Executive Summary : | Plant biomass is a heterogeneous matrix comprising of carbohydrates linked tightly with hemicellulose, lignins, and pectins. With the current advances in the use of biomass for sustainable production of biofuels and bio-based advanced materials, it is important to obtain a deeper understanding of the cell wall architecture and the interactions stabilizing various components of lignocellulosic biomass which limit cellulose accessibility. The cellulose microfibrils comprise of alpha-(1,4)-linked D-glucosyl units and have both (110) hydrophilic and (100) hydrophobic surfaces exposed for interactions. The most common hemicellulose, xylan is made up of alpha-(1,4)-linked D-xylosyl units and is further substituted by various side chains like glucuronic acid, 4-O-methylglucuronic acid, arabinose or acetyl (Ac) groups to mediate the function of cell walls. In particular, the presence of Ac groups affects the interchain attractions, solubility of xylan chain, and resistance of plant cell walls to enzymatic and microbial degradation. Thus, the pattern of acetylation on xylan is highly correlated with the extractability and recalcitrance of lignocellulosic biomass. Arabidopsis is one of the well-studied systems which has about 50% of the xylan residues acetylated evenly at O-2 or O-3 position or both. Experimental and molecular dynamics (MD) studies show that the presence of Ac groups at O-2 position favors 2-fold helical screw conformations of xylan to interact effectively with the (110) hydrophilic cellulose surface, while O-3 decorated xylan maintains a 3-fold helical screw probably to interact with lignin rather than cellulose. The proposed project aims at examining the effect of position of Ac substitution at O-2 and O-3 positions of xylan for Arabidopsis stems in influencing the xylan conformations and xylan-cellulose interactions on (100) hydrophobic cellulose surface by performing unrestrained MD simulations to monitor the spontaneous orientation of solvated xylan conformations on the cellulose surface. The second aim of the study is to examine the effect of degree of acetylation at O-2, O-3, or both positions simultaneously in influencing 2-fold helical screw conformations of xylan to stabilize interactions on both (110) hydrophilic and (100) hydrophobic surfaces of cellulose. This would provide insights about the necessity of the presence of an optimum number of unsubstituted residues on the xylan backbone and the subsequent implications on the formation of 2-fold helical screw conformations of xylan to interact effectively with the cellulose surface. The role of different specific atoms that stabilize the xylan-cellulose interactions by triggering 2-fold helical screw conformations of xylan on the cellulose surface will be elucidated. Thus, the role of macromolecular conformations of cellulose and hemicellulose in influencing the supramolecular interactions, function, and mechanical strength of plant cell walls will be highlighted. |