Model for eumelanin structure. This polymer is dark, from brown to black depending on the size and electronic conjugation. (a) The tetrone simplified model to account for empirical composition C₁₈H₁₀O₄N₂. (b) The classical model. Most units are indolic units. The most abundant units are DHI (5,6-dihydroxyindole, U1, U4, and U6) although some units of the 2-carboxylated analogue DHICA (5,6-dihydroxyindole-2-carboxylic acid, U2, and U5) are also found. The ratio DHI/DHICA depends on the dopachrome tautomerase activity and other conditions during the eumelanin formation. Oxidized units of IQ (5,6-indolequinone, U3), unaltered L-dopa units (U7), and carboxylated pyrroles due to partial degradation of indoles during the polymerization (U8) can also be incorporated during the polymerization. The units are linked by a variety of bonds involving positions 4, 7, 2, and 3. Arrows indicate some possible points for polymer growth.