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B1.2.6—Chemical diversity in the R-groups of amino acids as a basis for the immense diversity in protein form and function


Here are experiments our science specialists have selected to support this IB* topic.

Investigating Protein

Experiment #5 from Investigating Biology through Inquiry

In the Preliminary Activity, you will use a spectrophotometer and the Bradford protein assay to determine the protein content of milk. The Bradford protein assay is an extremely sensitive assay for protein. The Bradford reagent contains a dye called Coomassie G-250 that can interact with the R‑group of specific amino acids. The dominant protein in milk is called casein, which is composed of 224 amino acids. Thirteen of these amino acids react with the dye in the Bradford reagent. These amino acids include one tryptophan, four arginines, four tyrosines, and four histidines.

When the dye in the Bradford reagent interacts with these specific amino acids it turns the solution blue. The greater the concentration of protein in solution the deeper the color will be. If a set of known protein concentrations are allowed to react with a known concentration of Bradford reagent, we can measure the absorbance of the resulting solutions to create a standard curve. When a graph of absorbance vs. concentration is plotted for the standard solutions, a direct relationship should result. The direct relationship between absorbance and concentration for a solution is known as Beer’s law. To determine the protein concentration of an unknown solution, we can measure its absorbance and see where it falls on the standard curve. Because the relationship is linear, we could also calculate the protein concentration using the formula for the standard curve.

After completing the Preliminary Activity, you will first use reference sources to find out more about protein before you choose and investigate a researchable question. Some topics to consider in your reference search are:

  • macromolecule
  • protein
  • monomer
  • polymer
  • amino acid
  • alpha carbon
  • carboxyl group
  • primary structure
  • amino group
  • R group
  • peptide bond
  • polypeptide
  • Bradford protein assay
  • Beer’s law

Macromolecules: Experiments with Protein

Experiment #17 from Advanced Biology with Vernier

In this experiment, you will

  • Create a standard protein curve using the Bradford assay.
  • Determine the protein concentration of milk and a high protein drink.
  • Determine if the Bradford assay can detect both proteins and amino acids.
  • Determine if the Biuret assay can detect both proteins and amino acids.

Educational Standard
International Baccalaureate (IB) 2025
B Form and function
Level of organization
1 Molecules
B1.2 Proteins
Additional higher level

* The IB Diploma Program is an official program of the International Baccalaureate Organization (IBO) which authorizes schools to offer it. The material available here has been developed independently of the IBO and is not endorsed by it.