Dynamic Visualization of Lignocellulose
Integration of computer simulation with neutron scattering to examine the structure of lignocellulose.
1Oak Ridge National Laboratory, Oak Ridge, Tennessee
2Institute of Paper Science and Technology, Georgia Institute of Technology, Atlanta, Georgia
Lignocellulosic biomass is recalcitrant to deconstruction and saccharification due to its fundamental molecular architecture and multicomponent laminate composition. A fundamental understanding of the structural changes and associations that occur at the molecular level during biosynthesis, deconstruction, and hydrolysis of biomass is essential for improving processing and conversion methods for lignocellulose-based fuels production. The objective of this research is to develop and demonstrate a combined neutron scattering and computer simulation technology for multiple-length scale, real-time imaging of biomass during pretreatment and enzymatic hydrolysis.
ResearchSANS revealed an increase of the cross-sectional radius of the crystalline cellulose fibril in switchgrass lignocellulose resulting from dilute acid pretreatment. Supporting evidence for the increase in cellulose crystallinity was obtained by NMR. This important finding is counterintuitive given the prevailing paradigm that crystal packing contributes to recalcitrance while pretreatment increases digestibility. We confirmed that our pretreatment experiments indeed increase enzymatic saccharification. Concurrent with the observed re-annealing of cellulose our studies also show removal of hemicellulose and redistribution of lignin into aggregates with Rg~ 135 Å. It is possible that an increase in recalcitrance due to increased crystallinity is outweighed by a reduction in recalcitrance due to increased accessibility of enzymes to cellulose Together these results help close an important knowledge gap about the structuralconsequences of pretreatment. The degree of crystallinity by itself is a poor indicator for enzymatic digestibility; removal of hemicelluloses and lignin from the native lignocellulose composite morphology appear to be the determining factors for increasing biomass digestibility. This is an important insight because it suggests the possibility of optimizing the dilute acid pretreatment process to tune the balance of these counter effects to maximize digestibility.
Through technical innovation the molecular dynamics (MD) simulation codes for lignocellulosic biomass were scaled to petascale supercomputers. By combining SANS and MD, softwood lignin aggregates were shown to possess a highly folded and self-similar surface fractal dimension that is invariant under change of scale from ~1–1000Å. The MD, revealed extensive water penetration of the aggregates and heterogeneous chain dynamics corresponding to a rigid core with a fluid surface.
MD simulations demonstrate that lignin transitions from glassy, compact to mobile, extended states with increasing temperature at T≥1500C i.e., above typical pretreatment temperatures. The low-temperature collapse of lignin is thermodynamically driven by an increase in translational entropy and density fluctuations of those water molecules removed from the hydration shell. Thus, the results distinguish lignin’s collapse from enthalpy driven coil-globule transitions observed for other polymers and provide a thermodynamic role of hydration water density fluctuations in driving general hydrophobic polymer collapse (Petridis et al. et al., 2011).
SANS experiments demonstrated that, at the pH of optimal activity (pH 4.2), the solution conformation of cellobiohydrolase I (Cel7A) transitions from a compact structure at neutral pH to a more flexible structure. The catalytic core adopts a structure in which the compact packing typical of a fully folded polypeptide chain is disrupted (Pingali et al. et al., 2011).
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This research is funded by
Science Program, Office of Biological and Environmental Research, U. S. Department of Energy, under FWP ERKP752