Influence of Oxygen and Mediators on Laccase-Catalyzed Polymerization of Lignosulfonate

Abstract

Industrial utilization of lignin is of high interest since it represents around 30% of all nonfossil-based carbon sources worldwide. For various applications of lignosulfonates such as for dispersants or adhesives a larger molecular weight is essential. Here, we investigated laccase-catalyzed polymerization of lignosulfonate directly from the pulp and paper industry in the presence and absence of natural and synthetic mediators with and without oxygen supply. For example, laccase-mediated polymerization in the presence of a 2.5 mM TEMPO as mediator with a 10 cm 3 min −1 oxygen flow rate led to a 12-fold increase of the molecular weight, while without TEMPO a 13-fold increase was achieved. In contrast, without an external oxygen supply, only a 7-fold increase in molecular weight was achieved compared to a 4-fold increase for the TEMPO−laccase system. Fluorescence intensity, phenol content, and size exclusion chromatography measurements indicate that generally in the presence of high concentrations of mediators, such as TEMPO, vanillin, HBT, and 2,6-dimethoxyphenol, oxidation of other structural units in lignosulfonates may counteract desired polymerization reactions. In summary, for laccase-catalyzed polymerization of lignosulfonates, an external oxygen supply was found to be much more beneficial than the presence of laccase mediators.

High-Throughput Screening Assay for Laccase Engineering toward Lignosulfonate Valorization

“…Different studies have evidenced LS polymerization by laccase in the presence of synthetic or natural mediators [ 7 , 20 ]. Nevertheless, we demonstrated here that the presence of redox mediators does not promote the action of laccase on LS, as has been lately described [ 17 ], suggesting that mediators might indeed counteract polymerization by promoting degradation reactions. Lignosulfonate M r has been considerably augmented by direct oxidation with fungal laccases (without mediators) [ 21 ], particularly when high-redox-potential laccases are applied [ 5 ].…”

“…These bonds generate intramolecular linkages within the LS molecule, and intermolecular linkages with other LS molecules. The resulting lignosulfonates are more homogeneous in size, with larger M r , and consequently, expanded commercial usage [ 17 ]. The increment in M r inversely correlates with the phenolic content of the sulfonated lignin after the enzymatic treatment [ 18 ].…”

The impact of lignin sulfonation on its reactivity with laccase and laccase/HBT

“…Remarkably, when lignosulfonate is used as a substrate, both laccase and LMS incubations are shown to induce polymerization. 4,6,22,23,34 To date, there was no mechanistic explanation for this observation, since the effects of laccase and LMS have only been investigated on polymeric lignosulfonate. These studies reported changes in molecular weight or overall changes in chemical structure, but did not describe detailed reaction pathways.…”

“…1) were synthesized and used to mimic the end caps and backbone of lignosulfonate. We picked HBT as a mediator for two reasons: (i) it has been widely used in LMS incubations of both native lignin and lignosulfonate, 12,18,[22][23][24] which enables a direct comparison between our model compound study and results reported for LMS incubations of polymeric lignins, and (ii) it operates via the HAT mechanism, which makes it possible to extrapolate our results to incubations with natural phenolics as mediators.…”

"A Density Functional Theory Study Based on Monolignols: Molecular Structure, Homo-Lumo Analysis, Molecular Electrostatic Potential"

“…In the industry, only a small fraction of the lignin obtained as a waste by-product is used commercially, thus valorizing its various aromatic units, functional groups, conjugated linkages and molecular size. 7 Therefore, a detailed investigation of lignin and its components could provide solutions to the problem. While such a study would be expensive and time-consuming, a theoretical approach will bring both time and cost reducing benefits.…”