Biuret reaction is specific for: (A) –CONH-linkages, (B) –CSNH2 group, (C) –(NH)NH2 group, (D) All of these
Answer is A. 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