Enzymatic decomposition and electrochemical study of alkali lignin by laccase (Trametes versicolor) in the presence of a natural mediator (methyl syringate)

The aerobic-enzymatic decomposition of alkali lignin in the presence of laccase from Trametes versicolor (LTV) and the natural mediator methyl syringate in acetic acid-sodium acetate buffer solution (pH = 4.5) at 40 degrees C in an oxygen-rich (aerobic) environment is studied. SEM and BET analyses are used to characterize the changes in the surface area and morphology of lignin that occurred during the exposure to the laccase-mediator system (LMS) for 72 h. The LMS interaction causes a 2-fold improvement in the surface area from 4.9 to 9.8 m(2) g(-1), due to significant changes in the mesoporous structure of lignin within a pore size of 2-120 nm. This could be due to an efficient interaction of the surface phenolic groups and internal mesoporous beta-O-4 network of lignin with the LMS in an aerobic environment. To further understand the enzymatic degradation of lignin, electrochemical oxidation of a thin film of lignin on the surface of a glassy carbon electrode (GCE) is performed under aerobic vs. anaerobic conditions in the presence of the LMS. A synergistic lignin electrooxidation in the aerobic environment is observed due to the promotion of LMS activity by a parallel oxygen reduction reaction (ORR). Based on the electrochemical studies, a mechanism for understanding the role of oxygen in the enzymatic oxidation of lignin in an aerobic environment and the stability of the mediator radical (MS center dot) is proposed.