Reducing non-productive adsorption of cellulase and enhancing enzymatic hydrolysis of lignocelluloses by noncovalent modification of lignin with lignosulfonate

Highlights

• SL clearly reduced the non-productive adsorption of cellulase on lignin.

• SL with higher MW had stronger blocking effect on cellulase adsorption on lignin.

• Linear anionic aromatic polymers strongly blocked cellulase adsorption on lignin.

• Copolymer of lignin and PEG had stronger enhancement than PEG.

Abstract

Four fractions of one commercial sodium lignosulfonate (SXP) with different molecular weight (MW) and anionic polymers were studied to reduce non-productive adsorption of cellulase on bound lignin in a lignocellulosic substrate. SXP with higher MW had stronger blocking effect on non-productive adsorption of a commercial Trichoderma reesi cellulase cocktail (CTec2) on lignin measured by quartz crystal microgravimetry with dissipation monitoring. Linear anionic aromatic polymers have strong blocking effect, but they would also reduce CTec2 adsorption on cellulose to decrease the enzymatic activity. The copolymer of lignin and polyethylene glycol (AL-PEG1000) has strong enhancement in enzymatic hydrolysis of lignocelluloses, because it not only improves the cellulase activity to cellulose, but also blocks the non-productive cellulase adsorption on lignin. Apart from improving the cellulase activity to cellulose, the enhancements of enzymatic hydrolysis of lignocellulose by adding AL-PEG1000 and SXPs are the result of the decreased cellulase non-productive adsorption on lignin.

Introduction

Lignocellulosic biomass is a potentially renewable and abundant resource for biofuel and biomaterial production, which is significant to reduce human’s reliance on the limited global fossil fuel consumption (Himmel et al., 2007). Despite of the extensive research efforts, large-scale utilization of lignocelluloses through converting cellulose and hemicellulose into fermentable sugars is still limited by several factors (Jørgensen et al., 2007, Ding et al., 2012). The plant cell wall structure of lignocellulose is highly complex (Ding et al., 2012). Lignocellulose mainly contains of cellulose, hemicellulose, and lignin. Lignin can affect enzymatic hydrolysis by limiting access of the cellulase to the cellulose in cell wall (Ding et al., 2012, Leu and Zhu, 2013) as well as through the non-productive adsorption of cellulase (Palonen et al., 2004, Sewalt et al., 1997). Cellulase binds to lignin through hydrophobic (Eriksson et al., 2002), electrostatic (Berlin et al., 2006, Lou et al., 2013) and hydrogen bonding interactions (Sewalt et al., 1997). A pretreatment step is conducted to overcome recalcitrance of lignocelluloses (Ohgren et al., 2007). But lignin is usually partially removed and the cost is very high to achieve complete removal of lignin (Leu and Zhu, 2013). As a result, the non-productive adsorption of cellulase on lignin is unavoidable. Consequently, more cellulase dosages are required to achieve desired saccharification efficiency (Himmel et al., 2007, Zhu and Zhuang, 2012).

There are two routes to reduce cellulase non-productive adsorption on lignin, namely, covalent modification and noncovalent modification. Covalent modification of lignin, such as sulfonation (Lan et al., 2013, Lou et al., 2013) and carboxylation (Nakagame et al., 2011, Lim and Lee, 2013), would reduce cellulase non-productive adsorption on lignin to enhance the enzymatic hydrolysis. But it is usually achieved through pretreatment of lignocelluloses. By contrast, noncovalent modification of lignin can be more easily used. Surfactants, proteins and polymers have been reported to be effective to enhance enzymatic hydrolysis of lignocellulosic materials (Börjesson et al., 2007, Eckard et al., 2013, Kristensen et al., 2007). Further researches indicate that non-ionic surfactants are found more effective than cationic surfactants, whereas anionic surfactants lower cellulose hydrolysis (Eriksson et al., 2002, Kristensen et al., 2007). However, as an anionic polymer surfactant, lignosulfonate (LS) produced by sulfite pulping or sulfite pretreatment (Zhu et al., 2009) was reported to be effective to improve the enzymatic hydrolysis of lignocellulose (Wang et al., 2013, Zhou et al., 2013). The mechanism has yet to be fully elucidated.

Quartz crystal microgravimetry with dissipation monitoring (QCM-D) has been used to study cellulase adsorption on lignocellulosic and cellulosic films and monitor their enzymatic hydrolysis process (Hoeger et al., 2012, Sampedro et al., 2013). Well-defined and stable lignin model films have been utilized as substrates to investigate protein adsorption on lignin film (Salas et al., 2013) and competitive binding between proteins and blocking agents (Reimhult et al., 2008). Hence, it is of interest to study non-productive adsorption of cellulase on pure lignin films by QCM-D. In this work, a new rapid approach using QCM-D was developed to measure the cellulase adsorption on lignin. Then the impacts of SXP fractions with different MW on the cellulase non-productive adsorption on lignin film were investigated. Furthermore, three polymers, sulfanilic acid–phenol–formaldehyde condensate (ASP) and formaldehyde naphthalene sulfonate condensate (FDN) and sulfonated acetone–formaldehyde condensate (SAF), were used to elucidate the effect of the macromolecular structure feature on cellulase non-productive adsorption on lignin and enzymatic hydrolysis efficiency of pure cellulose. Furthermore, a copolymer of lignin and polyethylene glycol was prepared to reduce cellulase non-productive adsorption on lignin to improve enzymatic hydrolysis of lignocellulose.