Biuret reaction is specific for
Biuret Reaction : Cupric ions in an alkaline medium form a violet color with peptide bond nitrogen (Schiff, 1896). This needs a minimum of two peptide bonds, and so individual amino acids and dipeptides will not answer this test.
Reference: Vasudevan 7th  ed, pg 31
–CONH-linkages
–CSNH2 group
–(NH)NH2 group
All of these