The 24-chain core-shell nanostructure of wood cellulose microfibrils in seed plants

Abstract

Wood cellulose microfibrils (CMFs) are the most abundant organic substance on earth, but their nanostructures are poorly understood. There are controversies regarding the glucan chain number (N) of CMFs during initial synthesis and whether they become fused afterwards. Here, we combined small-angle X-ray scattering (SAXS), solid-state nuclear magnetic resonance (ssNMR) and X-ray diffraction (XRD) analyses to resolve these controversies. We successfully developed SAXS measurement methods for the cross-section aspect ratio and area of the crystalline-ordered CMF core, which showed higher density than the semi-disordered shell. The 1:1 aspect ratio suggested that CMFs remain mostly segregated, not fused. The area measurement revealed the chain number in the core zone (Ncore). The ratio of ordered cellulose over total cellulose, termed Roc, was determined by ssNMR. Using the formula N = Ncore / Roc, we found that the majority of wood CMFs contain 24 chains, conserved between gymnosperm and angiosperm trees. The average wood CMF has a crystalline-ordered core of ~2.2 nm diameter and a semi-disordered shell of ~0.5 nm thickness. In naturally and artificially aged wood, we only observed CMF aggregation (contact without crystalline continuity) but not fusion (forming conjoined crystalline unit). This further argued against the existence of partially fused CMFs in new wood, overturning the recently proposed 18-chain fusion hypothesis. Our findings are important for advancing wood structural knowledge and more efficient utilization of wood resources in sustainable bio-economies.