Millon's reagent

The Millon reaction ( too Millon sample) is a detection reaction for proteins which is specific for the amino acid tyrosine. It was named after the French chemist Auguste Nicolas Eugène Millon.

As a detection reagent is a solution of mercury (I ) nitrate ( HgNO3 ) in nitrous acid. With proteins to form a brick-red mercury -protein complex.

The Millon reaction is a group-specific detection reaction: it works with all compounds having a phenolic group with a free ortho position. This ortho- position is first nitrosated with nitrous acid to form the aromatic ring in the direct vicinity of a hydroxy group and a nitrosyl arise stabilized. Two such molecules can complex one Hg2 ion; the resulting complex has a brick-red coloring:

Of the naturally occurring amino acids, only tyrosine has such a reactive structure; The reaction can also be used to detect phenol or thymol.