The effect of the morphology and crystal polymorphic structure of cellulose on the activity of cellulase enzymes
The supermolecular structure and crystalline polymorphic state of pure cellulose were studied in relation to its susceptibility to cellulase enzymes. Cellulose, the most prevalent biopolymer on Earth, is a linear homopolysaccharide composed of β (1 → 4) linked D-glucose monomers. Cellulose is currently of great interest as a renewable feedstock for ethanol production. However, in order for lignocellulosic ethanol facilities to achieve economic viability, the natural recalcitrance of cellulose – its resistance to chemical and/or enzymatic degradation – must be overcome. The understanding of the natural solid-state of pure cellulose and how structural features relate to its recalcitrance were the basic themes of this dissertation. Previous research indicated that the conversion of native cellulose (cellulose I) to cellulose II via mercerization increased the enzymatic activity of pure individual cellulases. In this study, the supermolecular and crystalline structural features of cellulose (specific surface area, percent crystallinity, and crystallite size), hydration state (dry, wet, swollen, and hydrate) and crystalline polymorphic state (cellulose I, cellulose II, swollen cellulose I, cellulose II hydrate, and cellulose III) were investigated by powder X-ray diffraction, Congo red dye surface area measurements, differential scanning calorimetry, and 1H NMR.
The conversion of cellulose I to cellulose II hydrate was observed to enhance the enzymatic digestibility, measured as percent weight digestion, by individual cellulases while the conversion of cellulose I to cellulose II and cellulose III did not show improved enzyme hydrolysis. Further, the specific surface area, percent crystallinity, crystallite size, and cellulase binding were not observed to have a direct relationship to enzymatic digestion in this study. For cellulose II hydrate, the relationship between water types (free, bound, loosely bound) and the apparent increase in enzymatic activity for this substrate were studied using 1H NMR relaxation measurements. A critical and novel finding of this dissertation was that tightly bound water, defined as water that is bound to hydroxyl groups, was found in the cellulose II hydrate structure alone. This dissertation has shown that the structure of cellulose and the way in which water is incorporated into that structure may offer a new understanding of reducing the natural recalcitrance of cellulose and enhancing its enzymatic digestibility.
Keywords: cellulose, cellulose II hydrate, cellulose II, cellulose III, lignocellulosic biomass.
0495: Polymer chemistry