Laccase-Catalysed Homocoupling of Primary Aromatic Amines towards the Biosynthesis of Dyes

Coloured disubstituted benzoquinonimine trimeric structures are obtained as main reaction products of the oxidation of p-electron donor primary aromatic amines using two different laccases, CotA-laccase from Baccilus subtilus and TvL from Trametes versicolor. These orange-red to purple products, presenting high molar extinction coefficients, presumably result from oxidative homocoupling reactions, through the formation of NC bonds at positions 2 and 5, of the laccase oxidised intermediate as showed in the proposed oxidative pathway. The product of 1,4-phenylenediamine is shown to be the trimer known as Bandrowski's base which has an established role in hair and fur dyeing. Our results also show that the occurrence and/or rates of oxidation of aromatic amines are strongly dependent on the presence of p-electron releasing substituents in the aromatic ring and are independent on the properties of the enzyme used. Overall our data contribute for (i) understanding key features of laccase reactivity with p-substituted aromatic amines and (ii) establishing enzymatic processes that lead to the synthesis of coloured bio-products under mild conditions with potential impact in the cosmetic and dye industries.