Question

What part of the proteins structure produces a color with the reagents used a) in the biuret method? b) in the Lowry method?

Answer 1

a) Biuret Method:

  The reagent used in the Biuret Test is a solution of copper sulfate (CuSO4) and potassium hydroxide (KOH).

In proteins contain peptide bonds. the copper (II) ions will form a coordination complex with four nitrogen atoms involved in peptide bonds, when peptide bonds are present in the alkaline solution.

  

Copper Sulfate solution is a blue color, but when the copper (II) ions are coordinated with the nitrogen atoms of these peptide bonds, the color of the solution changes from blue to violet.

Colour changes depends on the number of peptide bonds

b) Lowry method:

The peptide bonds under alkaline conditions with the oxidation of aromatic protein residues. Lowry’s reaction in which the Folin Ciocaltaeu reagent which contains phosphomolybdic complex which is a mixture of sodium tungstate, sodium molybdate and phosphate, along with copper sulphate solution and the protein, a blue purple colour is produced which can be assessed by measuring the absorbance at 650-700nm.