Analytical Sciences

Aluminum Facilitation of the Iron-Mediated Oxidation of DOPA to Melanin

Junwei DI*^,** and Shuping BI*

*Department of Chemistry, State Key Laboratory of Coordination Chemistry of China, Nanjing University, Nanjing 210093, P. R. China
**Department of Chemistry, Suzhou University, Suzhou 215006, P. R. China

Aluminum, a trivalent cation unable to undergo redox reactions, is shown to faciliate iron-initiated DOPA oxidation in the melanin pathway under acidic condition of pH 5.5, which is a favored medium for aluminum facilitation of iron-induced lipid peroxidation. In the process of oxidation of DOPA to melanin in the presence of the metal ions, Fe³⁺ and H₂O₂ oxidize DOPA to dopachrome (DC), then Al³⁺ catalyzes the conversion of DC to 5,6-dihydroxyindole (DHI) and finally Fe³⁺ oxidizes DHI to indole-5,6-quinone (IQ), which polymerizes immediately to melanochrome and melanin. The reactions involve the intermediate complexes of metal ions and DOPA or its derivative. The present results indicate that aluminum can enhance the oxidative stress on iron-mediated DOPA oxidation in melanin pathway under acidic condition through the cooperation of iron and aluminum ions.

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