Effects of ionic liquids on the reaction kinetics of a laccase–mediator system

Abstract

Lignocellulosic biomass can potentially be transformed into a wide variety of products ranging from biofuels and bulk chemicals to high value products. Through enzymatic hydrolysis, cellulose and hemicellulose can be utilized. However, lignin remains difficult to degrade which is partially due to its insolubility in aqueous solutions. Ionic liquids (IL) such as [EMIM][EtSO₄] and [EMIM][Ac] increase lignin solubility and thereby, enzymatic access to lignin. Therefore, the reaction kinetics of the laccase–mediator system for the oxidation of lignin model compounds in IL solutions was investigated. Laccase in buffer solution had a higher activity compared to laccase in 5, 15, and 30% (v/v) [EMIM][EtSO₄] and [EMIM][Ac]. However, the presence of 15% (v/v) IL helped to stabilize laccase activity over time. Cyclic voltammetry was used to investigate the reaction kinetics between the mediator ABTS and the lignin model compound veratryl alcohol in different ILs and IL concentrations. The increased conductivity at low IL concentrations expectedly lead to a rising reaction rate of mediator with substrate, whereas further increasing IL concentrations lead to higher viscosities and correspondingly, lower reaction rates. As a result, the investigation of the laccase–mediator system's reaction kinetics in buffer and ionic liquids provides a basis for evaluating and optimizing the lignocellulosic biomass degradation process.