Biochemical Tests: Proteins (AQA AS Biology): Revision Note

Biochemical tests: proteins

• A liquid solution of a sample is treated with sodium or potassium hydroxide to make the solution alkaline

• A few drops of copper (II) sulfate solution (which is blue) are added to the sample

  • Biuret reagent contains an alkali and copper (II) sulfate and is commonly used

• If a colour change is observed from blue to lilac/purple, then protein is present

  • The colour change can be very subtle, it’s wise to hold the test tubes up against a white tile when making observations

• If no colour change is observed, no protein is present

  • For this test to work, there must be at least two peptide bonds present in any protein molecule, so if the sample contains only amino acids or dipeptides, the result will be negative

• The Biuret test is qualitative - it does not give a quantitative value as to the amount of protein present in a sample