Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes

Figure 8

Catechol-melanin formation. Catechol is oxidized by a catechol oxidase, generally in plants, to yield o-benzoquinone. Dismutation reaction between catechol and o-benzoquinone can generate semiquinonic radicals (bottom). Alternatively, o-benzoquinone reacts with water to generate 1,2,4-trihydroxybenzene (right). The semiquinone radical can spontaneously react in several ways to generate aromatic biphenolic dimers or diphenylene-dioxide-2,3-quinone species and finally an undefined catechol-melanin polymer. In some species, catechol can be replaced by other catecholic precursors, such as caffeic acid, but the subsequent reactions from the corresponding o-quinone are similar to this scheme.