Copper melanin formation catalyzed

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Kinetically slow steps in the formation of melanin from DOPA are the formation of dopaquinone from DOPA (step 1, kD), the reaction of dopachrome to dihydroxyindole (step 2), and the polymerization to form melanin (step 3, kM). Step 1 and step 2 proceed with about the same rate in the oxidative coupling polymerization catalyzed by tyrosinase. However, step 1 becomes remarkably slow when a macromolecule-metal complex is used as a catalyst. The copper complex in poly(l-vinylimidazole-co-vinylpyrrolidone) has been found [38] to act as an excellent catalyst and to exhibit the highest activity for melanin formation. The ratio of the rate constants ( m/ d) is approximately 3 (tyrosinase... 

The first two steps in the synthesis of melanin are catalyzed by tyrosinase, a copper-containing oxidase, which converts tyrosine to dopaquinone. All subsequent reactions presumably occur through nonenzymatic auto-oxidation, in the presence of zinc, with formation of the black to brown pigment eumelanin. The yellow to reddish brown, high-molecular-weight polymer known as pheomelanin and the low-molecular-weight trichromes result from addition of cysteine to dopaquinone and further modification of the products. Pheome-lanins and trichromes are primarily present in hair and feathers. 

Tables. Formation of melanin catalyzed by polymer-copper (II) complexes...

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