Secondary plant cell walls are complex composite biomaterials comprising of cellulose, hemicellulose, lignin and pectins. The interactions of xylan, an abundant non-cellulosic polysaccharide, with cellulose microfibrils are essential for secondary cell wall strength. The presence of substitutions on xylan backbone influences xylan-cellulose interactions. Characterizing these interactions along with investigating the role of position of substitution (O-2 and O-3) on the xylan backbone is crucial to improve upon our understanding of plant cell wall architecture and design alternate strategies to overcome plant cell wall recalcitrance for the production of biofuels and sustainable materials. In this work, we use unrestrained molecular dynamics simulations to understand the role of acetyl (Ac) substitutions on xylan interactions with the (110) hydrophilic face of cellulose fibers. In the presence of cellulose, O-2 acetylated xylan (2AcX) adopts a rigid 2-fold helical screw conformation, while O-3 acetylated xylan (3AcX) maintains 3-fold helical screw conformations. The interaction of acetylated oxygen and glycosidic linkage (X-O4, X-O5) with C-O6 of cellulose primarily stabilize the 2-fold 2AcX conformations on the cellulose surface. In contrast, the Ac groups for O-3 substituted xylan point away from the cellulose surface thereby favoring 3-fold helical screw conformations. Occasionally, Ac groups of 3AcX orient towards the cellulose surface resulting in the formation of 2-fold helical screw conformations but these conformations are not stable and revert to 3-fold helical screw. In comparison, in the absence of cellulose, all xylan decorations, independent of the substitution pattern adopt a highly flexible 3-fold helical screw conformation and show similar qualitative behavior as that of bare xylan. Our results show that evenly spaced chemical functionalization (Ac groups) and the position of substitution on the xylan backbone play a central role in controlling cellulose-xylan interactions by stabilizing 2-fold helical screw conformations of xylan backbone on the cellulose surface. A comparison with previous experimental findings further reveals that O-2 substituted xylan, irrespective of the chemical nature of the substituent (Ac, methyl glucuronic acid) trigger the formation of stable 2-fold helical screw xylan conformations on the cellulose surface, while O-3 substituted xylan plays a role in binding to lignin rather than cellulose in plant cell walls.

Spontaneous change in O-2 acetylated xylan conformations on the (110) hydrophilic cellulose surface

ACS Spring Meeting 2021

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