Point by point analysis: how ionic liquid affects the enzymatic hydrolysis of native and modified cellulose

Abstract

New strategies are needed to efficiently convert non-food biomass to glucose as a platform chemical. One promising approach is to use ionic liquids to first dissolve lignocellulose. Yet, in the presence of such solvents, the enzymes that catalyze cellulose hydrolysis become compromised in their activity. However, this decreased cellulase activity has not been examined in detail. Thus, the aim of this study was to investigate how the ionic liquid precisely affects cellulase activity and stability with regard to different cellulose substrates. Hereby, four ionic liquids were screened to identify which one best minimized the loss of enzyme activity. Then, this best ionic liquid was tested on one insoluble and two soluble cellulose substrates. Subsequently, the relevant parameters of solution viscosity and ionic strength were evaluated with respect to enzyme activity and stability. Finally the residual ionic liquid concentration from the precipitation of α-cellulose was varied. The best ionic liquid was found to be 1,3-dimethylimidazolium dimethylphosphate with the highest retained activity of 30% on the α-cellulose substrate in the presence of 10% (v/v) ionic liquid. Most importantly, an increase in viscosity and ionic strength contributed to the decrease in enzyme activity which nonetheless retained their stability. The hydrolysis of precipitated α-cellulose from ionic liquid showed significant higher reaction rates but reduced sugar yields when residual ionic liquid was present. None the less, it should be possible to effectively produce glucose from precipitated cellulose without needing to wash off all residual ionic liquid when optimized cellulase mixtures are used.