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Experiment 1: Testing for Proteins

 

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 5). 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. Materials

 

© 2013 eScience Labs, LLC.

 

All Rights Reserved (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

 

Permanent Marker

 

5 Pipettes 5 Test Tubes (Glass)

 

Test Tube Rack

 

5 mL Unknown Solution

 

*Tap Water

 

*Hot Water

 

*Egg White *You Must Provide Procedure

 

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

 

Sample Initial Color Final Color 1 - Albumin Solution

 

2 - Gelatin Solution

 

© 2013 eScience Labs, LLC.

 

All Rights Reserved Protein Present 3 - Glucose

 

4 - Water

 

5 - Unknown Post-Lab Questions

 

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, iodinepotassium iodide (IKI) may also be used when working with starch. IKI contains special triiodine 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? © 2013 eScience Labs, LLC.

 

All Rights Reserved © 2013 eScience Labs, LLC.

 

All Rights Reserved

 







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