Biuret Test

You: I remember that the Benedict test is another test for reducing sugars, while the Diphenylamine test indicates the presence of DNA. 

[Benedict test: Benedict’s reagent is a bright royal blue color. It is made up of a solution of copper sulfate, sodium carbonate and sodium citrate. In the presence of a reducing sugar, it accepts electrons causing the color change. The solution also turns cloudy due to the formation of a precipitate of cuprous oxide. Any color change indicates the presence of a reducing sugar, the degree of color change depends on the concentration of the reducing sugar. A yellow or green color indicate a low concentration of reducing sugar, while a red or orange color are indicative of a large concentration of reducing sugar. For more information click on the Benedicts test option.

It is carried out similarly to Fehling’s test in that 2mL of Benedict’s reagent is added to 2 mL of the sample solution in a test tube. The mixture is then heated in a boiling water bath for 2 minutes and color changes are observed. 

Diphenylamine test: DNA can be specifically detected using the Dische diphenylamine reagent. The deoxyribose moiety of DNA is converted to a molecule that binds with diphenylamine to form a blue color, which is proportional to the concentration of DNA. 

Experimental procedure: 0.1-2 ml of DNA solution is added to 2 ml diphenylamine reagent, and placed in a boiling water bath for ten minutes. Observe for a light blue color forming. For more information click on the Diphenylamine test option.]

Dr. Brown: That’s right! Which test should you use then for detecting proteins?

Monica: Well, we learnt about two identification tests for proteins, the Biuret test and the Xanthoproteic test.

You: I remember that; doesn’t the Biuret test indicate the presence of peptide ponds, while the Xanthoproteic test indicates the presence of aromatic amino acids?

Dr. Brown: You are absolutely right, so which one is more appropriate for medical diagnostics?

You: The Biuret test is simpler to carry out and can identify all proteins, since all proteins have peptide bonds. Since the Xanthoproteic test specifically detects proteins that contain aromatic amino acids, it is useful when you need to confirm the presence of these specific amino acids in a protein sample. The Biuret test further allows for quantitative analysis of protein in the sample by measuring the intensity of color change using a spectrophotometer. 

[Xanthoproteic test:

The procedure uses egg albumin as the protein sample. To 3 ml of 1% egg albumin, 1 ml of concentrated nitric acid is added. A white precipitate should appear. The solution is boiled until it turns yellow. The solution is cooled and 2 ml of concentrated 40% NaOH are added to give an orange color, indicating the presence of protein. For more information click on the Xanthoproteic test.]

Dr. Brown: That is exactly right. What color change do you expect will occur?