Abstract
Lignin is known to limit the enzyme-mediated hydrolysis of biomass by both restricting substrate swelling and binding to the enzymes. Pretreated mechanical pulp (MP) made from Aspen wood chips was incubated with either 16% sodium sulfite or 32% sodium percarbonate to incorporate similar amounts of sulfonic and carboxylic acid groups onto the lignin (60 mmol/kg substrate) present in the pulp without resulting in significant delignification. When Simon’s stain was used to assess potential enzyme accessibility to the cellulose, it was apparent that both post-treatments enhanced accessibility and cellulose hydrolysis. To further elucidate how acid group addition might influence potential enzyme binding to lignin, Protease Treated Lignin (PTL) was isolated from the original and modified mechanical pulps and added to a cellulose rich, delignified Kraft pulp. As anticipated, the PTLs from both the oxidized and sulfonated substrates proved less inhibitory and adsorbed less enzymes than did the PTL derived from the original pulp. Subsequent analyses indicated that both the sulfonated and oxidized lignin samples contained less phenolic hydroxyl groups, resulting in enhanced hydrophilicity and a more negative charge which decreased the non-productive binding of the cellulase enzymes to the lignin.
Highlights
Lignin has been proven to be a significant obstacle when trying to carry out effective enzymemediated hydrolysis of lignocellulosic substrates
As we anticipated that the presence of hemicellulose and the inherent acid groups associated with the biomass might influence substrate swelling (Sjöström et al, 1965; Ju et al, 2013) and cellulose hydrolysis, a pretreatment step was first carried out at 170◦C, to primarily solubilize the hemicellulose while minimizing lignin condensation (Garrote et al, 2001; Li et al, 2010)
As initial conductometric titration had indicated that the pretreated mechanical pulp (MP) still contained some weak acids, the pulp was subsequently acid washed to remove any residual uronic acid groups that were potentially located on the residual hemicellulose (Meng et al, 2015)
Summary
Lignin has been proven to be a significant obstacle when trying to carry out effective enzymemediated hydrolysis of lignocellulosic substrates. A considerable amount of work has focused on trying to mitigate lignins inhibitory effect by modifying it, such as using relatively mild reaction conditions that incorporate acid groups onto the lignin macromolecule, enhancing cellulose hydrolysis without the need for complete delignification (Eriksson et al, 2002; Berlin et al, 2006; Pan, 2008; Del Rio et al, 2011; Nakagame et al, 2011a). The deprotonation of the acid groups generates a negative charge, facilitating the electrostatic repulsion between the negatively charged lignin and cellulase enzymes (Nakagame et al, 2010; Del Rio et al, 2011)
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