Effect of residual lignins present in cholinium ionic liquid-pretreated rice straw on the enzymatic hydrolysis of cellulose

Abstract

To provide deep insights into cholinium ionic liquid (IL) pretreatment effect on the subsequent enzymatic hydrolysis, residual lignins (RLs) present in rice straw pretreated by cholinium glycinate ([Ch][Gly]), acetate ([Ch][OAc]), and glycolate ([Ch][Glc]) were isolated and characterized in this work. The effects of RLs on the enzymatic hydrolysis of cellulose and cellobiose, and their non-productive adsorption to β-glucosidase and cellulase were explored. Of the three RLs examined, RL isolated from [Ch][Gly]-pretreated straw (Gly-RL) had the highest carboxylic acid content (1.27mmol/g) and the lowest phenolic hydroxyl content (0.72mmol/g), resulting in the weakest non-productive enzyme adsorption. It might account well for the highest enzymatic digestibility of cellulose in the presence of Gly-RL. However, the strongest enzyme adsorption was found in the case of Glc-RL with the lowest carboxylic acid content (0.14mmol/g) and the highest phenolic hydroxyl content (1.19mmol/g). The enzymatic hydrolysis of cellulose was significantly affected by RLs. The negative effect of RLs on the cellulose hydrolysis mainly stemmed from their inhibition to cellulase-mediated reactions, instead of inhibition to β-glucosidase-catalyzed cellobiose hydrolysis.