Characterization of Products from the Reactions of Dopamine Quinone withN-Acetylcysteine

The reactions between electrochemically prepared dopamine (DA) quinone andN-acetylcysteine (NACySH), a protein model nucleophile, have been investigated at pH 7 and pH 2. Major products were purified by semipreparative reversed-phase liquid chromatography and identified by mass spectrometry and nuclear magnetic resonance spectroscopy to be nucleophilic addition products of the quinone with NACySH. The principal product is the monoaddition adduct of the quinone at C(5) of the ring, 5-S-(N-acetylcysteinyl)dopamine. The diaddition adduct of the quinone at C(2) and C(5) of the ring, 2,5-S,S′-di-(N-acetylcysteinyl)dopamine, and the monoaddition product at C(2) of the ring, 2-S-(N-acetylcysteinyl)dopamine, are produced to lesser extents. The relative molar ratio of 2-S-(N-acetylcysteinyl)dopamine, 5-S-(N-acetylcysteinyl)dopamine, and 2,5-S,S′-di-(N-acetylcysteinyl)dopamine produced at pH 7 is approximately 8:59:33, whereas at pH 2 the ratio is 10:89:1. The uv/vis spectroscopic analysis shows that the two monoaddition products have maxima at 258 and 294 nm, whereas the diaddition product has a maximum at 276 nm and a shoulder at 302 nm. Cyclic voltammetry and chronoamperometry demonstrate that these adducts are oxidized more readily than DA, which causes the anodic current for the oxidation of DA in the presence of NACySH to be kinetically controlled by the rates of the addition reactions. A reaction scheme is proposed for the formation of these products.