TESTS ON AMINO ACIDS AND PROTEINS

1. Ninhydrin Reaction Spot the solutions of amino acids provided on a filter paper, add a drop of ninhydrin reagent to each spot and hold the paper over a bunsen flame to dry. Note and comment on the colors produced. Expected results: Amino acids give purple colors with ninhydrin reagent. But proline (an imino acid) gives yellow color with ninhydrin.

2. Xanthoproteic Reaction Add 1 ml of conc. nitric acid to 1 ml of the amino acid solution. Warm the mixture in a boiling water bath and allow to cool. Note the formation of precipitates and colors. Add sufficient 40% NaOH to make the solution strongly alkaline. Expected result: A yellow color in acid solution which turns bright orange in alkali indicates the presence of one or more of the aromatic amino acids: tryptophan, phenylalanine, and tyrosine.

3. Millon’s Reaction Add 5 drops of Millon’s reagent to 1 ml of the test solution and warm the mixture in a boiling water bath for 10 minutes. Cool to room temperature and add 5 drops of sodium nitrate solution. Expected result: The formation of a brick-red color indicates the presence of a phenolic amino acid, i.e., tyrosine or its derivatives. Phenols and naphthols also give positive results.

4. Violet Ring Test Add 2 ml of glacial acetic acid that has been exposed to light to 2 ml of the test solution. Pour 2 ml of conc. sulfuric acid carefully down the sides of the test tube in a sloping position so as to form two layers. Expected result: A violet ring at the interface indicates the presence of tryptophan.

5. Nitroprusside Test Mix 0.5 ml of a fresh solution of sodium nitroprusside (C5FeN6Na2O or Na2Fe(CN)5NO) with 2 ml of the test solution. Add 0.5 ml of ammonium hydroxide. Note the color formed. Expected result: Thiol groups (cysteine) react with sodium nitroprusside in the presence of excess ammonia to give a red color.

6. Sagakuchi Reaction Mix 1 ml of 40% sodium hydroxide with 3 ml of arginine solution and add 2 drops of α-naphthol. Mix thoroughly and add about 4 drops of bromine water. Note the color formed. Expected result: The guanidine group of arginine reacts with α-naphthol and an oxidizing agent such as bromine water to give a red color.

7. Biuret Test Biuret Test for Proteins: Biuret reagent is a light blue solution which turns purple when mixed with a solution containing protein. When the copper ions of the Biuret Reagent react with peptide bonds in the polypeptide chains, a purple color complex is formed. To 2 ml of the test solution add 5 drops of biuret reagent. Mix thoroughly and note the colors formed. This is a test for the presence of two or more peptide bonds in solution. The intensity of the color produced is proportional to the number of peptide bonds present.

Determination of Protein Concentration: Biuret Method You are provided with protein samples, e.g., serum. Place the following reagents into various test tubes. Test (ml) Blank (ml) Test Sample 0.1 - 3%Sodium deoxycholate 0.4 Distilled water 0.5 0.6 Biuret reagent 4.0

Mix the contents of the test tubes by gentle shaking and let stand for 20 minutes for color development. Then read the absorbance of each test solution against the blank at 540 nm. You are also provided with a standard solution of albumin (bovine serum albumin) with a concentration of 10 mg/ml. Prepare a standard curve using various concentrations of the albumin. This can be done by replacing the test sample in the table above with various amounts, i.e., 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 ml, of the albumin. Adjust the amount of distilled water accordingly in order to keep the total volume constant at 5ml. Read the absorbance of the solution in each tube after 20 minutes and plot a graph of absorbance against concentration of albumin. Estimate the concentrations of the protein samples provided from your graph.