Mechanism of Alkaline Lignin Oxidation Using Laccase-methyl Syringate Mediator System

The mechanism of alkaline lignin oxidation in presence of laccase-methyl syringate (MS) mediator is discussed in terms of morphological changes that take place during exposure of the lignin to the phosphate buffer solution (pH=6.5) for 72hr at 70 o C. The SEM analysis of lignin before and after enzymatic treatment reveals the morphological changes explained by the interaction of the lignin surface groups with laccase-methyl syringate system. The BET analysis confirms that this interaction causes the change in the surface area from 2.75 to 5.50cm 2 /g. The corresponding pore-size distribution in lignin sample treated with laccase-methyl syringate is much broader in comparison to the untreated lignin and the pores within 25-150nm range are detected as a result of the BJH analysis. The electrochemical study of lignin, lignin with laccase, and lignin with laccase in presence of the mediator in the buffer solution has been performed in the potential range from -0.3 to +1.0V vs. Ag, AgCl, Cl - reference electrode. The cyclic voltammetry confirms reversible oxidation-reduction behavior of the methyl syringate natural mediator in anaerobic and aerobic environment. Specifically in anaerobic conditions three oxidation anodic peaks (0.265V, 0.474V and 0.884V) and two reduction peaks (0.421V and 0.103V) were detected out of which the oxidation peak at 0.474V was assigned to the formation of MS∙ radical. In aerobic conditions the methyl syringate demonstrates two oxidation peaks (0.473V and 0.812V) and two reduction peaks (0.410V and 0.135V). The mechanism of MS radical stability in oxygen vs. anaerobic environment is proposed based on formation of MS∙ radicals.