FLUORESCENCE OF CATECHOL AMINES AND RELATED COMPOUNDS CONDENSED WITH FORMALDEHYDE

TLDR

The secondary reaction likely involves protein binding and oxidation that introduces double bonds into the heterocyclic ring, or a combination of both processes. Under mild conditions, formaldehyde reacts with catechol amines—especially primary amines bearing 3,4‑dihydroxy groups—to form tetrahydroisoquinolines that become highly fluorescent and insoluble, while secondary amines produce weaker, slower fluorescence.

Abstract

The reaction under mild conditions between formaldehyde and phenylalanine and phenylethylamine derivatives has been studied. When the amines included in a dried protein film were exposed to formaldehyde vapour a very intense green to yellow fluorescence was give only by those that as well as being primary amines also have hydroxyl groups at the 3 and 4 positions (3,4-dihydroxyphenylalanine, dopamine, noradrenaline). The 3-OH group seems to be esssential for the reaction. The catechol amines, which are secondary amines (adrenaline, epinine), gave a much weaker fluorescence that developed more slowly. The results obtained on further examination of the reaction favour the view that the amines primarily condense with formaldehyde to 1,2,3,4-tetrahydroisoquinolines which are involved in a secondary reaction to become highly fluorescent and at the same time insoluble. This secondary reaction may be a binding to protein, and oxidation with the formation of double bonds in the heterocyclic ring, or both.