Abstract
Wood decayed by brown rot fungi and wood treated with the chelator-mediated Fenton (CMF) reaction, either alone or together with a cellulose enzyme cocktail, was analyzed by small angle neutron scattering (SANS), sum frequency generation (SFG) spectroscopy, Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Results showed that the CMF mechanism mimicked brown rot fungal attack for both holocellulose and lignin components of the wood. Crystalline cellulose and lignin were both depolymerized by the CMF reaction. Porosity of the softwood cell wall did not increase during CMF treatment, enzymes secreted by the fungi did not penetrate the decayed wood. The enzymes in the cellulose cocktail also did not appear to alter the effects of the CMF-treated wood relative to enhancing cell wall deconstruction. This suggests a rethinking of current brown rot decay models and supports a model where monomeric sugars and oligosaccharides diffuse from the softwood cell walls during non-enzymatic action. In this regard, the CMF mechanism should not be thought of as a “pretreatment” used to permit enzymatic penetration into softwood cell walls, but instead it enhances polysaccharide components diffusing to fungal enzymes located in wood cell lumen environments during decay. SANS and other data are consistent with a model for repolymerization and aggregation of at least some portion of the lignin within the cell wall, and this is supported by AFM and TEM data. The data suggest that new approaches for conversion of wood substrates to platform chemicals in biorefineries could be achieved using the CMF mechanism with >75% solubilization of lignocellulose, but that a more selective suite of enzymes and other downstream treatments may be required to work when using CMF deconstruction technology. Strategies to enhance polysaccharide release from lignocellulose substrates for enhanced enzymatic action and fermentation of the released fraction would also aid in the efficient recovery of the more uniform modified lignin fraction that the CMF reaction generates to enhance biorefinery profitability.
Highlights
Brown rot fungi in the three orders of Boletales, Gloeophyllales, and Polyporales are known to deconstruct wood in a highly efficient manner [1] using a nonenzymatic mechanism known as the chelator-mediated Fenton (CMF) system [2,3,4,5]
In addition to conventional chemical analysis, we explore the use of small angle neutron scattering (SANS) and sum frequency generation (SFG) analyses, to better assess both how the nanoscale structure within wood changes and how holocellulose crystallinity changes
This would be consistent with a moderate loss in glucose and concomitant partial loss of cellulose crystallinity (“SFG, X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis of fungal decayed lignocellulose”), together implying that the elementary fibril structure erodes as decay progresses
Summary
Brown rot fungi in the three orders of Boletales, Gloeophyllales, and Polyporales are known to deconstruct wood in a highly efficient manner [1] using a nonenzymatic mechanism known as the chelator-mediated Fenton (CMF) system [2,3,4,5]. Prior research has demonstrated that while the polysaccharide components are initially depolymerized during brown rot primarily by the action of hydroxyl radicals generated via the CMF reaction [10,11,12], lignin is depolymerized and rapidly repolymerized by these radicals [13,14,15,16]. Concurrent with the loss of these enzymes, the brown rot fungi have developed a non-enzymatic mechanism to deconstruct the lignocellulose framework by catalytically modifying lignin and depolymerizing cellulose. Because of the complex nature of the lignin molecule and how it encrusts and intersperses with holocellulosic components, the lignin must be depolymerized and modified, and much of the depolymerized fraction repolymerizes [13] This process has been assumed to open the wood cell structure to allow greater access to enzymes during the later stages of decay; this aspect of the mechanism has not previously been demonstrated. The production of a non-reducing chelator, oxalate, by the fungus, has been demonstrated to be required for solubilization and sequestration of iron prior to reduction by the redox cycling chelator [4, 16, 17, 19]
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