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Chemical structures of eumelanin components and representative oligomers. (a) DHI/DHICA (with atom numbering and ring labeling) and oxidized indolequinone (IQ), quinone methide (MQ), and quinone imine (NQ) forms; (b) linear trimer with two singly oxidized fragments and an interfragment double bond; (c) macrocyclic tetramer; (d) polycyclic pentamer.
