The protein molecules in many foods provide the amino acid building blocks required by our own cells to produce new proteins. To determine whether a sample contains protein, a reagent called Biuret solution is used. Biuret solution contains copper ions. However, the chemical state of the copper ions in Biuret solution causes them to form a chemical complex with the peptide bonds between amino acids (when present), changing the color of the solution. Biuret solution is normally blue, but changes to pink when short peptides are present and to violet when long polypeptides are present.
Figure 6: Biuret solution only is located on the far left side of the image (blue). Note the transition from blue to violet as proteins are added to the solution, causing the solution to transition from blue to violet.
(2) 250 mL Beakers
25 Drops Biuret Solution, H2NC(O)NHC(O)NH
(1) Knox® Gelatin Packet
5 mL 1% Glucose Solution, C6H12O6
(1) 10 mL Graduated Cylinder
(1) 100 mL Graduated Cylinder
5 Test Tubes (Plastic)
Test Tube Rack
5 mL Unknown Solution
*You Must Provide
1. Label five test tubes 1, 2, 3, 4 and 5.
2. Prepare your testing samples as follows:
a. Mix one egg white with 25 mL water in a 250 mL beaker to create an albumin solution. Pipette 5 mL of this solution into Test Tube 1. b. Mix the packet of Knox® gelatin with 50 mL hot water in a second 250 mL beaker. Stir until dissolved. Pipette 5 mL of this solution into Test Tube 2. 3. Pipette 5 mL of the 1% glucose solution into Test Tube 3. 4. Use the 10 mL graduated cylinder to measure and pour 5 mL of water into Test Tube 4. 5. Pipette 5 mL of the “Unknown Solution” into Test Tube 5. 6. Record the initial color of each sample in Table 1.
7. Develop a hypothesis regarding what you predict will happen when Biuret solution is added to Tubes 1 – 4. Record your hypothesis in the Post-Lab Question section. Then, pipette five drops of Biuret solution to each test tube (1 – 5). Swirl each tube to mix. 8. Record the final color in Table 1.
Note: Protein is present in the sample if a light purple color is observed. Table 1: Testing for Proteins Results
1 – Albumin Solution
2 – Gelatin Solution
3 – Glucose
4 – Water
5 – Unknown
1. Record your hypothesis about what will happen when Biuret solution is mixed with the solutions from test tubes 1, 2, 3, and 4 here. Be sure to use scientific reasoning to support your hypothesis.
2. Write a statement to explain the molecular composition of the unknown solution based on the results obtained during testing with the Biuret solution and each sample solution.
3. Diet and nutrition are closely linked to the study of biomolecules. How should you monitor your food intake to insure the cells in your body have the materials necessary to function?
4. There are other types of reagents used to determine what type of biomolecule a substance is. For example, copper ions present in Benedict’s reagent reacts with the free end of any reducing sugars, such as glucose, when heated. Originally blue in color, these copper ions are reduced by the sugar, and produce an orange-red colored precipitate. Alternatively, iodine-potassium iodide (IKI) may also be used when working with starch. IKI contains special tri-iodine ions which interact with the coiled structure of a starch polymer. Prior to a reaction, the IKI displays a yellow-brown color; however, after reacting with starch, a dark purple or black color is presented.
The molecule pictured below produced a blue color when tested with Benedict’s reagent, a yellow color when tested with IKI, and a violet color when tested with Biuret reagent. Based on the structure shown below and these chemical results, what kind of biomolecule is this?