Rd. Xu et al., CHARACTERIZATION OF PRODUCTS FROM THE REACTIONS OF DOPAMINE QUINONE WITH N-ACETYLCYSTEINE, Bioorganic chemistry, 24(1), 1996, pp. 110-126
The reactions between electrochemically prepared dopamine (DA) quinone
and N-acetylcysteine (NACySH), a protein model nucleophile, have been
investigated at pH 7 and pH 2. Major products were purified by semipr
eparative reversed-phase liquid chromatography and identified by mass
spectrometry and nuclear magnetic resonance spectroscopy to be nucleop
hilic addition products of the quinone with NACySH. The principal prod
uct 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, a
nd the monoaddition product at C(2) of the ring, 2-S-(N-acetylcysteiny
l)dopamine, are produced to lesser extents. The relative molar ratio o
f 2-S-(N-acetylcysteinyl)dopamine, 5-S-(N-acetylcysteinyl)dopamine, an
d 2,5,S,S'-di-(N-acetylcysteinyl)dopamine produced at pH 7 is approxim
ately 8:59:33, whereas at pH 2 the ratio is 10:89:1. The uv/vis spectr
oscopic 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, whi
ch causes the anodic current for the oxidation of DA in the presence o
f NACySH to be kinetically controlled by the rates of the addition rea
ctions. A reaction scheme is proposed for the formation of these produ
cts. (C) 1996 Academic Press. Inc.