Why does protein and Biuret reagent change from violet to blue when heated?

You don't need to heat a Biuret-protein solution to get the color change. It happens naturally within 30 minutes after mixing. The biuret technique for protein is based upon the coordination of cupric ion (Cu++) of the Biuret solution with unshared electron pairs of a protein's nitrogen atoms and water's oxygen atoms. The peptide chain must have at least three amino acids to yield a color complex.

Why does protein and Biuret reagent change from violet to blue when heated?
I think his question is why it changes colour from violet to blue when heated, not the other way round.





Anyway, it's probably because proteins denatures after being exposed to heat. Hence, the Biuret reagent goes back to its original colour (blue) when the Cu2+ ions form with the sulphate ions again.
Reply:I think it has to do with the dye binding to the peptide bonds.





Proteins=polypeptides





I may be wrong on this (it's been a while since I took chemistry), but I think the more peptides there are to bind to, the more defined the color change.

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