Biology – Testing for biological molecules | e-Consult

Reagents Required:

• Copper(II) sulfate solution (CuSO₄)
• Sodium hydroxide solution (NaOH)
• The sample containing the protein

Procedure:

1. Add a few drops of the sample to a test tube.
2. Add an equal volume of sodium hydroxide solution to the test tube.
3. Add several drops of copper(II) sulfate solution to the test tube.
4. Mix the solution thoroughly.

Expected Results:

• Positive Result: The solution will turn a violet or purple colour.
• Negative Result: The solution will remain blue.

Chemical Basis: Proteins contain peptide bonds, which are formed between amino acids. When copper(II) ions (Cu²⁺) react with peptide bonds in the presence of a strong alkaline solution (NaOH), they form a complex compound. This complex is responsible for the characteristic violet or purple colour. The more peptide bonds present (i.e., the more protein), the stronger the colour change. Amino acids without peptide bonds (free amino acids) will not give a positive result.