Hemicellulose binding and the spacing of cellulose microfibrils in spruce wood

Abstract

Cellulose microfibrils in conifers, as in other woody materials, are aggregated into loose bundles called macrofibrils. The centre-to-centre spacing of the microfibrils within these macrofibrils can be estimated from the position of a broad diffraction peak in small-angle neutron scattering (SANS) after deuteration. A known spacing of 3.0 nm, increasing with moisture content, is consistent with direct microfibril to microfibril contact. However recent evidence indicates that conifer microfibrils are partially coated with bound xylan chains, and possibly with lignin and galactoglucomannan, implying a wider centre-to-centre spacing as found in angiosperm wood. Delignification of spruce wood allowed a weak SANS peak to be observed without measurable change in spacing. By deuterating spruce wood in mildly alkaline D 2 O and then re-equilibrating with ambient H 2 O, deuterium atoms were trapped in a position that gave a 3.8 nm microfibril spacing under dry conditions as in angiosperm wood, instead of the 3.0 nm spacing normally observed in conifers. After conventional vapour deuteration of spruce wood a minor peak at 3.8 nm could be fitted in addition to the 3.0 nm peak. These observations are consistent with some microfibril segments being separated by bound xylan chains as in angiosperms, in addition to the microfibril segments that are in direct contact.

Nanostructural deformation of high-stiffness spruce wood under tension

“…Deuterated spruce wood gives two overlapped coherent SANS peaks: a major peak with a characteristic spacing of 3.0 nm when dry 9 , corresponding to close-packed 3 nm microfibrils and widening on hydration. A minor 3.7 nm peak has been interpreted as microfibrils separated by bound xylan 14 . To determine if microfibril spacings changed under stress, SANS data were collected during a stress-relaxation experiment on deuterated spruce wood (Fig.…”

“…940–950 cm −1 ) were crowded or had little slope so that correlation coefficients were low. However O-D stretching intensity at 2494 cm −1 in alkaline-deuterated samples is now thought to arise from cellulose chains underlying bound xylan 14 . In this region negative bandshifts were observed (negative because O-D stretching vibrations move to higher frequency when the associated hydrogen bonds elongate 14 ).…”

“…Is it the same as the ‘Molecular Velcro’ mechanism suggested 2 , 17 for wood with high microfibril angle? Where does it occur, within or between the microfibril bundles (macrofibrils) now known to occupy the main S2 layer of the softwood cell wall 12 – 14 ? Within the microfibril bundles, SANS experiments reported here and elsewhere 14 show that there is direct or hydrated contact between microfibril surfaces, some of which are xylan-coated 5 .…”

“…Cellulose microfibrils aggregate into loosely structured macrofibrils, typically 15–20 nm in diameter 12 . There are at least occasional close interactions between all the cell-wall polymers 5 , 13 , but much of the xylan appears to be within the macrofibrils and is bound to cellulose surfaces 5 , whereas more of the glucomannan and lignin lie between the macrofibrils 13 , 14 . There is little difference in chemical composition between wood with high and low tensile stiffness.…”

Surface modification effects on nanocellulose – molecular dynamics simulations using umbrella sampling and computational alchemy

“…In reality, the cellulose-cellulose contact points may be less perfect due to other configurations (anti-parallel or at an angle), nanofibril twisting (65,61), or contacts between different crystallographic planes (58). Such imperfections reduce the possibility to fuse completely, and make individual fibrils maintain their integrity as separate entities while still having direct fibril-fibril contact (18,66,67). Nevertheless, the present model is relevant for any material system where cellulose nanoparticles tend to aggregate, such as in suspensions of bacterial cellulose, tunicate whiskers, CNF or CNC.…”