Principle of the Biuret Test

What is the principle of the [biuret test](https://en.wikipedia.org/wiki/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](https://www.thermofisher.com/ca/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/chemistry-protein-assays.html)) 
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?




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


$\hspace{1.4cm}$[![complex](https://i.stack.imgur.com/mv9A7.png)](https://i.stack.imgur.com/mv9A7.png)


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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