How many of the given compounds will give a positive Biuret test ____________ ?
Glycine, Glycylalanine, Tripeptide, Biuret

Solution

Chemistry ma’am · 3 steps

Count the peptide-type -CO-NH⁻ links: the Biuret test needs at least two of them.

1. What the test needs. In the Biuret test, Cu²⁺ in alkaline solution binds to the nitrogens of neighbouring -CO-NH⁻ groups and turns violet. One such link cannot hold the copper ion, so a compound needs at least two of these links.

   Compound	-CO-NH⁻ links	Biuret test
   Glycine (amino acid)	0	negative
   Glycylalanine (dipeptide)	1	negative
   Tripeptide	2	positive
   Biuret, H₂N-CO-NH-CO-NH₂	2	positive

2. About biuret itself. Biuret is not a peptide, but it has two -CO-NH⁻ units side by side, just like a peptide chain. The test is named after it because it gives the same violet colour.
3. So. Only the tripeptide and biuret give a positive test, so the answer is 2.

The common slip: 3. Counting the dipeptide glycylalanine as positive gives 3. It has only one peptide bond, and the copper ion needs two.

The idea to keep. The Biuret test detects two or more -CO-NH⁻ links, so it picks out tripeptides and larger. Amino acids and dipeptides fail it, and biuret itself passes.

Answer from the NTA final answer key. Solution by PrepWiser.