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Abstract

The detailed structure of melanin remains elusive due to the complexity and insolubility of the pigment. Herein we describe a novel oxidation of 5,6-dihydroxyindole (DHI) as a means to characterize soluble intermediates formed prior to oligomerization. The approach entails the use of a biomimetic copper-peroxo oxidant, at ultra-low temperature (−78°C). DHI oxidized by [LCuII(O2)CuIIL]B(C6F5)4 (L = 2,6,10-trimethyl-2,6,10-triazaundecane) under argon produces the one electron oxidation product, semiquinone radical, which is spectroscopically observed at −78°C. MS analysis of the reaction mixture reveals the DHI dimer as well as other extensively oxidized DHI units.