Proteins: Biuret test. Proteins are made of amino acids, each of which has an
amino group (–NH2), a carboxyl (acid) group (–COOH), and a variable side chain. A
peptide bond forms between the amino group of one amino acid and the carboxyl group
of an adjacent amino acid. Proteins are identified by a Biuret test. Peptide bonds (C – N
bonds) in proteins complex with Cu2+ in Biuret reagent and produce a violet color. A Cu2+
must complex with four to six peptide bonds to produce a color; therefore, individual amino
acids do not react positively. See results below.
Several solutions and their test results for the Biuret test are presented in the table below:
Which of the solutions is a positive control? Which is a negative control?
Which contains more protein (C – N bonds), egg albumen or amino acid solution?
How can you tell?
Color Reactions for the Biuret Test for Protein
tube solution color
1 2 ml egg albumen purple
2 2 ml amino acid solution blue
3 2 ml distilled water blue
4 2 ml protein solution purple
Do free amino acids have peptide bonds?
What are the functions of proteins in living organisms?
