Effects of the Non-ionic Surfactant Tween 80 on the Enzymatic Hydrolysis of Model Cellulose and Lignocellulosic Substrates

Non-ionic surfactants are known to enhance the biochemical conversion of lignocellulosic (LC) biomass to bioethanol. Their mechanisms of action, however, are incompletely understood. This research was conducted to elucidate the effects of the non-ionic surfactant Tween 80 on the enzymatic hydrolysis of cellulose and LC substrates. Model cellulose substrates were prepared from cellulose nanocrystals (CNCs) obtained by sulfuric acid hydrolysis of wood pulp. Two methods were developed for the removal of the sulfate groups on the CNCs, resulting from the use of sulfuric acid in their preparation. The effect of sulfate groups, which may be introduced into LC biomass during pretreatment with sulfuric acid, on the enzymatic hydrolysis of cellulose was studied with model cellulose substrates prepared from CNCs with different sulfate group densities. Adsorption of cellulases onto sulfated substrates increased with increasing sulfate group density but their rate of hydrolysis decreased. The decrease indicated an inhibitory effect of sulfate groups on the enzymatic hydrolysis of cellulose, possibly due to non-productive binding of the cellulases onto the substrates through electrostatic interactions instead of their cellulose binding domains. The effect of Tween 80 on the adsorption of cellulases onto lignin, often present as residual lignin in pretreated biomass, was studied with model lignin substrates, prepared from kraft lignin, organosolv lignin, and milled wood lignin. Cellulases appeared to adsorb onto the lignin substrates via both hydrophobic and polar interactions. Tween 80 molecules on the lignin substrates seemed to hinder cellulase adsorption via hydrophobic interactions and reduced the adsorption rate. Finally, the effects of lignin and Tween 80 on the enzymatic hydrolysis of cellulose and LC substrates were studied. Lignin hindered both the adsorption of cellulases onto the substrates and the enzymatic hydrolysis of the substrates. Tween 80 was found to form surfactant–protein complexes with cellulases in solution without compromising cellulase activity. Either substrate-adsorbed or in solution, Tween 80 had no effect on the hydrolysis of cellulose by cellulases. Substrate-adsorbed Tween 80 increased the apparent enzymatic hydrolysis rates of LC substrates but the ability of Tween 80 to increase their apparent hydrolysis rate depended strongly on their structural properties and the chemical properties of the lignin. Hence, Tween 80 may be able to mitigate the inhibitory effect of lignin on the enzymatic hydrolysis of pretreated biomass.