Dji

indole-5,6-quinone ch dihydroxyindole

melanin

Figure 8.14 The formation of melanin from DOPA, showing a fragment of the melanin structure with two of the resonance forms that contribute to the conjugated system of double bonds that gives it its strong light absorption melanin

Figure 8.14 The formation of melanin from DOPA, showing a fragment of the melanin structure with two of the resonance forms that contribute to the conjugated system of double bonds that gives it its strong light absorption and then cyclized to leucodopachrome. This in turn is oxidized to another o-quinone, dopachrome, which is re-aromatized by loss of C02, and oxidized yet again to indole-5,6-quinone. Oxidative dehydrogenation of this quinone links the aromatic units together to give the polymer, a section of which is represented in Figure 8.14. Very little is known about this final stage or the degree of cross-linking. The extended conjugated system of double bonds absorbs all over the visible spectrum so that the polymer appears black. The black pigment is also called eumelanin, while the yellow to red cuticle pigment containing sulphur is called phae-omelanin. It is not clear whether the yellow or red cuticle of some ants is coloured with phaeomelanin.

The oxidative dehydrogenation of phenols and quinones to give dimers and polymers (as here in melanin and in the aphins, see later) is a common reaction in both animals and plants. Compare the production of lignin and tannins in plants. The reaction is usually seen as a radical reaction. The in vitro coupling of /?-cresol to give the compound known as Pummerer's ketone, using ferricyanide, peroxidase or phenol oxidase provides something of a model (Figure 8.15).

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