Question

A student heats a test tube containing a large amount of protein and Biuret reagent; she notices a color change from violet to blue. Explain why heating causes a color change

Answer 1

Heating disrupts the proteins' form and conglomeration (if that's the word for it, not sure).

Basically, the protein's structure changes, which means it will react differently with the reagent, and hence the color change.

Answer 2

^on the right track. The thermal energy from heating disrupts the weak forces that hold a protein in it's globular form, this allows for the copper ions to coordinate to the backbone (amide bonds) - this changes the ligand field around the copper ions and hence the electronic transitions.