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 groupof 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?