What is the principle of the biuret test? While reading online, it is unclear to me how a bond is created between the $\ce {Cu^{2+}}$ and the proteins? What are the chemical reactions that lead to the color change?


The biuret test is a chemical test combining two products, $\ce{CuSO_{4}}$ and $\ce{OH^{-}}$ in my case. While exploring online the possible effects on protein, some sites suggest that the $\ce{OH^{-}}$ is responsible of removing on hydrogen from the amine and that is why there is a color change while other suggest that the $\ce{OH^{-}}$ would rather act as a redactor for the copper and the release of electrons from the copper would transfer the copper ion from a cupric form to cuprous one. (Copper-based assay chemistries) I have also understood that the copper geometry is changing during the reaction, but the idea of the reduction is unclear to me.

I will ask my question again, what are the chemical reactions that lead to the color change?

  • $\begingroup$ It is a coordination complex $\endgroup$
    – getafix
    Oct 7, 2016 at 5:57

1 Answer 1


The $\ce{Cu^{2+}}$ ion is capable of coordinating to the lone pairs of the nitrogens of the amide bonds.


The formation of this complex is associated with a certain energy change in the orbitals of the $\ce{Cu^{2+}}$ ion, and hence a color change.

  • 4
    $\begingroup$ In the typical violet complex that is formed under the strong alkaline conditions of the biuret test (not the blue-green complex that may be initially formed at lower pH), the binding amide groups are deprotonated. $\endgroup$
    – user7951
    Oct 7, 2016 at 13:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.