If the Folin phenol reagent produces a color change when mixed with a protein sample, what underlying mechanism explains this observation?

The Folin phenol reagent contains phenol and a strong oxidizing agent that reacts with the aromatic rings and certain side‑chain groups in proteins, especially tyrosine, tryptophan, and phenylalanine residues. When the reagent contacts a protein, these side chains donate electrons, reducing the oxidant and forming colored phenolic compounds. The degree of color intensity depends on how many of these reactive groups are present, so proteins rich in aromatic amino acids give a deeper color. For example, a protein with many tyrosine residues will turn a darker blue than one with few tyrosines, showing that the color change reflects the oxidation of specific amino‑acid side chains. Thus, the color change is due to the oxidation of aromatic side chains by the Folin reagent, producing a visible chromophore.