Cellulose occurs with few exceptions together with other cell wall polysaccharides, which structure and monosaccharide composition differs between species and cell wall location. Common such polysaccharides include xylans, glucomannans, β (1→4) glucans, mixed glucans, galactanes, galacturonans, and rhamnogalacturonans. These are somewhat whimsical divided in hemicelluloses and pectins based on, among others, extrability. Here, we propose a stricter structural and functional classification of non-cellulose wood polysaccharides taking into account their molecular and conformational features, which in turn affect their association with cellulose and lignin and their function in the cell wall. We suggest that hemicelluloses are polysaccharides with a backbone chain that can adapt the same conformation as the cellulose I-type structure, i.e., a flat 2-fold helical screw, whereas pectins cannot. This structural division might also reflect a functional difference, where hemicelluloses with their possibility to line up, or “co-crystalize”, with cellulose chains might be designed for a very close contact with cellulose – maybe as a part of a “composite” material forming flexible bridges between cellulose fibrils, whereas pectins are also located more in regions with lower cellulose content, such as middle lamella, – maybe more act as a filling substance. However, within both groups there are differences between different polysaccharides; for instance, the organization of hydroxyl groups and side chains around the glycosidic linkage give different flexibility of the main chain and ability to adapt to other structures than the “cellulose-type” 2 fold screw, which have been studied by molecular dynamics simulations and NMR studies. Furthermore, different pectins have structural and flexibility differences in their main chains. Also, side chains/groups can affect main chain conformation. Another way to divide non-cellulose wood polysaccharides are for neutral respectively negatively charged polysaccharides. The role of the charges might differ; in the pectin galacturonan they form crosslinking” egg box” complexes with calcium ions, and in xylan they might play a role in affecting lignin structure and form covalent bonds to lignin, and maybe also disperse cellulose.

Non-cellulose wood polysaccharides – a need for a stricter structural and functional classification?

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.