(111b) Evolution of Cellulose Structure throughout Biomass Deconstruction in Gamma-Valerolactone
Biomass recalcitrance during deconstruction remains a key inhibitor to successful implementation of affordable biomass processing technologies. A clear connection between the cell wall structure and biomass deconstruction is necessary to understand how lignocellulosic material is broken down, and to identify defining features of residual cellulose. In this work, we have employed solid-state 13C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy to track domain changes in cellulose microfibrils throughout various chemical and enzymatic hydrolysis treatments. We also demonstrate the capability of an NMR observable metric to predict enzymatic hydrolysis sugar yields of pretreated cellulose, regardless of pretreatment co-solvents or biomass feed. P39 poplar was pretreated in GVL-water co-solvents with mild sulfuric acid concentrations at increasing temperatures and subsequently hydrolyzed with an engineered cellulase. Residual solids from each step were characterized using 13C CP/MAS NMR and spectra were fit with subpeaks corresponding to different cellulose microenvironments at the C-4 carbon center. The changes of the C-4 carbon subpeaks were tracked throughout each treatment to understand the physical changes occurring within different cellulose microfibril domains. We show that the xylan-cellulose and inaccessible fibril surface resonances decrease significantly upon hydrolysis with mild acidic GVL-water co-solvent pretreatment. Enzymatic hydrolysis lead to further depletion of the inaccessible fibril surface peak. This behavior can be interpreted as an opening of bound microfibril surfaces previously inaccessible to the solvent. The cleaving of xylan-cellulose linkages and opening of inaccessible fibril surfaces by acid co-solvent pretreatment primes the cellulose for enzymatic attack. This technique can monitor the evolution of structural changes to the cell wall and allows for comparison between different cellulose deconstruction methods. It can guide larger questions of recalcitrance and give insight to improvement of subsequent steps for specific deconstruction methods based on residual solids structures.