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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?

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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?

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

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

$\hspace{1.4cm}$complex

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.

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    $\begingroup$ 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

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