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. 2021 Oct 20;22(11):4669–4680. doi: 10.1021/acs.biomac.1c00937

Figure 8.

Figure 8

Schematic model of a cellulose microfibril with a hydrated matrix and the changes occurring during the drying and rehydration processes as interpreted from the experimental solid-state NMR and MD simulations. (a) Transversal section of a cellulose microfibril with a 2-3-4-4-3-2 habit surrounded by the hemicellulose and lignin matrix. The cellulose is surrounded by a continuum of water molecules due to the presence of hydronium ions as the pH of the softwood cell wall is around 5.5. The water molecules present at the cellulose xylan interface (outlined in red) are tightly bound and exchange with the bulk of the water molecules at a slow rate. AGX and GGM molecules close to the cellulose microfibrils are in a two-fold screw configuration. GGM is divided into two categories (Man/Glc/Gal = 3:1:1.2 and DSAc = 0.1) and (Man/Glc/Gal = 4:1:0.3 and DSAc = 0.4).26 Glucuronic acid interacts with the lignin complex via electrostatic interactions. (b) Upon complete dehydration, a general collapse of the matrix surrounding the cellulose microfibril is observed. It leads to shrinkage in size with the lignin moving closer to the cellulose. The xylose now directly interacts with the cellulose surface and GGM moves closer to the cellulose surface. While the xylose residues of xylan are barely affected, the arabinose side chains start interacting with surrounding molecules in an aspecific manner. (c) Upon rehydration as the lignin–cellulose and hemicellulose–cellulose distances increase, the microfibril unit expands with its size becoming comparable to its original value. However, some changes are observed between the never-dried state and rehydrated state. The changes are highlighted with red dashed circles. ① The xylan–cellulose interface is not rehydrated. ② The rewetting of the cellulose is almost complete, but some of the cellulose does not revert to a domain 2 conformation. ③ The GGM regains its original position. However, a fraction of the GGM molecules becomes more mobile with a configuration similar to that in solution. ④ The arabinose side chains become more ordered and some arabinose becomes more mobile.