooled and few ml of concentrated sulphuric acid are allowed to flow down the side of the inclined test tube, a purple colour develops at the junction of two liquids within a short time. The reaction is due to presence of tryptophan in protein molecule.