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

Figure 3

Model for pheomelanin structure. This pigment is fair color, from reddish to yellowish. It consists of two main units, benzothiazines (U1 + U2, U6 + U7) and benzothiazoles (U4 + U5). Both units are derived from the spontaneous addition of L-cysteine to L-dopaquinone during the melanin biosynthetic pathway and subsequent reactions and cyclization of cysteine residues originate the sulfur-containing rings. Side chains of dopaquinone remain opened or sometimes cycled (such as U3). These last cycles could also give place to dihydroisoquinoline units. Arrows indicate possible points for polymer growth.