Investigations of ionic-liquid-mediated hydrolysis of cellulosic compounds with and without cellulase enzymes

Abstract

Improved processing of biomass as a feedstock for plastics, liquid fuels, and other value-added products is an essential step towards an environmentally sustainable energy economy. The deconstruction of biomass is regarded as one of the main hurtles to the economic viability of biomass processing facilities. Ionic Liquids (ILs) are a promising class of solvents for the degradation of cellulosic biomass due to their ability to dissolve/solvate the cellulose substrate and improve access for enzymes and other catalysts. One favorable trait of ILs is that they have negligible vapor pressure and are relatively stable at elevated temperatures, which allow for increased reaction kinetics and increased lifetime through solvent recycling. It is believed that a synergistic system of cellulase enzymes, water, and ILs and/or a catalytically enabled IL water binary mixture could greatly enhance the efficiency of biomass processing for production of biofuels and value-added chemicals/materials. This research provides novel molecular-level insights into various interactions between cellulase enzymes, cellulosic substrate, ionic liquids and water. Initial studies of the protonatable residues within the active site of a thermophilic cellobiohydrolase, Melancarpus albomyces (Ma Cel7B), were found to impact the substrate conformation, which are tied closely to the optimum pH for hydrolytic activity. The activity of hyperthermophilic endoglucanase enzymes in IL are shown to be diminished through competitive binding, dampened protein fluctuations, and decreased conformational sampling of the bound substrate. The solvation of cellulosic compounds in various ILs was also studied, and indicated preference for dihedral orientations around the glycosidic bond and ring puckering conformations similar to those in cellulase active sites. Subsequent studies have shown that adding triethoxy to the cation decreases viscosity and inhibition of enzymes relative to dialkyl-imidazolium ILs while retaining the ILs favorable solvation and conformational characteristics. Expanding on our previous studies, a carboxylic acid functionalized IL was developed to mimic the inverting glycoside hydrolase active sites for simultaneous solvation and saccharification of cellulose without enzymes or added catalysts. This novel IL has been evaluated with ab initio calculations and long-time molecular dynamics, and indicates a promising new approach to the solvation and sachharification of biomass.