Laccase/Mediator Systems: Their Reactivity toward Phenolic Lignin Structures

Abstract

Laccase-mediator systems (LMS) have been widely studied for their capacity to oxidize the nonphenolic subunits of lignin (70–90% of the polymer). The phenolic subunits (10–30% of the polymer), which can also be oxidized without mediators, have received considerably less attention. Consequently, it remains unclear to what extent the presence of a mediator influences the reactions of the phenolic subunits of lignin. To get more insight in this, UHPLC-MS was used to study the reactions of a phenolic lignin dimer (GBG), initiated by a laccase from Trametes versicolor, alone or in combination with the mediators HBT and ABTS. The role of HBT was negligible, as its oxidation by laccase occurred slowly in comparison to that of GBG. Laccase and laccase/HBT oxidized GBG at a comparable rate, resulting in extensive polymerization of GBG. In contrast, laccase/ABTS converted GBG at a higher rate, as GBG was oxidized both directly by laccase but also by ABTS radical cations, which were rapidly formed by laccase. The laccase/ABTS system resulted in Cα oxidation of GBG and coupling of ABTS to GBG, rather than polymerization of GBG. Based on these results, we propose reaction pathways of phenolic lignin model compounds with laccase/HBT and laccase/ABTS.

ABTS/PP Decolorization Assay of Antioxidant Capacity Reaction Pathways

“…Interestingly, that arbutin−ABTS •+ hydrazinediylidene-like adduct (19) revealed no antioxidant activity in contrast to 3-ethyl-2-oxo-1,3-benzothiazoline-6-sulfonate (6), which revealed radical-scavenging activity, though was almost negligible. The model compound mimicking the phenolic subunits in lignin, 1-(4-hydroxy-3methoxyphenyl)-2-(2-methoxyphenoxy) propane-1,3-diol (guaiacylglycerol-β-guaiacyl ether, GBG), reported by Hilgers at al., was also able to couple with ABTS •+ ( Figure 10) [130]. This study set out with the aim of assessing the reactivity of different laccase/mediator systems towards phenolic lignin structures.…”

“…The results partly corroborated the findings of the aforementioned reports-the ABTS •+ , obtained in reaction with laccase (pH 4, 40 °C) and then centrifuged over centrifugal filter to get rid of laccase, alone oxidized GBG to finally form the coupling product (20), oxidation products of GBG (GBGox) (22), GBG dimer (23), and oxidized GBG dimer (24), which were detected by employing reversed-phase ultrahigh-performance liquid chromatography with photodiode array and electrospray ionization mass spectrometry detection (RP-UHPLC-PDA-ESI-MS n ). The model compound mimicking the phenolic subunits in lignin, 1-(4-hydroxy-3-methoxyphenyl) -2-(2-methoxyphenoxy) propane-1,3-diol (guaiacylglycerol-β-guaiacyl ether, GBG), reported by Hilgers at al., was also able to couple with ABTS •+ (Figure 10) [130]. This study set out with the aim of assessing the reactivity of different laccase/mediator systems towards phenolic lignin structures.…”

“…The reactant and product abundance timeline clearly demonstrated that GBG was converted within the first minute of incubation, giving rise to GBGox (22) and GBGox with an ABTS coupling product similar to the GBG-ABTS coupling adduct (21), whose concentrations peaked and then almost totally decreased by the 20th min. The concentration of the hydrazinediylidene-like adduct (21) of ABTS •+ and GBG was also the highest after 1 min, but [130]. Copyright 2018, adopted with permission from American Chemical Society.…”

“…Sci. 2019, 20, x FOR PEER REVIEW 17 of Reaction pathways of ABTS •+ with guaiacylglycerol-β-guaiacyl ether (GBG)[130]. Copyright 2018, adopted with permission from American Chemical Society.…”

The Role of Natural Laccase Redox Mediators in Simultaneous Dye Decolorization and Power Production in Microbial Fuel Cells

“…The mechanism of laccase mediation varies between each mediator. It was, for example, observed for lignin oxidation that ABTS/Laccase carried out alpha carbon oxidation and coupling of the lignin subunits, whereas HBT/laccase polymerized them [14].…”