Introduction
Proteins are important components of all organisms. A protein molecule is made up of one or more amino acids linked together by polypeptide chains. The lowest members are derived from only two amino acids forming simple proteins (e.g. albumin, globulin, prolamin).Complex proteins incorporates other groups as part of the molecule other than amino acids, examples are carbohydrates in mucoproteins, chlorophyll molecules in the protein chloroplasts, phosphorus containing proteins such as casein, nucleoproteins in which proteins are combined with nucleic acids and the lipoproteins of the cytoplasm, in which proteins are combined with lipids. All these complex compounds have two or more molecules of amino acids united by a peptide linkage which results from the elimination of water, an –OH coming from one amino acid and an –H from the other. Each protein has its unique sequence of amino acids.
Identification tests for proteins
- Biuret test:- To a solution in a test tube containing the sample, add 2ml of sodium hydroxide and 5 to 6 drops of copper sulphate solution. Shake the test tube gently to mix the contents thoroughly and allow the mixture to stand for 4-5 minutes. The appearance of a bluish-violet colour, indicates the presence of proteins.
- Xanthoproteic test:- To a test tube containing the sample, add a few drops of concentrated sulfuric acid and shake test tube, heat gently on a Bunsen burner.The appearance of a yellow precipitate confirms the presence of proteins.
- Millon’s test:- To a test tube containing the sample, add 2-3 drops of Millon’s reagent and shake well. The appearance of a white precipitate which turns to brick red on heating confirms the presence of proteins.
- Ninhydrin’s test:- To a test tube containing the sample, add 1-2ml of Ninhydrin solution and shake the test tube. Boil the mixture and observe the change. The appearance of a deep blue or purple colour confirms the presence of proteins.