Cellulose hydrolysis with thermo- and alkali-tolerant cellulases in cellulose-dissolving superbase ionic liquids

Abstract

Pretreatment with ionic liquids (ILs) is known to greatly increase the subsequent biomass hydrolysis with enzymes. However, the presence of even low amounts of ILs has negative effects on cellulase action. Most studies on cellulase inactivation by ILs have focused on imidazolium-based ILs, which until recently were one of the few IL classes known to dissolve cellulose. In this article we describe results of cellulase action in matrices containing ILs belonging to two IL classes recently reported as cellulose solvents. These ILs are based on the organic superbases 1,1,3,3-tetramethylguanidine (TMG) or 1,5-diazabicyclo[4.3.0]non-5-ene (DBN). In this study commercial thermo- and alkaline stabile cellulase products were employed, as these were anticipated to also have a higher stability in ILs. For comparison, hydrolysis experiments were also carried out with a well-characterised endoglucanase (Cel5A) from Trichoderma reesei and in matrices containing 1-ethyl-3-methylimidazolium acetate, [EMIM]AcO. Two different substrates were used, microcrystalline cellulose (MCC) and eucalyptus pre-hydrolysis kraft dissolving grade pulp. The hydrolysis yields were on the same level for both of these substrates, but decreases in molecular weight of the cellulose was observed only for the dissolving grade pulp. By using commercial cellulases with good thermo- and alkali-stability some benefits were obtained in terms of IL compatibility. Enzyme thermostability correlated with higher hydrolysis yields in IL-containing matrices, whereas activity at high pH values did not offer benefits in terms of IL tolerance. The new classes of cellulose-dissolving superbase ILs did not differ in terms of cellulase compatibility from the well-studied imidazolium-based ILs. Of the novel superbase ILs tested, [TMGH]AcO was found to inhibit the enzymatic hydrolysis the least.