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 we re initiated by potato-polyphenoloxidase (PPO) or horseradish peroxidase/H2 O2 system (HRP/H2O2). The insolubilization processes occurred after a molec ular weight increase of the lignin, as shown by gel permeation chromatograp hy (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 pe r unit of enzyme activity was also evaluated. Chemical characterization of the macromolecules obtained under optimized conditions and the soluble lign in fragments present in the effluent suggests that the polymerization react ions occur by oxidative cleavage of alpha-beta unsaturated bonds of the sol uble lignin fragments. Methoxyl group analysis showed that p-hydroxycoumary l units were preferentially oxidized by PPO. In contrast, HRP oxidized pref erentially guaiacyl and siringyl units giving more condensed polymers. (C) 2001 Elsevier Science Inc. All rights reserved.