Molecular weight distribution and structural characteristics of polymers obtained from acid soluble lignin treated by oxidative enzymes

Brown precipitates were obtained by polymerization of low molecular weight lignin fragments contained in a model effluent. Polymerization reactions were initiated by potato-polyphenoloxidase (PPO) or horseradish peroxidase/H2O2 system (HRP/H2O2). The insolubilization processes occurred after a molecular weight increase of the lignin, as shown by gel permeation chromatography (GPC). The effect of reaction time, pH and amount of soluble lignin per unit of enzyme activity on the molecular weight distribution was evaluated for PPO-initiated reactions. For HRP-initiated system the amount of H2O2 per unit of enzyme activity was also evaluated. Chemical characterization of the macromolecules obtained under optimized conditions and the soluble lignin fragments present in the effluent suggests that the polymerization reactions occur by oxidative cleavage of α-β unsaturated bonds of the soluble lignin fragments. Methoxyl group analysis showed that p-hydroxycoumaryl units were preferentially oxidized by PPO. In contrast, HRP oxidized preferentially guaiacyl and siringyl units giving more condensed polymers.