Explain the Principle or Theory of biuret method?

If a strongly alkaline solution of Biuret is heated with very dilute copper sulphate, a violet colour is obtained. The substances which contain two carbamyl (-CONH₂) groups joined together either directly or through a single carbon or nitrogen atom and those which contain two or more peptide links give a blue or purple coloured complex with alkaline copper solution.

Proteins respond positively since there are pairs of -CONH₂ groups in the molecule. The reaction is due to coordination of cupric anion with the unshared electron pair of peptide nitrogen and the oxygen of the water. A coordination complex is formed which is purplish coloured and can be colorimetrically measured. When protein solution is made strongly alkaline with NaOH or KOH and very dilute CuSO₄ is added, a purplish to pink violet colour is obtained, the intensity of the colour depending upon the concentration of protein or the number of peptide bonds undergoing the reaction.