Fungal Biotransformation of Insoluble Kraft Lignin into a Water Soluble Polymer

Abstract

Low substrate solubility and slow decomposition/biotransformation rate are among the main impediments for industrial scale lignin biotreatment. The outcome and dynamics of kraft lignin biomodification by basidiomycetous fungi, Coriolus versicolor, were investigated in the presence of dimethyl sulfoxide (DMSO). The addition of 2 vol % DMSO to aqueous media increased the lignin solubility up to 70%, while the quasi-immobilized fungi (pregrown on agar containing kenaf biomass) maintained their ability to produce lignolytic enzymes. Basidiomycetous fungi were able to grow on solid media containing both 5–25 g/L lignin and up to 5 vol % DMSO, in contrast to no growth in liquid media as a free suspended culture. When a fungal culture pregrown on agar was used for lignin treatment in an aqueous medium containing 2–5% DMSO with up to 25 g/L lignin, significant lignin modification was observed in 1–6 days. The product analysis suggests that lignin was biotransformed, rather than biodegraded, into an oxygenated and cross-linked phenolic polymer. The resulting product showed the removal of phenolic monomers and/or their immediate precursors based on gas chromatography and thermal desorption–pyrolysis–gas chromatography–mass spectrometry analyses. Significant intramolecular cross-linking among the reaction products was shown by thermal carbon analysis and 1H NMR spectroscopy. An increase in polarity, presumably due to oxygenation, and a decrease in polydispersity of the lignin treatment product compared to untreated lignin were observed while using liquid chromatography.

Production of lignin based insoluble polymers (anionic hydrogels) by C. versicolor

“…The calculated number-average and weight-average MW values corroborated this observation showing an increase of MW n from 1,750 Da to 4,780 Da and that of MW w from 4,690 Da to 28,760 Da, respectively. This observation indicated that intermolecular cross-linking (presumably, oxidative cross-linking) is the major reaction path in lignin biomodification 19 .

Figure 1 GPC profiles of untreated lignin and APPL obtained by a 6-day lignin treatment with C. vercicolor, along with the elution maxima of MW calibration standards whose MW values are shown on the graph.

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“…The IP-H yield was rather low while using water as a medium, without DMSO (Table 1 ). However, we discovered that the addition of 2% DMSO to this medium significantly increased the yield of not only APPL 19 but also IP-H, from 20 to 72% (Table 1 ). Thus, DMSO as a solubilizing solvent is not essential for the IP-H formation but increases the lignin conversion as a result of fungal biotreatment.…”

“…First, the corresponding water-soluble polymer, APPL, was synthesized by cultivation of pre-grown and quasi-immobilized (on agar) fungi, primarily, Coriolus versicolor , as described in Section 4.2 of Materials and Methods. Enzymation 20 , i.e., uncoupling the culture growth and the subsequent feedstock treatment, was used conducting the latter in a solvent (distilled water, with or without DMSO) as opposed to an elaborate culture medium 19 .…”

“…Apparently, lignin oxidative cross-linking resulting from the fungal treatment is essential to render the polymer insoluble. As a result, the highly water soluble product of the fungal treatment (APPL) 19 becomes highly insoluble during the dehydration process (drying) (Table 1 ).…”

“…To our best knowledge, no lignin-based hydrogels have been synthesized via biomodification. Recently, we described the production of a water-soluble lignin-based polymer 19 . Then, we discovered that this polymer can be converted into less soluble polymers; this information is reported in this article.…”