Protein coagulation and the biuret test
ChemistryBiomolecules & PolymersAges 17–18Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Biomolecules & Polymers
Five test tubes of egg-white solution are heated or treated with HCl, ethanol and CuSO₄, and the fifth undergoes the biuret test (NaOH + CuSO₄). Students see the protein coagulate into a white solid or precipitate and the biuret tube turn violet; switch the sample to a tripeptide, dipeptide, glycine or glucose to see that only molecules with at least 2 peptide bonds give the violet color. A results table records the observation and explanation for each tube.
Lesson: Peptides, proteins and enzymes – denaturation (coagulation) and the biuret test
What it shows
A protein keeps its function only while its folded three-dimensional structure is intact. Heat, acids, alcohol and heavy-metal ions break the hydrogen bonds and other interactions that hold this structure together, so the protein is irreversibly denatured and coagulates out of solution. The biuret test detects peptide bonds: in alkaline solution, Cu²⁺ ions form a violet complex with molecules that contain at least two peptide bonds. Comparing egg white with a tripeptide, a dipeptide, an amino acid and glucose shows exactly what each test responds to.
How to use
Choose a Sample, then tap each test tube or press Do all 5 tubes in turn and read the results table. Set the Heating temperature, tube 1 below 60 °C to show that coagulation needs enough heat. Repeat with the other samples and compare tube 5: only egg white and the tripeptide turn violet. Increase the Drops of 2 % CuSO₄ to see how excess reagent adds a blue precipitate that makes the color harder to judge.
Parameters you can change
- Sample Egg white (protein), Tripeptide Gly-Ala-Gly, Dipeptide Gly-Ala, Glycine, Glucose
- Heating temperature, tube 1 40–100 °C
- Drops of 2 % CuSO₄ 1–5 drops
- Simulation speed 0.5–3 ×
Questions to explore
- Why is milk or raw egg white given as first aid to someone who has swallowed a heavy-metal salt?
- Which test distinguishes Gly-Ala from Gly-Ala-Gly, and what do you observe with each one?
- Glucose and egg white both react with Cu(OH)₂. How do the colors of the resulting solutions differ, and why?
