Physico-chemical properties of lignin fractions from acid pretreated corn stover and their effects on enzymatic hydrolysis of microcrystalline cellulose

Abstract

Lignin has been shown to be a recalcitrance factor in many biomass conversion studies. To better understand the effects of lignin on cellulose conversion, different lignin fractions were extracted from the same dilute acid pretreated corn stover by three sequential isolation methods, namely ethanol extraction, dioxane extraction, and enzyme purification. The physicochemical properties of each lignin fraction including molecular weight distribution, surface area, surface charge, and other structural features varied, depending on the isolation methods. All three lignin fractions had negative surface charges, and ethanol-extracted lignin carried the highest surface charges, followed by dioxane-extracted lignin and cellulase-purified residual lignin. These physicochemical properties of lignin fractions also resulted in different extent of inhibitory effects on enzymatic hydrolysis of microcrystalline cellulose (MCC). Dioxane-extracted lignin exhibited the highest inhibitory effect on glucose release from MCC, followed by the cellulase-purified residual lignin fraction and ethanol-extracted lignin. Furthermore, lignin fractions with higher contents of syringyl (S) substructure and β-O-4 aryl ether interunit linkages showed a stronger negative effect on cellulase hydrolysis of MCC.

Toward a Fundamental Understanding of the Role of Lignin in the Biorefinery Process

“…During DA pretreatment, lignin has been reported to be significantly depolymerized by acid catalyzed hydrolysis of β-O-4 linkages as reported in several earlier studies (Lundquist et al, 1972;Karlsson et al, 1988). Usually, the molecular weight of lignin after DA pretreatment was decreased (Yang et al, 2020), and the lower molecular weight favored CBH (Cellobiohydrolase I) adsorption to lignin (Yao et al, 2018a). Poplar lignin was only partially degraded after DA pretreatment at 170 °C for 8 min using 0.5% H 2 SO 4 as catalyst (Hu et al, 2013).…”

“…Studies showed that the inhibitory effect of lignin from DA pretreated biomass was more significant than the untreated one (Xu et al, 2020). It is well known that both polymerization and depolymerization can happen during acid-catalyzed pretreatment (Schutyser et al, 2018;Yang et al, 2020).…”

“…Cellulases and lignin will bind together if they possess opposite charges by electrostatic interactions. At pH 4.8, which is the typical optimum pH of cellulases, lignin showed a negative zeta potential, and cellulases (such as Cel6A and Cel5A) were positively charged (Saini et al, 2016;Yang et al, 2020) while others (such as Cel7A and Cel7B) were negatively charged (Lai et al, 2018b). Thus, depends on the pH and the types of enzymes, there could be either attractive or repulsive forces between cellulase and lignin substrates.…”

“…An elevated pH in the cellulases hydrolysis process could significantly reduce the nonspecific binding of cellulases to lignin residues after various pretreatment (Lou et al, 2013). However, some studies in the literature report that electrostatic interactions were not significantly involved in the cellulases lignin interactions (Lou et al, 2013;Yang et al, 2020) which suggests further investigations are needed on this aspect.…”