:Millon's reagent
{{Short description|Reagent used to detect water soluble proteins}}{{distinguish|Millon's base}}
Millon's reagent is an analytical reagent used to detect the presence of soluble proteins. A few drops of the reagent are added to the test solution, which is then heated gently. A reddish-brown coloration or precipitate indicates the presence of tyrosine residue which occur in nearly all proteins.{{Cite book |last=Walsh |first=Edward O. |url=https://books.google.com/books?id=WgvuOwAACAAJ |title=An Introduction to Biochemistry |publisher=The English Universities Press; Hodder & Stoughton |year=1961 |isbn= |location=London |pages=406–407 |oclc=421450365 |url-status=live |access-date=2022-05-16 |archive-date=2022-05-16 |archive-url=https://web.archive.org/web/20220516123548/https://www.google.co.in/books/edition/An_Introduction_to_Biochemistry/WgvuOwAACAAJ?hl=en }} The test was developed by the French chemist Auguste Nicolas Eugene Millon. The structure of the metal complex is usually misrepresented. It is an nitroso complex, with M-N bonds.{{cite journal |doi=10.1246/bcsj.43.3070 |title=The Crystal Structure of a Colored Complex, C7H7OHg·C7H6O2N, Obtained in the Millon Reaction of p -Cresol |date=1970 |last1=Kobayashi |first1=Yoshiko |last2=Iitaka |first2=Yoichi |last3=Kido |first3=Yasumasa |journal=Bulletin of the Chemical Society of Japan |volume=43 |issue=10 |pages=3070–3078 }}
File:Millon Reaction Principle V.1 (cropped).svg
The reagent is made by dissolving metallic mercury in nitric acid and diluting with water, forming mercuric nitrate (Hg[NO3]2).{{Cite web |title=Millon's reagent, for the detection of Tyrosine, Thermo Scientific {{!}} Fisher Scientific |url=https://www.fishersci.com/shop/products/millon-s-reagent-the-detection-tyrosine-thermo-scientific/AC458541000#:~:text=mercuric,reagent |access-date=16 May 2022 |website=www.fishersci.com |archive-date=16 May 2022 |archive-url=https://web.archive.org/web/20220516123453/https://www.fishersci.com/shop/products/millon-s-reagent-the-detection-tyrosine-thermo-scientific/AC458541000#:~:text=mercuric,reagent |url-status=live }} In the test, the phenol group in the side chain of tyrosine gets nitrated, and that product then complexes with Hg(I) or Hg(II) ions to give a red colored precipitate. Millon's test is not specific for proteins; it also gives a positive test for other compounds containing the phenol functional group. Therefore, the biuret test or the ninhydrin reaction are used along with it to confirm the presence of proteins.