L-DOPA, L-3,4-dihydroxyphenylalanine; DHICA, 5,6-dihydroxyindole-2-carboxylic acid; IQCA, indole 5,6-quinone carboxylic acid; DHI, 5,6-dihydroxyindole; IQ, indole 5,6-quinone.
In the model of eumelanin structure, U1, U4, and U6 indolic units are DHI, but U2 and U5 are DHICA. Adapted from (Sugumaran and Barek, 2016, Solano, 2014, Ito and Wakamatsu, 2008).