Biuretic test is answered positive by proteins and peptides that posses atleast two peptide linkages (3 aminoacids) Histidine is the only aminoacid that gives positive by uret test. this test is used to detect the presence of proteins in biologoical fluids by formation of colored copper coordinated complex. REF :BIOCHEMISTRY-Dr.U.Satyanarayana and Dr.U.Chakrapani.3rd EDITION .Page-61

Histidine. ref: Biochemistry by U.Satyanarayana, Pg-61