“…For ADR, at physiological pH, partial deprotonation of the amine group of the side chain leads to an irreversible 1,4-intramolecular cyclization, a reaction that occurs through nucleophilic attack of the nitrogen atom at position 6 of the quinone ring, to form "leucoadrenochrome"; leucoadrenochrome is subsequently oxidized to form adrenochrome (Heacock and Mahon, 1958, Bindoli et al, 1992, Bindoli et al, 1999. This indole is often represented as a zwitterionic structure in aqueous solutions (Heacock and Mahon, 1958, Costa et al, 2007 (Figure 5). In summary, the oxidation of catecholamines occurs through two-stages whereby a total of four electrons is removed and an indole is formed by cyclization (Costa et al, 2011).…”