Oxidative degradation of melanins to pyrrole acids: a model study.

The origin of pyrrole-2,3,5-tricarboxylic acid (PTCA), the most characteristic degradation product of melanins, was investigated by use of synthetic pigments prepared from the major biosynthetic precursors, 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Under the reported conditions, i.e. acidic permanganate, oxidative degradation of DHI- and DHICA-melanins afforded PTCA in 0.15 and 3.0% w/w yield, respectively. A significant improvement of PTCA yields up to 7% was obtained using alkaline hydrogen peroxide as the oxidising agent. Under these conditions pyrrole-2,3-dicarboxylic acid (PDCA) was also obtained in significant yields. Investigation of the oxidative degradation of some model indole oligomers (1-4) provided unambiguous evidence that PTCA may originate from 2-linked DHI-units in the pigment polymer as well as from DHICA-derived units, whereas PDCA arises from DHI-units not substituted at 2-position. Monitoring of the oxidation of dimer 1 to PTCA showed the intermediate formation of DHICA. Accordingly, a mechanistic route is proposed for the degradation of 2-substituted DHI-units in melanin polymer to PTCA which also accounts for the observed yields of formation of the pyrrole acid from the model oligomers.