Question

A solution of several different kinds of amino acids will not give a positive reaction with Biuret Reagent. However, a solution of protein made from those same amino acids will give a positive reaction with Biuret Reagent. Explain the reason why.

Answer 1

Biuret reagent reacts with one or more peptide bonds of proteins to give positive result.

Cu(II) ions biuret reagent reacts with nitrogen of peptide bond and gets reduced into Cu(I). Mixture of amino acids do not have peptide bonds, while proteins made up of those amino acids have peptide bonds.

Therefore, amino acid mixture would not give positive result for biuret reagent, while protein made of those amino acids would give positive result due to the presence of peptide bonds.