Biuret test is not given by

The biuret is ${\mathrm{NH}}_2\mathrm{CONHCONH}$ When an aqueous biuret solution is treated with sodium hydroxide and a drop of copper sulphate solution, a violet colour is produced. This is known as biuret reaction, which is characteristic of all compounds containing the grouping $-\mathrm{CONH}-\text{, e.g., }$ proteins, polypeptide and urea (which on gently heated form biuret).