Abstract
The autoxidation, periodate oxidation, and tyrosinase-mediated oxidation of 6-hydroxydopamine, dopamine, norepinephrine, and epinephrine were studied by absorption spectroscopy. Autoxidation and tyrosinase-mediated oxidation of the three catecholamines resulted in dopachrome analogues—aminochrome from dopamine, noradrenochrome from norepinephrine, and adrenochrome from epinephrine—without evidence for the expected intermediates, the o-quinones and the corresponding leukochromes. The use of periodate as an oxidant, on the other hand, allowed visualization of the o-quinone intermediates and the subsequent conversion to the dopachrome analogues. Cyclization of the o-quinones appeared to occur in the order epinephrine > norepinephrine > dopamine, while the rate of autoxidation occurred in the reverse order. The oxidation of 6-hydroxydopamine to its p-quinone was visualized under all three oxidizing conditions. However, the oxidation of 6-hydroxydopamine by periodate gave evidence for a transient intermediate, the o-quinone, which rapidly tautomerized to the p-quinone. The p-quinone product of 6-hydroxydopamine was seen to undergo cyclization to aminochrome, with subsequent polymerization.
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