Dopaquinone

Dopaquinone is an o-quinone derived from L-3,4-dihydroxyphenylalanine (L-DOPA) and is a key intermediate in melanin biosynthesis. It is formed mainly by the oxidation of L-DOPA by the copper-containing enzyme tyrosinase, though autoxidation can contribute under physiological conditions. The dopaquinone molecule features a highly reactive quinone moiety, making it an electrophile that participates in several competing reaction pathways.

In the absence of nucleophiles, dopaquinone undergoes rapid intramolecular cyclization to form leukodopachrome, which is then oxidized to dopachrome. Dopachrome can be rearranged by dopachrome tautomerase to 5,6-dihydroxyindole-2-carboxylic acid (DHICA); an alternative decarboxylation route leads to 5,6-dihydroxyindole (DHI). These indolic monomers are subsequently oxidized and polymerized to form eumelanin, the dark pigment responsible for brown to black coloration in skin, hair, and eyes.

If nucleophiles such as cysteine are present, dopaquinone reacts with them to form 5-S-cysteinyldopa and related cysteinyl adducts, steering the pathway toward pheomelanin, which imparts reddish-yellow colors. The balance between the pheomelanin and eumelanin branches influences pigment type and overall coloration.

Dopaquinone is short-lived and highly reactive, with its fate determined by the surrounding chemical milieu. Its chemistry underpins fundamental aspects of mammalian pigmentation and has implications for understanding pigmentary disorders and the design of melanin-inspired materials.