Presence of peptide bonds is detected by performing a chemical test named biuret test. . In this test the sample is first heated and then Sodium Hydroxide is mixed into it.
Drops of 1 percent copper (II) sulphate is added slowly. Copper II is reduced to copper I in a positive test.
This helps to form a complex with the nitrogen and carbons of the peptide bonds in an alkaline solution. A change in the color of the test sample will give out a positive or negative result. When the sample turns into purple it means that the sample contains proteins. Peptide bonds occur with approximately the same frequency for most proteins per gram of material. So to determine the concentration of proteins biuret reaction can be used. If the concentration is the more, the sample will change into a deeper purple.
Many proteins contain sulfur. They are complex with molecules composed of carbon, hydrogen, oxygen, and nitrogen. Amino acids are the result of these blocks of proteins and they are connected by peptide bonds. There is a lot of similarity between these amino acids and the biuret molecule and both of them react in the same way. Biuret reagent is a light blue solution, which turns purple when mixed with a solution containing protein. A purple color complex is formed when the copper ions of the Biuret Reagent react with peptide bonds in the polypeptide chains.
Since proteins are made up of amino acids, presence of peptide bonds during Biuret test for proteins will always give a positive result for all kinds of protein based foods.
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