During the Biuret Test, what indicates the presence of proteins?

In the Biuret Test, the presence of proteins is indicated by a color change in the solution. When a protein is present, the solution changes color from blue to pink or purple due to the reaction between the copper ions in the Biuret reagent and the peptide bonds present in the proteins. The copper ions form a complex with the proteins, resulting in this distinct color change, which is a positive indicator of proteins in the sample.

This color transformation is specific; a solution that remains blue suggests a lack of peptide bonds, indicating no proteins are present. Similarly, if the solution changes to yellow, this is not characteristic of protein presence and typically indicates the presence of other substances, such as reducing sugars or does not relate to proteins at all. Thus, the transition to pink or purple distinctly signals the presence of proteins in the tested solution.