Proteins are about 50% of the dry weight of most cells, and are the most structurally complex macromolecules known. Each type of protein has its own unique structure and function.
Polymers are any kind of large molecules made of repeating identical or similar subunits called monomers. The starch and cellulose we previously discussed are polymers of glucose, which in that case, is the monomer. Proteins are polymers of about 20 amino acids (the monomer).
In the approximately-20 amino acids found in our bodies, what varies is the side chain. Some side chains are hydrophilic while others are hydrophobic. Since these side chains stick out from the backbone of the molecule, they help determine the properties of the protein made from them
Test for Protein and Amino Acids 1.Biuret test The Biuret test can be used qualitatively to detect protein and can be used quantitatively with the colorimeter to detect the increasing purple colour of the positive test. Relatively large quantities of protein need to be present for this test to be effective
Add 1 ml of 40% sodium hydroxide solution and 2 drops of 1% copper sulphate solution till a blue color is produced, and then add to the 1 ml of the extract. Formation of pinkish or purple violet color indicates the presence of proteins.
2.Ninhydrin test Add about 2 ml of the extract to 1 ml of a solution of 0.2 g of ninhydrin (1,2,3indanetrione monohydrate) in 50 mL of water. The test mixture is heated to boiling for 15-20 sec. This reaction is important not only because it is a qualitative test, but also because it is the source of the absorbing material that can be measured quantitatively by an automatic amino acid analyzer. This color reaction is also used to detect the presence and position of amino acids after paper chromatographic separation.
A blue to blue-violet color is given by a-amino acids and constitutes a positive test. Other colors (yellow, orange, red) are negative
3.Xanthoproteic test To 1 ml of the extract, add 1 ml of concentrated nitric acid. A white precipitate is formed, it is boiled and cooled. Then 20% of sodium hydroxide or ammonia is added. Orange color indicates the presence of aromatic amino
4.Million’s test 1 ml of test solution is made acidify with sulphuric acid and add Million’s reagent and boil this solution. A yellow precipitate is formed indicates the presence of protein.
An alternative test uses the Bradford method and is able to detect microgram amounts of protein. With care this test will permit accurate measurements of protein in different foods, plant and animal tissues, etc.
