Biuret reagent reacts with peptide bonds in proteins Peptide

Biuret reagent reacts with peptide bonds in proteins. Peptide bonds form between amino acids by the action of a polymerase enzyme. If you have a 0.1M solution of the amino acid glycine and test it for peptide bonds, would the reaction be positive (purple) or negative?

Solution

Biuret reagent contains 3 components:

1) Hydrated copper suplhate: This gives Cu²⁺ ions which chelates with the peptide bonds present in glycine (amino acid) giving characteristic blue colour.

2) Potassium hydroxide solution: This only provides alkaline medium for this reaction

3) Potassium sodium tartarate: This stablizes the chelate complex.

The chelate complex absorbs light at 540 nm so appears deep purple. Therefore 0.1 M solution of glycine will give purple colour in Biuret test. But Glycine alone when tested for Biuret won\'t provide purple colour as for formation of chelate complex 2 amino acids are needed.

We can conclude, that proteins provide positive result with Biuret test but only amino acid (1 amino acid) will give negative rest with Biuret test.