Effects of Xylan Side-Chain Substitutions on Xylan–Cellulose Interactions and Implications for Thermal Pretreatment of Cellulosic Biomass

Abstract

Lignocellulosic biomass is mainly constituted by cellulose, hemicellulose, and lignin and represents an important resource for the sustainable production of biofuels and green chemistry materials. Xylans, a common hemicellulose, interact with cellulose and often exhibit various side chain substitutions including acetate, (4-O-methyl) glucuronic acid, and arabinose. Recent studies have shown that the distribution of xylan substitutions is not random, but follows patterns that are dependent on the plant taxonomic family and cell wall type. Here, we use molecular dynamics simulations to investigate the role of substitutions on xylan interactions with the hydrophilic cellulose face, using the recently discovered xylan decoration pattern of the conifer gymnosperms as a model. The results show that α-1,2-linked substitutions stabilize the binding of single xylan chains independently of the nature of the substitution and that Ca ions can mediate cross-links between glucuronic acid substitutions of two neighboring xylan chains, thus stabilizing binding. At high temperature, xylans move from the hydrophilic to the hydrophobic cellulose surface and are also stabilized by Ca cross-links. Our results help to explain the role of substitutions on xylan-cellulose interactions, and improve our understanding of the plant cell wall architecture and the fundamentals of biomass pretreatments.

Regular Motifs in Xylan Modulate Molecular Flexibility and Interactions with Cellulose Surfaces

“…Although the conformation of the backbone is the same on both surfaces considered here, the hydrophobic surfaces seem to stabilize the conformation of the side groups docked on them compared with the hydrophilic surfaces. This conclusion is in agreement with previous observations reporting the migration of an AGX chain from the hydrophilic surface to the hydrophobic surface of cellulose upon thermal treatment at 160°C using MD simulations (Pereira et al, 2017).…”

“…It is noteworthy that the latter two were positioned with their side groups pointing away from the surface, whereas XXXU m U m X, which exhibited the strongest interaction, has one side group pointing directly to the cellulose. This is in agreement with a recent study that showed that a-(1→2)-linked substitutions improve the interaction between xylan and cellulose (Pereira et al, 2017). These results also clearly show that the interaction with the hydrophobic (200) surface is stronger compared with the hydrophilic (1-10) one.…”

“…These results also clearly show that the interaction with the hydrophobic (200) surface is stronger compared with the hydrophilic (1-10) one. This was also suggested previously (Pereira et al, 2017), where it was seen that xylan diffused from hydrophilic to hydrophobic surfaces at elevated temperatures. Therefore, a substituted structure is a clear advantage for the interaction with the (200) hydrophobic surface, where the weakest interaction was observed for the unsubstituted structure.…”

Diffuse Growth of Plant Cell Walls

“…This depiction is a synthesis based on the most recent results from AFM, FESEM, solid-state NMR, and mechanics. the hydrophilic faces of cellulose (Simmons et al, 2016;Grantham et al, 2017;Pereira et al, 2017). Primary cell walls generally contain only small amounts of xylan, e.g.…”

Enzymatically Debranched Xylans in Graft Copolymerization

“…37 Both the empirical and computational analyses confirmed that high molecular weight and unsubstituted xylans show highest affinity to cellulose; 35−38 molecular dynamics simulations also predict impacts of substituent distribution on the stability of cellulose associations. 38,39 In the present study, two different arabinofuranosidases were used to selectively remove distinct α-L-Araf substituents from wheat arabinoxylan (WAX) prior to graft copolymerization with glycidyl methacrylate (GMA). GMA was selected as the monomer for the grafting experiments as it was previously shown to react well with polysaccharides, including hemicelluloses, 18 and because the epoxy groups introduced through GMA grafting creates sites that can be further chemically modified depending on the end use.…”