Dopaquinon

Dopaquinone is an o-quinone that arises from the oxidation of L-DOPA (3,4-dihydroxyphenylalanine). In vertebrates, it is a key intermediate in the melanogenesis pathway, formed when the enzyme tyrosinase oxidizes L-DOPA. It can also result from auto-oxidation under certain conditions.

The compound is highly reactive due to its quinone structure. It readily engages nucleophiles such as thiols and amines. Reaction with cysteine and related thiols channels metabolism toward pheomelanin, the red‑yellow pigment. In the absence of significant nucleophiles, dopaquinone undergoes intramolecular cyclization and subsequent rearrangements to form dopachrome, which can be converted to DHICA or, via spontaneous decarboxylation, to DHI. These indole intermediates polymerize to produce eumelanin, the dark pigment. The relative flux through the pheomelanin or eumelanin branches influences pigmentation in skin, hair, and eyes.

In a broader context, dopaquinone is studied as a reactive intermediate in enzymatic browning reactions, including those in foods, where polyphenol oxidases generate quinone species that contribute to browning and flavor changes. Because of its instability and high reactivity, dopaquinone is typically examined indirectly through downstream pigments or enzyme activity rather than isolated in pure form.

Summary: Dopaquinone is a short-lived, highly reactive oxidation product of L-DOPA that sits at a branching point in melanin biosynthesis, directing production toward eumelanin or pheomelanin depending on the available nucleophiles and enzymatic context.