Obtaining aromatic compounds through lignin oxidation with laccase immobilized in alginate

Lignin is an abundant biopolymer in nature; because of its polymeric structure, it is a potential source of aromatic compounds with high added value in the chemical, food and pharmaceutical industries. Lignin can be used with chemical and biological procedures that depolymerize it in a gradual and selective manner. Peroxidases and laccases are enzymes used as oxidizing agents in processes of oxidative depolymerization; however, the use of H₂O₂ as secondary electron acceptor to increase the oxidizing effect of immobilized laccase is not reported so far. The objective of this study was to evaluate the catalytic capacity of laccase (1.25 mUI) immobilized in alginate pearls, with a particle diameter (Dp) of 3 mm, in order to obtain aromatic compounds from lignin oxidation. The reactions were kept with agitation and controlled temperature, the pH varied and H₂O₂ was used as secondary electron acceptor. The experimental design was factorial 3 x 4: three pH values (5.8, 6.5 and 8.6) and four concentrations of H₂O₂ (25, 50, 75 and 100 mM); the trials were performed in triplicate. ANDEVA was carried out with the data as well as means comparison tests (Tukey, p≤0.05). The highest conversion of lignin to aromatic compounds was obtained with a pH of 8.6: benzoic acid (63.5 %), vanillic acid (25 %) and vanillin (11.5 %). The immobilizing support for the enzyme was disintegrated and dissolved in the medium in the reactions catalyzed with a pH over 7.0. The results allow suggesting that the degree of lignin depolymerization with immobilized laccase depends directly on the concentration of H₂O₂ and the medium's pH.