Ultra-low temperature oxidation of 5,6-dihydroxyindole: a novel approach to study synthetic melanogenesis.

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 degrees 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 degrees C. MS analysis of the reaction mixture reveals the DHI dimer as well as other extensively oxidized DHI units.